Light is the single most important need your plants have. Without it they cannot photosynthesise and they will die.
Full disclosure: I am not looking at supplemental and artificial light here. It is becoming more readily available and affordable, but is not at the stage where it is unobtrusive or easy to use for all your plants. So I am concentrating on natural light.
Get thee to the app store
Light is a complex topic, it isn’t just a matter of how weak or strong it is. The relationship of plants to light varies with species, strength of light, wavelength of light, time of year, age and stage of plant life, and number of hours of light. This is too much to get into if you just want to grow a succulent on your windowsill.
What we are going to look at here is strength of light, measured in lux. This is easy to measure for free if you have a smart phone or tablet. So get yourself down to the app store, find a free lux meter, and a compass too if you can. Most iPhones have a compass anyway. Obviously, doing it this way is not going to produce professional results, but it will give you an idea of how light various parts of your home are.
What you are trying to do is use the app to measure the natural light falling on that spot, using the camera in your phone/tablet. Place your phone/tablet exactly where the plant is (or will be) and take a reading with the camera/sensor facing the light. Don’t go nearer or further away from the light source or this will change the reading.
I have measured the light available at home today (mid-November, south-east England, rainy with some brightness). I haven’t cleaned my windows!
Patio doors: Face: EastNorthEast; Lux: 563 (cloud cleared a little)
Kitchen window (obscure glass, faces brick wall 2m away) Face:WNW; Lux 380
Spare room (bungalow about 4 m away) Face: ESE; Lux 300
Bedroom (net curtain) Face: WSW; Lux: 380
Porch (double aspect) Face: WSW: Lux: 385; Face: SSE: Lux: 725 (open view up the road, rather than more houses).
Greenhouse (shade paint still on, one layer of bubble wrap Lux: 843.
Your results will vary according to the time of day, the weather and the time of year.
What does this mean in terms of plants?
We can translate lux into terms that are used commonly when referring to indoor plants (thanks to Wikipedia).
500-2500 lux = low light conditions.
2500-10,000 lux = medium light conditions.
10,000-20,000 lux = high light conditions.
20,000+ = very high light conditions.
And we can find a range of plant that will grow in these conditions (thanks to Missouri.edu).
Low light conditions
Most plants that will tolerate these conditions will grow only slowly and may not flower. If they are moved into medium light conditions they will perform better.
Aspidistra elatior, Aglaonema, Pilea, Draceana, Ferns, Asparagus sprengeri, Syngonium, Crassula argentea, Araucaria heterophylla, Epipremnum/Scindapsus, Sansevieria trifasciata, Spathiphyllum wallisii, Chlorophytum comosum & amaniense, Plectranthus verticillatus, Monstera deliciosa.
Medium light conditions
Peperomia, Begonia, Ficus, Dieffenbachia, Hoya (not to flower), Schefflera, Palms, Philodendron, Bromeliads, Gynura, Maranta, Phalaenopsis, Tradescantia.
High light conditions
These are needed for some plants in the other sections to flower, and for most flowering plants.
Aphelandra, Beaucarnea, Coleus, Cordyline, Gardenia, Hibiscus. Impatiens, Hoya (flowering) Spathiphyllum (flowering).
Very high light conditions
Most cacti and succulents.
There are oddities, like the Jade Plant (Crassula argentea) will put up with all light conditions above, but it will grind to a halt in low light. Araucaria will tolerate everything but very high light conditions.
The lower the light conditions, the less your plant will want in terms of water and food, the less it will grow. Within reason. This does not mean that you can grow a cactus in the dark on nothing.
You have at this point spotted the flaw. If I have well over a hundred houseplants at home, but barely enough light to sustain an aspidistra, how does it work?
Plants are kind enough to average the light received over a period. So mine are counting the good light from the last time it was sunny, in my favour. I also perk a lot of them up by putting them in the garden (in the shade) in the summer. Outdoor light levels are a lot higher, full sun being between 32,000 -100,000 lux.
And remember I have measured the light on a nasty day, at the back end of the year, so it isn’t always this dull. If I have a chance I will measure the light on a few days to show the range.
I grow most of my cacti, succulents and pelargoniums in the porch or the greenhouse. The foliage plants needing more light are in the spare room or by the patio doors (no obscure glass or curtains). I try to cluster everything as close to the windows as possible in the winter, as the light intensity drops even a few centimetres back from the window. The air plants are in the spare room or the porch, except for a few in the kitchen window.