GB2382014A

GB2382014A - Illumination of Plants

Info

Publication number: GB2382014A
Application number: GB0226549A
Authority: GB
Inventors: John Walter Allen
Original assignee: University of St Andrews
Current assignee: University of St Andrews
Priority date: 2001-11-14
Filing date: 2002-11-14
Publication date: 2003-05-21
Anticipated expiration: 2022-11-14
Also published as: GB2382014B; GB0226549D0

Abstract

A method is described for illuminating a horticultural growing environment without interfering with plant development, comprising illuminating the environment with light having a peak wavelength in the range 510-535 nm. Also described is an associated apparatus, comprising at least one cluster of LED's having a spectral peak centred in the range 510-535 NM. Preferably, the radiation intensity is not more than 15mWm<SP>-2</SP> at the plant. There is also a monitoring system comprising an LED light source with a peak wavelength in the range of 510-535 NM, at least one CCTV camera which is sensitive to the above wavelength range, and a way of activating the light source in response to movement, time, date or demand from an operator. A method is also described for promoting the flowering of a plant, by illuminating the plant with light having a peak wavelength in the range of 605-645 NM as a supplement to the light the plant is normally exposed to. There is also a method of promoting root growth of plant cuttings, by illuminating the cuttings with red light at a first illumination intensity and blue light at a second illumination intensity, as a supplement to the light the cutting is normally exposed to.

Description

Title: Improvements in and relating to rooting and flowering of plants and illuminating of plant growing environments Field of invention This invention concerns the illumination of plant growing environments and in particular a methods and apparatus by which such environments can be illuminated either so as not to affect the development of the plants therein, or to promote rooting and/or flowering.

The invention also concerns the production of flowers by plants and to a method for promoting flowering.

The invention also concerns the production of roots by plant cuttings and in particular to a method and apparatus for promoting rooting of cuttings.

Background to the invention Plants obtain the energy required for their life processes by absorbing radiant energy from sunlight and converting it by photosynthesis into chemical energy. In addition to photosynthetic chlorophyll there are many other photoactive pigments in plants which control such things as shape or time of flowering. Plant photobiology is hence a complex subject. For the incident light one must consider e. g. spectrum, intensity and timing of application, while different plants have very different responses and different parts of a single plant may also respond differently: for example a leaf may respond differently from a leaf-node. Some useful sources of information are the following.

"Light and plant responses"T. H. Attridge (Edward Arnold) 1990 "Photoperiodism in plants"Brian Thomas and Daphne Vince-Prue (Academic Press) 2nd Edition 1997 "Phytochromes and light signal perception by plants-an emerging synthesis"Harry Smith, Nature 407 585-591 (2000) Gardeners and horticulruralists use plant photoresponses in a variety of ways. At its simplest one classifies plants as shade tolerant or not and plants them accordingly. A more interventionist approach is a crude control of light, as in forcing rhubarb. Beyond this, one can have illuminated greenhouses to give an artificial daylight or, on a smaller scale, domestic use of fluorescent lamps to encourage flowering in Saintpaulia. For commercial horticulture, sodium lamps or fluorescent lamps, with or without specially designed phosphors, are used with the choice dependent on economic factors. Timing is used not only to control day-lengths but also, for example, to control flowering in chrysanthemums by a short period of light during the night.

Light-emitting diodes (LED's) are semiconductor devices which convert electrical power into radiant power. They emit in a narrow wavelength band and are produced with peak wavelengths which range from near ultraviolet through the visible to the infrared. The current efficiency of red LED's is greater than that of fluorescent lamps. Estimated operating times of 10 years are normal. Large-scale production has brought prices down. The combination of high efficiency of red LED's and their long operating life leads to lower operating costs in maintenance and electrical power.

There is now great interest in extending the use of LED's from display to illumination. Two developments which have driven this interest are the major increase in efficiency over the past few years and the recent availability of green and blue LED's which has extended the spectral range to cover the whole of the visible region. Additionally and more importantly this has allowed plant growth characteristics such as leaf formation, flower

production, root production and the like to be investigated in relation to the wavelength of light to which the plant is subjected.

However, it has also become apparent that even a low level of exposure of plants to light of wavelengths other than those which control or promote particular characteristics of growth and development, can adversely affect the controlled growth and/or development of the plants.

It is one object of the present invention to provide a method and apparatus by which a horticultural plant growing environment can be illuminated without affecting the development or growth of the plants.

In addition to the foregoing there is now great interest in extending the use of LED's from display to illumination. Two developments which have driven this interest are the major increase in efficiency over the past few years and the recent availability of blue LED's which has extended the spectral range to cover the whole of the visible region.

