An introduction to light.
Updated: Aug 24
By James Freeman
As the sun comes up or as it’s replaced by moonlight our beloved houseplant’s circadian clock ticks alongside our own. Houseplants hungry for the bright blue light produced by the rising sun will grow towards its energy laden light, an action called positive phototropism (a quick think about plant roots diving into the dark soil and we’ve discussed negative phototropism).
Plants sense light in varying wavelengths just the same as we see different colours. This means your houseplant knows when you’re wearing your favourite red shirt or your trusty blue jumper . We’ve both evolved separate places to keep our light sensing cells but we both have photoreceptors to help us do this.
I mention all this as we have a problem, before we jumped into existence on the planet there were no roofs or walls in the way, just rock faces and taller plants. Handily plants evolved to live on dark forest floors and in deserts, but most of our house plants will have always wanted to have as much view of the sky as possible. With that said periods of darkness are just as important for plants to rest as they are for us.
The ability to convert light into energy is an amazing evolutionary trait we never got the hang of, but plants have evolved to know which way to grow, when to flower and when to rest. All from their interactions with light. They can even tell what season it is from the length of the day.
As we’ve discovered these facts, we’ve learnt to manipulate these traits with grow lamps, helping us have year round crops, and the ability to force flower production with different coloured bulbs.
Anyway, back to our plants...
Ideally if you want to understand the light surrounding your plant, go and stand next it to and pretend you’re the plant and have a good look around;
Where is the closest window?
Will the sun be seen, and for how long?
Will the lower winter sun be an issue?
I get asked a lot about plants in reference to windowless rooms. These shady places tend to be cold so temperatures will have to be thought about. Plants enjoy a fairly warm and constant temperature.
If the room is dark, using a liquid fertilizer will become more important too, ideally once every 2 weeks during summer, as this will help to compensate for the minimal carbohydrate storage the plant will have due to lack of photosynthesis. And by all means if you have a windowless room and don’t want to use grow lights, just replace your plant every couple of months and treat it like you would a vase of cut flowers.
A good shade tolerant plant be would an Aspidistra elatior (cast iron plants). One of my books has it described as suitable for living anywhere that isn’t poisonous to humans! Handy to know.
Direct midday summer sunlight will burn leaves
Too much sun can cause stem and leaf wilting
Not rotating your plants will cause lopsided growth
Not dusting leaves will prevent photosynthesis
Not enough light can prevent flowering
Too little light can cause pale or yellow leaves
Stems can end up etiolated (leggy) if forced to reach for the light
With all this to keep in mind, if you’re worried about your plant or it’s displaying signs of unhappiness, move it to a new home and observe. I see a lot of over watering accidents occurring, when light levels are the real issue.
I could talk about light forever but I’ve tried to just be informative and tackle the issues I hear about. I’ve seen too many pictures of sad plants already this spring.
As always it comes down to right plant, right place. Read your plants care instructions, or search for one on the web.
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