Light: The Essential Ingredient for Good Plant Growth

, written by Bob Flowerdew gb flag

Cordon apples on a sunny south-facing fence

It may seem obvious but light is crucial to plants. One of the least solvable problems handicapping many if not most gardeners is insufficient sunlight - often because a small urban or city plot is in the shade of surrounding buildings for too much of the day. Likewise, an idyllic hideaway in a pretty wood, or another nestling in the bottom of a steep valley may be too shady. The majority of our garden plants require full daylight all day long for the greatest success, although luckily many ornamentals perform adequately given less. However nearly all our herbs, fruits and vegetables need the absolute maximum of light. Given less and they grow lanky, drawn, perform badly, even fail to crop entirely.

Why Good Light is Essential

Photosynthesis is the process whereby plants turn the energy of light into their own building materials. They combine water and carbon dioxide (and a minute amount of other elements) into the complex chemicals their cells are made of. When you burn a log of wood all heat coming off is the sun’s light the tree absorbed now being re-emitted. (With the fire carbon dioxide and water are re-formed and a trace of ash left.) When more light is available more energy is absorbed, more sugars are formed, more growth takes place, flowers wax bigger, fruits get sweeter and crops burgeon.

Plants look green because all other colors from the sun’s spectrum are being absorbed (thus green light is the least useful to plants). Young leaves, especially in the tropics, often look red. They are not absorbing red light as well so they don’t over-heat before they have toughened enough to withstand the full force. Variegated, yellow and white patterned leaves absorb less light, performing less well than all-green leaves and so will usually be less vigorous cultivars. You can usually tell a shade lover from a sun loving plant by their leaf size. Shade enduring plants frequently have much bigger leaves to make the best use of the dimmer light, whereas plants adapted to full sun often have smallish leaves or have reduced them to many smaller leaflets. Most cacti have effectively dispensed with them entirely.

Tomatoes need plenty of sun to grow well

Light Intensity and Endurance

The brightness of our sun varies with dust and clouds, and throughout the year. Obviously grey overcast days have less light than clear ones, and dust from the day’s activities means morning light is usually brighter than evening. A south facing wall is warmest and sunniest through winter in the northern hemisphere, a north facing one remaining in near perpetual shade. In the southern hemisphere it’s opposite, while in the tropics no wall is always self shaded as through the year the sun will strike from every direction.

Endurance is as important to many plants as brightness. Most temperate plants measure the changing day-length and many adjust their growth pattern accordingly. Thus the necessity for sowing certain crops at specific times as otherwise they are sure to fail or bolt into early unproductive flower. Some plants can be ‘fooled’ into flowering at the ‘wrong’ time by manipulating the length of light and dark artificially.

Often the length of the dark period is more important than the light period. Thus something to bear in mind is the effect of other light. Now moonlight may be very weak but does alter plant growth so there may be a basis to some astrological planting. However less often considered is how street lighting could cause problems- some plants might fail or flower at the ‘wrong’ time if kept permanently without dark. Likewise sudden very bright bursts of security lights or car headlights could affect others.

Practical Steps for Increasing Available Light

But the commonest problem with light as suggested at the start is a shortage of it overall. Under cover this can be worst as even the cleanest glass or plastic necessarily absorbs or reflects some light. All dirt and grime must be regularly cleaned off such surfaces. Indoors and out it’s probably worth painting almost everything inanimate white as this will reflect most light. Even paths and drives can be lightened to throw back more. And do watch out for the relentless incursion of trees and shrubs- they keep growing and if you are not ever-vigilant a small sunny garden can too soon become a shady grotto!

A grow light for raising seedlings in winter
A grow light for raising seedlings in winter

There is the option of adding artificial light. Now outdoors this may not be practical as the amount needed may be too expensive and also annoy the neighbours. But under cover, especially for winter crops and early sowings, a ‘grow’ light may be well worth having. These have to be the type especially made for the job otherwise the wrong spectrum is emitted. They are not cheap however by adding a little extra light in winter and through early spring you can seriously improve your plants’ performance.

By Bob Flowerdew

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