It is a second independent object of the present invention to provide a separate method and apparatus by which LED's can be introduced into horticultural illumination for the purpose of promoting the flowering of plants.

It is a third independent object of the present invention to provide a third separate method and apparatus by which LED's can be introduced into horticultural illumination for the purpose of promoting root formation by plant cuttings.

Summary of various aspects of the invention According to a first aspect of the present invention linked to the first object a method of illuminating a horticultural growing environment so as to minimise interference with plant development or growth comprises the step of illuminating the environment with light

having a peak wavelength or wavelengths in the range 510-535 nm, typically in the range 510-530 nm and preferably substantially monochromatic light centred on 525 nm..

Apparatus also linked to the first object for illuminating a horticultural growing environment so as to minimise interference with plant growth or development comprises at least one cluster of LED's having a spectral peak centred in the range 510-535 nm, preferably in the range 510-530 nm, and more particularly in the range 515-525 nm, and preferably substantially monochromatic light centred on 525 run.

Preferably"high brightness"LED's are employed.

The number of LED's and/or their spacing and/or operating current and/or the distance to the plants is selected so that a radiation intensity of not more than 15mWm-2 of the illuminating radiation exists at the plants.

The first aspect of the invention allows growing plants to be inspected and/or monitored by using light in the range 510-535 nm and at an illumination intensity of not more than 15 mWm' which has been found not to affect the growth or development of the plants which is being controlled by illumination at other wavelengths to which the plants are subjected for some or all of the time.

A cluster of 6 LED's each having a 450 view angle, producing light for example at 525 nm, with a 3-6% power conversion efficiency and powered by an operating current of 25 mA per LED will give an illumination intensity at a distance of 1.5m of less than 15 mWm' over a circular area of 1m diameter.

Although illumination at 525 nm and at an illumination intensity of the order of 15mWm-2 can probably be left on for indefinite periods of time without significantly affecting growth of most plants, it is preferred that a control system is provided to turnoff the LED's when not required, so as to minimise illumination even at this relatively safe wavelength and intensity.

By locating clusters of such LED's around a growing environment subjected to primary illumination for controlling growth and/or development characteristics of the plants therein, it is possible for the environment to be monitored for security purposes, and/or allows the plants to be inspected either in person or by cameras or other inspection devices, without affecting the growth or development characteristics which the primary illumination is controlling.

The first aspect of the invention also lies in a system comprising in combination at least one LED light source which when energised will produce light of wavelength or wavelengths as specified herein, at least one CCTV camera, chosen or modified to have high sensitivity to light of the aforesaid wavelength or wavelengths, and means for energising the light source in response to a time and/or date determined signal or a demand signal from a human operator wishing to view the environment via the CCTV camera, or from a movement sensing device such as are employed in security systems.

The first aspect of the invention also lies in the combination of such a system with a video recorder.

According to a second aspect of the present invention linked to the second object of the invention a method of promoting the flowering of a plant comprises the step of illuminating the plant with supplementary light in the range 605-645 nm wavelength as a supplement to the normal illumination to which the plant would normally be exposed.

The normal illumination may be daylight and/or sodium lamp light and/or fluorescent lamp light.

The supplementary illumination may be for less than, the same as or longer than the duration of the normal illumination.

Best results have been obtained where the supplementary exposure period is longer than the duration of the normal illumination.

Apparatus also linked to the second object of the invention for providing the supplementary illumination for use in a method according to the said second aspect comprises clusters of LED's having a spectral peak in the range 605-645 nm, preferably centred on 640 nm.

Preferably"high brightness"LED's are employed.

Preferably a radiation density of the order of 1.5 watts per square metre at the plants is employed, and the number of LED's and/or the operating current and/or the height of the LED's above the plants and/or the spacing of the LED's or clusters of LED's is are selected and/or adjusted to achieve this density.

In a preferred arrangement for a glass-house growing environment LED luminaires each comprised of 50 high brightness LED's with 300 view angle in 5 rows of 10, each luminaire requiring 24V at 125mA, has been found to be very suitable. Where the plants cover a significant area, substantially uniform and adequate supplementary illumination has been obtained by positioning 4 luminaires per square metre at a height of 1 metre or more above the plants. Such an arrangement has been employed to accelerate the flowering of Fuchsia cultivars, but the method and apparatus is not limited to fuchsias and can be employed to accelerate the flowering of other plants.

In the case of Fuchsias the supplementary illumination extends beyond the normal illumination period and typically is supplied from 7 am to 11 pm BST at the density of 1.5 watts per square metre.

