Defining Various Degrees of Light for Plants
Bright Light – Horticulturally speaking, a “Full Sun” plant is defined as any plant that requires 6 hours or more of direct sunlight per day to perform its best. A rose, for example, is a full-sun plant. For it to grow and bloom its best, it needs at least 6 hours of direct sunlight per day.
But since many or most of us are growing indoors, let’s talk about windows.
A southern or a western window will often provide a full-sun setting (southern will usually offer the most light).
An eastern exposure is next best for bright light, though in many cases the addition of artificial lighting for a few hours per day may be needed for a full-sun or bright-light plant. If your eastern window only gets 3 hours of direct light, for example, you may need to add 6 or so hours of artificial lighting to supplement the deficit.
If a mixture of lighting like this seems best for you, just remember that 3 hours of sunlight cannot be replaced by 3 hours of artificial light since the lamp’s production is much less than that of the sun. I’d factor the artificial light at half the value of the sun. So, if you have no windows at all, but want a full-sun or bright-light plant, you’ll need to provide 12+ hours of good lamp light.
Note: In the growrooms at Flyleaf we get very little direct sun, so we keep our growlights on for around 14 hours per day for the bright light plants.
Medium Light – We know that full-sun or bright light indicates 6+ hours of direct sun (or a blend of sun and lamps), so medium light is going to be something less than that. 4-6 hours.
Medium light may be direct light for that period of time, or it may be filtered light (for a few hours longer – this would depend on the degree of filtering that the light is receiving). I have a couple of west-facing windows at home, but also have large trees across the street which filter the light that my windows receive. I’d call these medium light windows.
I also have a west-facing front porch. It gets the same filtering from the trees across the street, but it also gets more of the indirect light that happens before the sun crosses my roof. I’d call this a medium to bright light setting.
Low Light – This is usually going to consist mostly of filtered or indirect light for several hours per day. I have a north-facing window at home. This window gets several hours of nothing but indirect light throughout the day. This is a low-light setting (the jade with the 2 haworthia pictured are in this northern window).
Note: While there is science to the lighting needs of plants, getting it done is a little bit more of an art.
You may have trees across the street, or a need to keep the shades drawn at certain times of the day. You may have awnings or a slight twist to the direction of your home. You may have 80 watt rather than 100 watt bulbs. Maybe you have a skylight. Whatever your situation, the guidelines presented should give you a great starting point as you work on lighting your own plants. Just get the gist of it worked out and count on your plants to let you know when they need a little more or a little less light.