Home Gardening Greenfingers 20- Artificial Light

Greenfingers 20- Artificial Light


By William Mills

photo of Citrus Tree
Citrus Tree in bay window after a few weeks under CFL light in October


By January the longest night, 22nd December is past, and the three coldest winter months lie ahead. But by their end spring’s first blooms will be arriving.

Of course spring can come early for our indoor plants if we treat them carefully and take care not to shock them with any drastic changes.

It is necessary for plants to have some down time resting after a long period of growing. During this time it is important not to water excessively as it could result in the roots going soggy and rotten, or spindly fresh growth indicating it has started growing again too early. It is a mistake to use fertiliser during a rest period unless a particular variety has special needs.

Temperature wise most homes are warm enough all winter to sustain continuous growth, although if a plant gets a sudden change such as a window being left open on a sunny day the shock of cold air can be enough to kill it! Plants like fresh air and ventilation but draughts can kill.

Light is the most crucial ingredient in promoting new plant growth. The shorter days of autumn act as a time clock for plants to stop growing. Once Christmas is past and the days start to lengthen again it’s possible to encourage growth with some extra artificial light.

It is an idea to regularly clean windows so as to make full use of what little natural sunshine there is over winter.

photo of citrus with spindly growth
Citrus has continued to grow during winter rest period


Spindly growth of long stems and small pale leaves is indicative of too little light. So unless you are lucky enough to have a south facing conservatory or a roof greenhouse artificial light will be needed unless we want to wait until spring.

Commercial growers advocate using 600 watt light bulbs or more for all year around growth and harvests. As well as using large amounts of electricity they become extremely hot are not necessary for recreational house plants.

A simple 50 w halogen reading lamp gives of enough light to help a plant grow. However the cheaper tungsten household bulbs, or low energy fluorescent tubes aren’t powerful enough to make any difference. A light meter from a garden centre or online store is an invaluable, as well as inexpensive tool.

One of the most cost effective sources of artificial light are the 250w Compact Fluorescent Lamps.

Bought with a reflector and lamp holder they can be suspended from a ceiling and directly plugged into the mains electricity. As they don’t get hot they can be placed in among the top leaves of a plant.

Artificial light should be used to augment natural daylight rather than replacing it. Using a time clock enables us to give our plant the same number of hours every day which is important for the plant’s natural cycle.

photo of bushy growth on citrus
Good growth from primary branches.


Try turning the light on for six hours between 10am and 4pm for the first week, then nine hours for the second, and twelve for the third and onwards. Carefully write the hours down then compare with an online sunrise to sunset timetable, bearing in mind that by June 22 daylight stretches from 4am to 10pm. Use artificial light to boost up the intensity of the existing natural light.

If the light is strong enough within four or five days new shoots will appear with leaves growing from primary branches rather than elongated stems. This is a good time to snip off any branches not supporting new leaf growth.

Pruning is the subject we will consider next time.

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