A similar extension to normal illumination using supplementary light in the same range of 605-645 nm, may be employed to accelerate the flowering of other so-called long day plants.

Where the LED's are to supplement daylight or other illumination, the LED's are preferably arranged in transparent or translucent supports so as to minimise shading of the plants from daylight or other illumination.

Linked to the said second aspect of the invention, the wavelength of the supplementary illumination is chosen to match the spectral response of the flowering pigment of a particular plant, for the purpose of controlling the time to flowering.

According to a third aspect of the present invention linked to the said third object of the invention a method of promoting the production of roots by a plant cutting comprises the step of illuminating the cutting with supplementary red light at a first illumination intensity at the cutting together with supplementary blue light at a second illumination intensity at the cutting, as a supplement to the normal illumination to which the cutting would normally be exposed.

The intensity of the red light at the cutting may be the same as or more preferably different from the intensity of the blue light at the cutting.

In one embodiment the red light illumination intensity is 20Wm-2 while that of the blue light illumination is 1. 5Wm-2, at the plants.

Typically the red light is centred on 660 nm.

Typically the blue light is centred on 470 nm.

Typically the red and blue light is obtained from red and blue LED's.

When using LED's as is preferred, the desired radiation intensity such as 1.5 Wm-'for blue light and 20 Wu. 2 for red light at the plant cuttings may be achieved by adjusting the overall number and/or relative numbers of the LED's and/or the operating current and/or the height of the LED's above the cuttings and/or the spacing of the LED's or clusters of LED's.

The supplementary illumination may be for less than, the same, as or longer than the duration of the normal illumination.

Typically the supplementary light is supplied for 16 hours in each period of 24 hours.

The method may be, and typically will be, applied to more than one cutting but if so, it is important to ensure that the two wavelengths of light are received generally uniformly by the cuttings. If the LED light source does not produce uniform illumination over an area of cuttings, then either the LED source or the cuttings may be rotated so as to remove at least some of the non-uniformity of illumination.

Apparatus for providing the supplementary illumination for use in a method provided by the said third aspect of the invention comprises a cluster of 660 run and 470 nm LED's in the ratio of approximately 25: 4 respectively.

Preferably"high brightness"LED's are employed.

Preferably clusters of LED's are mounted in a transparent or at least translucent support so that when positioned above a cutting or cuttings the support material does not create a shadow or shade the cuttings.

In a preferred arrangement for a glass-house growing environment an LED luminaire suitable for such an environment, at a height of 50 cm above the cuttings, comprises fifty high brightness 660 nm LED's in five rows of ten, together with eight high brightness 470 nm LED's which may be arranged symmetrically around, or to one side or centrally of the five rows of ten 660 nm LED's.

Alternatively the luminaire may contain five rows of LED's each containing ten 660 nm LED's and two 470 nm LED's.

Where the plants cover a significant area, substantially uniform and adequate supplementary illumination has been obtained by arranging four such luminaires per square metre. Such an arrangement has been employed to accelerate the rooting of Fuchsia magellanica Riccartonii, but the method and apparatus is not limited to fuchsias and can be employed to accelerate the rooting of other plant cuttings.

If required, the luminaires may be rotated in a plane parallel to the growing medium containing the cuttings (or the cuttings may be rotated relative to the luminaires) so as to render the mean illumination incident on the cuttings to be substantially more uniform than might otherwise be the case. To this end in one embodiment drive means is provided for rotating either the luminaires or containers planted up with the cuttings.

In the case of Fuchsia magellanica Riccartonii the supplementary illumination is preferably switched on for a period of 16 hours within or extending beyond the normal daylight hours of each period of 24 hours.

Experiments suggest that shorter periods of exposure to the supplementary illumination may be sufficient to accelerate root formation of cuttings of other plants.

An advantage of employing supplementary illumination as aforesaid is that rooting of the cuttings appears to occur at lower average temperatures than are normally assumed to be necessary to achieve reliable rooting to occur. Thus Fuchsia m. R. cuttings subjected to 16 hours of supplementary illumination as aforesaid rooted at average temperatures in the range 14-16OC whereas it is usually assumed that an average temperature of 18 C is required for such cuttings to form roots.

The following definitions and parameters and arrangements of apparatus apply to the first aspect of the invention.

Colour An LED emits in a narrow wavelength band, the peak wavelength depending on the chemical composition of the semiconductor from which it is made.

