It was Boxing Day when it hit me. The lack of light is really starting to get to me. It happens every year, I’m fine up until Christmas and then with January looming, the short days and the lack of sun begins to feel oppressive. The weather so far this winter has been particularly dull with this corner of Wales swathed under a thick layer of cloud. I can’t remember when we last had some sunshine.
We were out walking the other day and I was looking up at the sky wondering how much light was actually getting through the cloud. Light levels are measured in lux. On a sunny summer’s day light will measure around 100,000 lux compared with only 5,000 on a dull winter’s day but worse still is that minimum lux levels for general office work is only 500 lux rising to 1,000 if more detailed work is being carried out. Is it any wonder people feel so gloomy in winter or when cooped up in offices?
Plants need sunlight to produce their own food but most need only around 10,000 lux to photsynthesise and reach saturation point at 12,000 lux. This sounds very low when compared to the 100,000 lux of a sunny day in June but then again is probably just as well bearing in mind the sort of summers we get here in Britain.
It’s not just the amount of light that plants receive that is important for growth but also the amount of time during the day that they are exposed to the light. Different plants have different points, known as the ‘critical period’ when the change in exposure to light triggers flowering. For some plants it is when the days start to shorten and there are longer nights, such as Chrysanthemums these are known as ‘short day plants’, for others it is when the days start to lengthen and nights get shorter such as Calendula, these are known as ‘long day plants’. There are also plants such as roses which don’t appear to be affected by these changes and they are known as ‘day neutral plants’.
Commercial plant growers can exploit the way plants respond to light in several ways. Some growers will artificially create short days when it is summer by excluding light for a specified time using blackout screens and likewise long days can be achieved by using artificial lighting that simulates actual daylight. These methods can be used to produce flowers when they would not ordinarily be available and also hold back plants so they don’t flower until required, for instance Poinsettias for Christmas.
Some growers use ‘supplementary lighting’ adding to the natural light levels to boost plant growth and crop production. This is mainly used at the end of winter and start of summer for seedlings and young plants.
Bedding plants such as Begonias and Tagetes are often germinated in ‘growing rooms’. These are spaces where all natural light is excluded allowing the grower complete control. Often the seedlings are exposed to light for 24 hours a day until germination has taken place.
Knowing how plants respond to light is not just important for commercial growers though. We’ve all experienced how young plants bend towards the light. This is because the plant’s stem contains a growth hormone called auxin. Auxin collects mainly on the side of the stem that is in the shade and the bending towards the light is in fact the shady side of the stem growing more quickly that the other side. As growers we know to turn our plants and seed trays to allow even light exposure.
I find it fascinating how plants respond to light and that it is so complex and how humans have discovered how to exploit it. Although I do find it a bit depressing that some plants are pampered in glasshouses receiving the perfect amount of light when millions of humans are stuck in poorly light offices and spending a lot of their lives not getting the exposure to daylight they need to be happy and healthy.
If I was a plant I would definitely be a long day plant ready and waiting for those long summers’ days to start flowering.
For more information http://www.biology-online.org/ is a very good website.