'Isn't this a nice light room. It must be ideal for growing indoor plants.' How many times have you heard such a remark. How many times have you said it. But it is the kind of remark that would make a plant pull up its roots and die laughing. For some plants it will be too dark, while others feel that they would benefit from a pair of sunglasses. When we go into a room we get an instant impression of the overall brightness or gloom, whereas plants are only interested in the spot where they happen to be standing. No room, small or large, is uniformly lit, and we, with our poor eyes, are largely unaware of these varying intensities of light unless we are trying to read or do embroidery. We are much better in responding to differences in temperature, but, while we have thermometers to confirm how hot or cold we feel, there aren't many people that carry light meters to check if they have a chance of reading. If they did they would have their eyes opened to the enormous variation of light and shade in a room - variations, not only in different parts of the room but in the same part at different times of the day, and from day to day throughout the year. Now life is too short to go trailing around the house with a light meter making notes of light intensity changes from hour to hour all the year round. But if you get into the habit of looking at a room from time to time from the point of view of the plants you have in it you will realise where some plants may safely be placed and where you dare not put others if you want them to thrive, or even live.
The level of light in a room depends in the first place on how big the windows are and which way they face. Then the levels will vary depending on whether the sun is rising, high in the sky, or sinking. Light levels drop dramatically as you move away from the windows, and they are obviously lower in all rooms in winter. Windows in a south facing wall let in most direct sunlight. Those in a south-east or south-west facing wall are only slightly worse off. In winter, when the sun is low, the sunlight moves around the walls of such rooms, but in the middle of summer, when the sun is high in the sky, it is only a relatively small area of the floor near the window which catches the sun. The rest of the room has bright light without direct sunlight. Second in the league for light and warmth is a window which faces west. Such a window may get a little winter sun, but in summer the sunlight floods into the room as the sun sets, and this sun can be quite strong and hot. An east facing room, however, gets its bright sunshine in the early morning. This may be only a little in the winter, but in summer can be for quite a few hours before you even think about getting up. This light may be bright or it may be hazy, but it is generally not very fierce until later on in the morning as it moves around the room. The light in a north facing room is remarkably steady. It is never really bright, although at mid-summer some direct sunlight may get in but this is never strong enough to harm any plant. Mind you, all I have said assumes that there is nothing to obstruct the sun's rays. Any tall buildings or trees in the near vicinity could cause the light getting into the room to be reduced, sometimes drastically. So there are no hard and fast rules. One persons north facing window could actually get more light than somebody else's east window.
But what happens within a room. You may not realise it but away from the direct rays of the sun the light level can be as low as one twentieth of what it is just inside the window. Most of this drop happens in the first six feet. And there are also dark patches to the side of the windows where the sun never shines. Light walls help to retain levels of light further into the room and dark walls lower them. Mirrors also help to keep the light levels up, but be careful - a plant in front of a mirror will benefit from the extra light but the reflected rays of the sun could cause serious damage.
Some plants, like cacti, really love the sun. Others, like ferns, much prefer the shade. Most plants that we try to grow indoors prefer something in between. During their growing period they need good or even bright light, whether the room is warm or cool. Some of them even benefit from a little direct sunlight, especially in winter and spring. As a rule, plants are usually more tolerant of too little light than too much, though not too much of the little, if you know what I mean. The reaction to too much sunlight - scorching and wilting - is more rapid than the reaction to too little light - growing straggly or leaning over to get near what light there is. But always bear in mind that a variety of a plant which has variegated leaves needs better light than a variety with plain green leaves, because the pale coloured parts of the leaves are short of chlorophyll and to compensate for this the rest of the leaf needs brighter light for more efficient photosynthesis. One of the difficulties about growing plants indoors is getting them to grow into a decent shape. Unless a plant has reasonable equal light from all sides and from above (as it has when growing out of doors) its natural response is to bend towards the source of light - indoors this will be the nearest window. If allowed to indulge the habit the plant grows permanently disfigured. To avoid this turn the plant a little, but regularly. You can put a mark on the side of the container as a guide and a reminder. But be careful if the plant is in bud or in flower. Some plants are very touchy about being moved and are quite likely to drop their buds and flowers if you move them, just to spite you.
And here is another point to be careful about. When a plant is resting in winter it is in a period of no growth and it needs not only less warmth, less water and no food but, by and large it needs less light. And here is the catch. Since winter levels of daylight are so much poorer than in summer a plant may have to be moved nearer to the window so that it still gets adequate light - not further away from the window, as you might expect.
And just to confuse the issue (in case you weren't confused enough already) some flowering plants react to the duration of the light as well as the intensity - the so-called short day and long day plants. Short day plants, those which flower in the winter and early spring, only produce flower buds after they have had a few weeks when the hours of darkness are longer than the hours of light - like fourteen hours of darkness to ten hours of light, whereas long day plants will only flower when they have had at least twelve hours of light per day for two months or more. Who said growing plants indoors is easy?
Happy Gardening.
Bill Hutchings
Return to the November 1999 Features page
return to Home page and main index
page last updated 1 NOVEMBER 1999