Nearly all visible and near infrared LED's are made from III- V compounds, containing equal numbers of atoms from Group III and Group V of the Periodic Table. By using "mixed"crystals the manufacturer has flexibility in choosing the wavelength. For example, by using AlxGal-xAs one can choose a wavelength between red and 900nm by choosing the value of x. There are two groups of materials. The nitrides are most efficient in the blue and green spectral regions. The non-nitride III-V materials do not produce blue light. size Typically the emitting semiconductor is 0.35 mm across. It is encapsulated in a special epoxy resin which protects it and also is shaped as an optical element. For experimental work standard diodes with diameters of either 3 mm or 5 mm have been used. Their length is typically 9 mm.

Beam size The epoxy encapsulant is in the form of a slightly tapered cylinder with a hemisphere at the end acting as a lens. The object is to collect as much light as possible from the emitting semiconductor and to put it out in a conical beam. By choosing the

optical parameters of the encapsulant (length and taper, curvature of the lens, refractive index of the epoxy) the manufacturer can determine the angle of the cone of the beam and this is normally given in the diode specification. Diodes have been used with a 45 cone angle. The cone angle is sometimes referred to as a view angle.

Where more localised illumination is required in relation to the first and second aspects of the invention, LED's of smaller view angle such as 30 or even 15 may be employed.

For illumination of plants the radiant power density (e. g. W m-2) at the plant is a relevant quantity. The suppliers of visible LED's usually specify the view angle, i. e. the angle of the cone of light, and the on-axis brightness in photometric units (candelas). To achieve high on-axis brightness the view angle is made small. This is inefficient in light collection.

Empirically, for a given semiconductor emitting element, the efficiency (radiant power in the beam divided by electrical power) is approximately proportional to the inverse of the view angle. It follows that, for a given wavelength, the optimum choice is not the brightest LED. Instead, the view angle should be the maximum acceptable for the application, then given this angle the maximum brightness should be chosen.

LED variation Although LED's are supplied as RED or GREEN or BLUE light emitting devices, the actual wavelength of light emitted by any particular LED can vary from batch to batch of the same manufacturer and model number. Since the present invention requires light centred on particular wavelengths to be used, it may be necessary to check the actual wavelength of light emitted by each LED to be employed and to select and reject LED's accordingly before they are incorporated into an illumination source for use in methods and apparatus embodying the invention. Therefore although examples of LED's are given these are intended merely as an indication of devices which have been used. Since new devices are being brought to market every year and existing ones rendered obsolete or being phased out of production, LED's must be selected from those available at the time for light output.

Electrical drive An individual green nitride LED requires a drive of about 3.6V DC at a current of 20-30 mA. (Because an LED is a rectifier it is also possible to use an AC voltage). It is convenient to use series-parallel combinations of diodes. For example, three LED's could be connected in series and two of such series connections could be put in parallel, giving a total of six LED's requiring a power supply of 50 mA at 12V with a 25 ohm dropping resistor. In the unlikely event of one LED failing, the others in the series connection would be affected and the light output would dip by 50% but the other three LED's would continue to supply illumination.

An advantage of LED's is that the light output is proportional to the current so one can control the illumination intensity simply by controlling the drive current, without affecting the spectrum of the light emitted.

A Table of properties of some LED's is contained in Table 1 and power conversion efficiencies for these LED's are shown in Fig 1.

The following definitions and parameters and arrangements of apparatus apply to the said second aspect of the invention.

Colour An LED emits in a narrow wavelength band, the peak wavelength depending on the chemical composition of the semiconductor from which it is made. Nearly all visible and near infrared LED's are made from III-V compounds, containing equal numbers of atoms

from Group III and Group V of the Periodic Table. By using"mixed"crystals the manufacturer has flexibility in choosing the wavelength. For example, by using AlxGal-xAs one can choose a wavelength between red and 900nm by choosing the value of x. There are two groups of materials. The nitrides are most efficient in the blue and green spectral regions. The non-nitride III- V materials do not produce blue light: their efficiency increases as one goes from green to infrared. These properties are shown in Table I and

Figure 1. At present the ultraviolet and infrared LED's are only available from specialist suppliers, the others are easily available.

Size Typically the emitting semiconductor is 0.35 mm across. It is encapsulated in a special epoxy resin which protects it and also is shaped as an optical element. For experimental work standard diodes of either 3 mm or 5 mm diameter and 9mm length have been used.

