Let there be light

Growing plants from seeds is one of the most satisfying (and cheap!) ways of propagating.

Seeds are little power-packs of biological wonder. To get them growing they need moisture and a certain degree of warmth. The seed itself provides the initial food to enable the first root (radicle) and shoot to form. During the early stages when biological activity is inside the seed, light is fairly unimportant (although some seeds do respond to light). However, as soon as the first seed leaves (cotyledons) have popped their heads above the soil, light becomes the most important factor.

Light powers photosynthesis, which in turn powers the plant. Without adequate light, your seedlings will be unable to make the food they need to develop and grow. Even if you provide the ideal temperature, humidity and watering, if there isn’t enough light, your seedlings won’t thrive.

The physics of lights are complex, and I’m no physicist, but from a plant’s perspective, the quality, the intensity and the duration of light are all important.

Quality refers to the wavelengths of the light. At the seedling stage, blue wavelengths are particularly important; red ones are more important for flowering.
Light intensity, for plants, usually refers to the part of the spectrum that is “useful” for photosynthesis – called photosynthetically active radiation – PAR. This is classed as light with a wavelength between 400 and 700 nanometers. (Humans can generally see light between 380 and 770 nanometers.) Confusingly, in horticulture, the unit of measure most widely used is the micromole per metre square per second. Not something that trips off the tongue and more confusingly you will also find reference to foot candles, lux and lumen.

There’s no need to worry over much about the jargon, what is important to know is that your seedlings will need light which is between 10 and 20 times more intense than is found inside the average house. Dirty glass in a greenhouse or coldframe, or dirty film on a polytunnel will also significantly reduce the intensity (and affect the quality) of light reaching your seed trays.

So, when choosing where to site your seed trays look for somewhere with the best possible light exposure, and also good air circulation to prevent stagnation and ensuing problems with rot and mould. If you can’t find adequate natural light, consider supplementing the light: LED lights with low running costs, and light wavelengths specifically developed for plants are widely available.

As soon as the seed leaves appear, make sure that your seedlings have optimal light for around eight to 12 hours each day. That way you should grow robust, well coloured seedlings that will develop into strong healthy plants.


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