January gardening, sowing and lighting systems.


Some of you may know that in a previous life I grew seedlings and young plants, hundreds of thousands of them, for the bedding plant industry. Our nursery, Opax Farm in Headley, on the Hampshire / Surrey border, was a state-of-the-art affair consisting of 6 ½ acres of glasshouses on a nine acre site. It had computers controlling everything from the ventilators which opened according to wind direction, the percentage of CO2 in the air, the pH of the water and of course, the air temperature. In case anything broke down we had two of everything and, in event of a power failure, a generator, which could have lit a small town, automatically kicked in, powered by a huge, Rolls Royce engine. This piece of equipment was thoroughly tested in the week we took over, when a passing hurricane cut off our valley from the rest of the world for more than a week!

Begonia seedlings

These days my growing is a little less high tech but I will shortly be sowing Geraniums and Begonias in the propagator I have set up in the attic and I’m concerned that they may not receive the light they need to grow well. But why do we need light in plant cultivation? The major horticulture issues are:

  • photosynthesis — converting light, air and water into carbohydrates and oxygen to support plant growth.
  • chlorophyll synthesis —building the plant cells that perform photosynthesis.
  • photoperiod — sensitivity to the length of day.
  • phototropism — movement toward a light source.

Back in my days at Opax Farm we had a number of lighting systems for a range of plant growing tasks. Strings of ordinary tungsten bulbs had been used by the previous owners to control flowering in crops like Chrysanthemum. This plant requires a very specific day-length to flower and if you need to produce flowers throughout the year you will at times have to completely block out the light, while at other periods of the year create light. Plants that bloom in the winter, such as Christmas cactus, poinsettias, gardenias and chrysanthemums, don’t flower unless the nights are longer than the days. They are referred to as long-night plants and for the most sensitive long-night plants even one minute of bright light during the night is enough to prevent them from blooming. In general, long-night plants need a maximum of 10-13hours of light per day to flower.

Last year's Begonias

Plants that typically bloom during the summer don’t flower unless the nights are shorter than the days, so they are called short-night plants and include many bedding plants and vegetables. Short-night plants need 14-18 hours of light per day in order to flower. Other plants bloom regardless of the length of the photoperiod, so they are called night neutral plants. Many of these plants are sensitive to temperature variations however, and bloom when the nights are cooler than the days. For early growth and development, plants need the opposite photoperiod: young long-night plants should have long days for the first month or two to encourage full growth before blooming, while young short night plants should have short days. Day length can be manipulated for other reasons, such as the selection of male or female plants early in the cycle, by inducing flowering.

We had a number of growth rooms at Opax in which we germinated seed in optimum conditions. Some plants need light to germinate while others prefer the dark. Trays of recently sown seed went into different growth rooms depending on their requirement for light and heat. With sufficient light levels it is possible to grow plants without any natural light.

Greenhouse lighting

Several of the plants we grew were needed early in the season when light levels are low: we started sowing Begonia sempervirons for bedding plant growers before Christmas. These plants were placed in an area of the greenhouse where we had lighting to supplement that of the sun, ensuring good, strong, compact growth at the darkest time of the year. Attempting the same thing in our attic later this month, I have assembled a lighting rig to increase the light levels for the seedlings I will grow there. This structure incorporates a number of domestic halogen lamps which I trust will provide a useful boost in light levels but be inexpensive to run. I’ll let you know how we get on. I did have a quick look at the new LED Grow Light units currently being promoted for indoor growing but the cost was far too high for me to justify on my hobby, so my Heath-Robinson construction will have to do for now.

Several Garden Design Academy courses look at the complex subject of plants and light, also considering the types of light most suitable for horticulture by cost, light spectrum, heat emissions and other factors. Some of our RHS courses and the Certificates / Advanced Certificates in Horticulture, are particularly good on this.

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