Beam size The epoxy encapsulant is in the form of a slightly tapered cylinder with a hemisphere at the end acting as a lens. The object is to collect as much light as possible from the emitting semiconductor and to put it out in a conical beam. By choosing the optical parameters of the encapsulant (length and taper, curvature of the lens, refractive index of the epoxy) the manufacturer can determine the angle of the cone of the beam and this is normally given in the diode specification. Diodes have been used with a 30 cone angle. The cone angle is sometimes referred to as a"view angle", For illumination of plants the radiant power density (e. g. W m-2) at the plant is a relevant quantity. The suppliers of visible LED's usually specify the view angle, i. e. the angle of the cone of light, and the on-axis brightness in photometric units (candelas). To achieve high on-axis brightness the view angle is made small. This is inefficient in light collection.

Although LED's are supplied as RED or GREEN or BLUE light emitting devices, the actual wavelength of light emitted by any particular LED can vary from batch to batch of

the same manufacturer and model number. Since the present invention requires light centred on particular wavelengths to be used, it may be necessary to check the actual wavelength of light emitted by each LED to be employed and to select and reject LED's accordingly before they are incorporated into an illumination source for use in methods and apparatus embodying the invention. Therefore although examples of LED's are given these are intended merely as an indication of devices which have been used. Since new devices are being brought to market every year and existing ones rendered obsolete or being phased out of production, LED's must be selected from those available at the time for light output.

Electrical drive An individual visible LED requires a drive of about 2V DC at a current of 20-30 mA.

(Because an LED is a rectifier it is also possible to use an AC voltage). It is convenient to use series-parallel combinations of diodes. For example, 10 LED's could be connected in series and 5 of such series connections could be put in parallel, giving a total of 50 LED's requiring a power supply of 125 mA at 24V. In the unlikely event of one LED failing, only the others in the series connection would be affected and the light output would only dip by a fairly small percentage. Blue and green nitride LED's require a higher individual voltage, typically 3.6V.

At 20 mA per LED the expected operating life is of the order of ten years. Because LED technology is advancing so rapidly it may be economically advantageous to run the LED's at say 50 mA. This reduces the initial capital cost by reducing the number of LED's required for a given illumination level. The operating life will be shortened but this does not matter if it is economic to replace the LED's by improved ones after say three years.

Clusters or arrays Experiments have been performed using clusters of from 3 to 50 LED's. Each cluster was of LED's of a particular wavelength. When illumination at two wavelengths was required appropriate clusters were placed side by side. If required, clusters containing a mixture of wavelengths can be assembled. For illumination at 660nm, commercially-available clusters of 50 LED's, wired and encapsulated, are available. These are driven at lO. 5V, 250 mA.

For domestic use or for the amateur gardener it is possible to use smaller clusters of e. g. 6 LED's at a height of 30cm to illuminate a plant or plants in, for example, a single 12 cm pot. At this distance, the boundary of the illuminated area is reasonable well-defined, so it is possible to illuminate a single pot with little overspill of light onto a plant or plants in adjacent pots.

The following advantages are attributable to the use of LED's as opposed to other light sources for plant flowering control.

The good efficiency of LED's combined with the fact that the radiation can be in a required wavelength band, results in very good overall efficiency. This, combined with the long operating life, leads to lower operating costs to offset the higher capital cost.

Because the radiation intensity is proportional to the electrical current it is easy to make control systems. For example, LED's may be used to supplement natural daylight, with a control system to give a total radiation intensity at a desired level which can be a function of time.

The directionality of the light beam from an LED allows the light to be directed onto the plant to minimise light wastage.

Results of early experiments in St Andrews relating to the said second aspect of the invention Preliminary experiments on plants such as Saintpaulia, Fuchsia varieties and Campanula isophylla indicated that the time to flowering could be affected using modest arrays of LED's. Typically combinations of wavelengths were employed using e. g. 660 nm + 470 nm LED's. The directionality of LED light output enabled the light to be used effectively, so adjacent plants or groups of plants could be given different light treatments.

A more extensive series of experiments was begun in March 2000 using daylight as the control, 660 nm + 470 nm LED combinations, and red-light only LED's nominally producing 660 nm light, a radiation density at the plants of the order of 20 Wm for the 660 nm light and much less for the 470 nm light. Plants in plug form, 10 plants of each kind, were potted into 12 cm or 14 cm pots, the 10 being distributed 3,3, 4. After two weeks those kinds which had unacceptable variation in growth rate from plant to plant were removed from the experiment. The plants were placed in an unheated south-facing room, in trays containing a bed of Hortag. Clusters of LED's were mounted 30 cm above the soil surface. For each kind of plant, the pot with 4 plants was used as a control with just ambient light, the other two were used with different illuminations augmenting the ambient light. (The pots were large enough that the use of 4 to a pot did not lead to appreciable competition for resources). At a later stage those plants which grew large enough to have a substantial part outside the illuminated areas were transplanted into individual pots. In this particular series of experiments the main characteristics measured were the length of the longest stem and the time of first flowering: where appropriate, other characteristics were measured.

