Understanding Light Levels in the Greenhouse

When it comes to growing plants, few things are more essential than light. Although there are a some real sun-worshippers and others which do best in shade, most varieties will be happy in filtered sunlight, or conditions which are brightly lit, but not in full sun. Unfortunately, judging light levels isn’t a particularly easy thing to do – largely because the human eye is remarkably good at seeing well in a whole range of different strengths of illumination. The upshot of that is that we often aren’t immediately aware just how bright or dingy things really are, as anyone who has ever tried to take a photograph without using flash in a seemingly “brightly” lit room, and then been disappointed by the dull, yellow result knows only too well!

Aside from being a big benefit when it comes to positioning plants with particular needs, understanding a little about how and why light levels vary within a greenhouse can also help you get the best out of the growing conditions for everything you cultivate.

Light and Distance

First a little science, but don’t panic – there’s only one golden rule to remember and while it’s an important one, it’s also simple and, frankly, pretty obvious. Illumination is inversely proportional to distance – physics-speak for the further away you are from the window, the less light there is, which is why growing plants indoors can often be such a problem. It’s back to the thing about the eye – that corner may look light enough to us, but it very probably isn’t. Clearly in many traditional greenhouses, this is seldom too much of a problem, after all, a 6ft x 4ft frame with glass on all sides means that nothing is ever more than two feet from the light, though it can still pose difficulties for larger, half-glassed, lean-to designs. A plant six feet away from the glass in such a set-up may only receive as little as a fifth of the light of one placed right beside the pane itself, so it’s definitely something to bear in mind.

How Bright is it Really?

Having established that it’s hard to tell by eye, someone had to come up with a way of measuring light to get around the problem and tell us how bright “bright” really is – and for generations of horticulturists and gardeners, that’s been the foot-candle. Although it is a distinctly old-fashioned – and strictly-speaking obsolete – unit of measure, you’ll still find books that discuss the light needs of particular plants in foot-candles, so it’s worth having at least a passing familiarity with the name and what it’s all about.

The simplest way of thinking about the foot-candle (fc), at least for most practical purposes, is as the amount of light a standard candle would shine on an object from a distance of one foot – although the proper technical definition is, of course, a bit more complex. For plant growing, however, how it is defined is less important than the ability it gives us to compare light levels directly; full midday summer sun, for instance would be around 10,000fc; light levels on an overcast winter day might be a low as 1000fc. As a result, light levels can be categorised into useful ranges that help make sense of the growing conditions from a plant’s perspective:

  • Very Low – below 1,000fc (deep shade/grey, overcast day)
  • Low – between 1000 and 1,800fc (too little light to form clear shadows)
  • Medium – between 1,000 and 4,000fc (dappled sunlight)
  • High – between 4,000 and 5,000fc (a good sunny position).
  • Very High – 5,000fc and more (full, unobstructed sunlight)

As a general rule, most kinds of plants photosynthesise most efficiently at around 5,000fc though there are some notable exceptions amongst greenhouse favourites – most notably, some of the orchids – which makes it pretty clear why knowing a bit about light levels can be so helpful. It also helps to make the point that although we all know that grey days aren’t as light as sunny ones, it’s not quite the same thing as realising that they may be five or even ten times as dark!

Light Needs

The particular light needs of the plants you grow are every bit as important as their preferred soil type, feeding regime and watering demands. Some of them are obvious – no prizes for knowing that cacti enjoy bright sunny conditions, and ferns like it shady – while others require a little more research and sometimes a bit of a balancing act too. Plants such as Hippeastrum, for instance, need good light if they are going to bloom properly, but too much can shorten flower-life, so there can often be some trial and error involved in getting conditions just right.

Once you’ve successfully mastered dealing with natural light, there’s always the question of supplementary artificial lighting to consider, particularly if you’re trying to keep continuously flowering plants such as African Violets in good show all year round. As a general rule, most plants will need around 12 to 14 hours of light a day to maintain healthy growth and recent research suggests that some kinds of seedlings grow best with a “day” of 16 hours, so there’s always something new to learn about lighting. One thing’s for sure – if you’ll forgive the pun – it’s never dull!

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