In the experiments a 300 angle was found to be suitable for illuminating plants in 12cm pots with the LED's 30cm above them. Given the required wavelength and this view angle, LED's from different manufacturers could be compared to give the maximum brightness.

Early spring was cold and frequently overcast, with daytime temperatures about 11 C until mid-April 2000 (about day 20). Initial growth was therefore slow. Thereafter, day temperatures increased to 14-16 c.

Whilst flowering patterns were found to be influenced accelerated flowering in some species seemed to be linked to significantly increased (and undesirable) leaf production.

Results for individual plants are given in the following examples.

Example I F. "Spion Kop". In February 2000 ten plants were used, again in a heated laboratory. The three illumination regimes were (a) control, (b) 660 nm at 20 Wm, 2 and (c) 660 nm at 20 Wu'2 plus 470 nm at 0.8 Wu-2. Illumination started on 3rd February 2000 and was switched off on 22nd March 2000 (48 days). Up to 64 days the 660 run plants grew more strongly while the addition of 470 nm counteracts this. These effects persisted after switching off the LED's.

The plants with red + blue (660 + 470 nm) illumination were appreciably darker at the time the LED's were switched off but this effect had disappeared two weeks later. The

plants which had 660 nm first flowered on 6""May 2000 (day93), those with 660 + 470 nm a month later on 6"June 2000 (day 124) and the controls nearly two weeks later again on 18"June 2000 (day 136).

Example II F."Snowcaps". These were in the main sequence of experiments beginning 25th March 2000. The illumination regimes were (a) control, (b) infrared 740 nm at 1.8 Wu~2 ana (c) 660 nm at 20 Wu, 2 plus 740 nm at 1.8 Wm-2. It is interesting that the plants with 740 nm grew much faster than the control, which suggests that in this variety turning Pfr to Pr

increases growth rate. The 660 + 740 run caused even faster growth initially but after 73 days some plants had grown large enough to leave their light cones, confusing the results.

The LED illumination not only produced longer main stem length but also produced larger leaves. On 8th July 2000 (day 105) the length of the leaves between 10 cm and 20 cm along the stem from the earth was measured with the following results.

Control 740 nm 660 + 740 nm Mean (cm) 29.3 32.4 49. 4 Standard deviation (cm) 5.7 4.8 5.3 The first flowering was on the 660 + 740 nm plants on 26'July 2000,19 days after the LED's were switched off.

Example III F."Son of Thumb". These also were in the main sequence of experiments beginning on 25 March 2000, with the LED's switched off on 6'July 2000 (day 103). The illumination regimes were (a) control, (b) 660 nm at 20 Wm' and (c) 660 nm at 20 Wm-2 and 470 nm at 1.2 cm-2. The red 660 nm illumination produced an increased growth rate but red 660 nm plus blue 470 nm produced an even greater increase. (Again, at the longest times the results are confused by the plants growing out of the light cones). This is in marked contrast with F."Spion Kop"where 470 nm illumination appeared to inhibit growth.

On 6'July 2000 (day 103) the plants with 660 + 470 nm illumination had darker and smaller leaves than the control, despite the increased stem length, and had flower buds, which others did not.

F."Coachman","Pink Galore","Bon Accord"and"Natasha Sinton"were also grown, but there was too much variability among the plants for firm conclusions to be drawn.

Results of Year 2001 experiments also relating to the said second aspect of the invention At the beginning of 2001 systematic experiments were undertaken using a single type of plant but in large numbers and with replication of the experiment. This was to demonstrate that effects of interest to the commercial grower could be achieved and could be reproduced.

High efficiency LED's in the 605-645 nm region had become available and for comparison 640 run (nominal) LED's were employed in one test, 660 nm (nominal) LED's were employed in another simultaneous test, and ordinary daylight was used in a third simultaneous test as the control.

The 2001 experiments used the Fuchsia cultivar"Beacon", a commercially important plant.

A first experiment lasted from the beginning of February to the end of May. The LED's were on from 7 am to 11 pm, i. e. a 16-hour day, supplementing daylight in a south-facing room in a building in St Andrews, Scotland.

The first fully-opened flower from each of the three tests was recorded as follows: - after 97 days with 640 nm LED's - after 117 days with 660 nm LED's - after 130 days with daylight The replication of the first experiment used the same Fuchsia cultivar, employed the same three tests, was set up in the same room in St Andrews, Scotland, but under more precisely controlled conditions. Thus the temperature was maintained at 19 C +/-2 C and the radiation power density at the plants was maintained in the range 1. 0-1. 5 Wu-2.

The second experiment lasted from 29"'June to the end of October 2001. The first fully- opened flower from each of the three tests making up the second (replication) experiment was recorded as follows: - after 77 days with 640 nm LED's - after 84 days with 660 nm LED's - after 116 days with daylight.

Both experiments indicated a shortened period to flowering using Red LED supplemented light relative to daylight, and the shorter period in both experiments was obtained using 640 nm LED's as opposed to 660 nm LED's.

At the level of supplementary radiation density used, no statistically significant difference was obtained in the stem length of the plants as between one test and another. Thus it appeared that flowering is being controlled without affecting stem length.

The following definitions and parameters and arrangements of apparatus apply to the said third aspect of the invention.

Nearly all visible and near infrared LED's are made from III-V compounds, containing equal numbers of atoms from Group III and Group V of the Periodic Table. By using "mixed"crystals the manufacturer has flexibility in choosing the wavelength. For example, by using AlxGal Gal As one can choose a wavelength between red and 900nm by choosing the value of x. There are two groups of materials. The nitrides are most efficient in the blue and green spectral regions. The non-nitride 111- V materials do not produce blue light: their efficiency increases as one goes from green to infrared. These properties are shown in Table I and Figure 1. At present the ultraviolet and infrared LED's are only available from specialist suppliers, the others are easily available.

Size Typically the emitting semiconductor is 0.35 mm across. It is encapsulated in a special epoxy resin which protects it and also is shaped as an optical element. For experimental work standard diodes with diameters of either 3 mm or 5 mm have been used. Their length is typically 9 mm.

Beam size The epoxy encapsulant is in the form of a slightly tapered cylinder with a hemisphere at the end acting as a lens. The object is to collect as much light as possible from the emitting semiconductor and to put it out in a conical beam. By choosing the optical parameters of the encapsulant (length and taper, curvature of the lens, refractive index of the epoxy) the manufacturer can determine the angle of the cone of the beam and this is normally given in the diode specification. Diodes have been used with a 30 cone angle. The cone angle is sometimes referred to as a view angle.

For illumination of plant cuttings the radiant power density (e. g. W m-2) at the plant is a relevant quantity. The suppliers of visible LED's usually specify the view angle, i. e. the angle of the cone of light, and the on-axis brightness in photometric units (candelas). To achieve high on-axis brightness the view angle is made small. This is inefficient in light collection. Empirically, for a given semiconductor emitting element, the efficiency (radiant power in the beam divided by electrical power) is approximately proportional to the inverse of the view angle. It follows that, for a given wavelength, the optimum choice is not the brightest LED. Instead, the view angle should be the maximum acceptable for the application, then given this angle the maximum brightness should be chosen.

Height of LED's above plants/cuttings For small scale (e. g. laboratory or amateur gardener) applications it is proposed that diodes be mounted at least 30 cm from the plants at which distance the boundary of the illuminated area is reasonable well-defined. At this distance it is possible to illuminate a cutting or cuttings in a single 12 cm pot from above, with little overspill of light onto cuttings in adjacent pots.

LED variation Although LED's are supplied as RED or GREEN or BLUE light emitting devices, the actual wavelength of light emitted by any particular LED can vary from batch to batch of the same manufacturer and model number. Since the present invention requires light centred on particular wavelengths to be used, it may be necessary to check the actual wavelength of light emitted by each LED to be employed and to select and reject LED's accordingly before they are incorporated into an illumination source for use in methods and apparatus embodying the invention. Where examples of LED's are given these are intended merely as an indication of devices which have been used. Since new devices are being brought to market every year and existing ones rendered obsolete or being phased out of production, LED's must be selected from those available at the time for light output.

Electrical drive An individual red LED requires a drive of about 2V DC at a current of 20-30 mA. (Because an LED is a rectifier it is also possible to use an AC voltage). It is convenient to use series-parallel combinations of diodes. For example, 6 LED's could be connected in series and 16 of such series connections could be put in parallel, giving a total of 96 LED's requiring a power supply of 320 mA at 12V. In the unlikely event of one LED failing, only the others in the series connection would be affected and the light output would only dip by a small percentage. Blue and green nitride LED's require a higher individual voltage, typically 3.6V.

At 20 mA per LED the expected operating life is of the order of ten years. Because LED technology is advancing so rapidly it may be economically advantageous to run the LED's at say 50 mA. This reduces the initial capital cost by reducing the number of LED's

required for a given illumination level. The operating life will be shortened but this does not matter if it is economic to replace the LED's by improved ones after say three years.

Clusters. arrays When illumination at two wavelengths is required, appropriate clusters of LED's can be placed side by side or LED's of one type arranged randomly or in a symmetrical pattern relative to LED's of another type. For illumination at 660nm, commercially-available clusters of 50 LED's, wired and encapsulated, are available. These are driven at 10. 5V, 250 mA.

The following advantages are attributable to the use of LED's as opposed to other light sources for control of the rooting of cuttings.

The directionality of the light beam from an LED allows the light to be directed onto the plant/cutting to minimise light wastage.

Promotion of rooting by supplementary illumination, using Red and Blue LED arrays, can reduce the time in the glass-house required to get cuttings to marketable condition.

Claims

1. A method of illuminating a horticultural growing environment so as to minimise interference with plant development or growth comprises the step of illuminating the environment with light having a peak wavelength or wavelengths in the range 510-535 nm, typically in the range 510-530 nm and preferably substantially monochromatic light centred on 525 nm..

2. Apparatus for illuminating a horticultural growing environment so as to minimise interference with plant growth or development comprises at least one cluster of LED's having a spectral peak centred in the range 510-535 nm, preferably in the range 510-530 nm, and more particularly in the range 515-525 nm, and preferably substantially monochromatic light centred on 525 nm.

3. A method as claimed in claim 1 or apparatus as claimed in claim 2 in which the number of LED's and/or their spacing and/or operating current and/or the distance to the plants is selected so that a radiation intensity of not more than 15mWm-2 of the illuminating radiation exists at the plants.

4. A method as claimed in claim 1 or apparatus as claimed in claim 2 in which a control system is provided to turn off the LED's when not required.

5. A system comprising in combination at least one LED light source which when energised will produce light of wavelength or wavelengths as specified in claim 1, at least one CCTV camera, chosen or modified to have high sensitivity to light of the aforesaid wavelength or wavelengths, and means for energising the light source in response to a time and/or date determined signal or a demand signal from a human operator wishing to view the environment via the CCTV camera, or from a movement sensing device such as are employed in security systems.

6. A system as claimed in claim 5 in combination with a video recorder.

7. A method of promoting the flowering of a plant comprising the step of illuminating the plant with supplementary light in the range 605-645 nm wavelength as a supplement to the normal illumination to which the plant would normally be exposed.

8. A method as claimed in claim 7 in which the normal illumination is daylight and/or sodium lamp light and/or fluorescent lamp light.

9. A method as claimed in claim 7 or 8 wherein the duration of the supplementary illumination is less than, the same as, or longer than the duration of the normal illumination.

10. A method as claimed in claim 7 or 8 wherein the duration of the supplementary exposure period is longer than the duration of the normal illumination.

11. Apparatus for providing the supplementary illumination for use in a method as claimed in any of claims 7 to 10 comprising clusters of LED's having a spectral peak in the range 605-645 nm, preferably centred on 640 nm.

12. A method as claimed in any of claims 7 to 10 wherein the wavelength of the supplementary illumination is chosen to match the spectral response of the flowering pigment of a particular plant, for the purpose of controlling the time to flowering.

13. A method of promoting the production of roots by a plant cutting comprising the step of illuminating the cutting with supplementary red light at a first illumination intensity at the cutting together with supplementary blue light at a second illumination intensity at the cutting, as a supplement to the normal illumination to which the cutting would normally be exposed.

14. A method as claimed in claim 13 wherein the intensity of the red light at the cutting is the same as or different from the intensity of the blue light at the cutting.

15. A method as claimed in claim 13 wherein the red light illumination intensity is 20Wm-2 while that of the blue light illumination is 1. 5Wm-2, at the plants.

16. A method as claimed in any of claims 13 to 15 wherein the red light is centred on 660

nm.

17. A method as claimed in any of claims 13 to 16 wherein the blue light is centred on 470 nm.

18. A method as claimed in any of claims 13 to 17 wherein the red and blue light is obtained from red and blue LED's.

19. A method as claimed in any of claims 13 to 18 wherein the normal illumination is daylight and/or sodium lamp light and/or fluorescent lamp light.

20. A method as claimed in any of claims 13 to 19 wherein the supplementary illumination is for less than, the same as, or longer than the duration of the normal illumination.

21. A method as claimed in any of claims 13 to 20 wherein the supplementary light is supplied for 16 hours in each period of 24 hours.

22. Apparatus for providing the supplementary illumination for use in a method as claimed in any of claims 13 to 21 comprising a cluster of 660 nm and 470 nm LED's in the ratio of approximately 25: 4 respectively.