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I’ve done a fair bit of research on this, and the consensus seems to be that cuttings do best with long hours of indirect light.
On a cloudy day, put them in your brightest spot, on sunny days, pull them back from the window.
If your cuttings have leaves, they do need light
Plants get their energy from photosynthesis, and appreciate additional energy when they’re trying to form roots is important. If your cutting has leaves, you may as well put them to work.
They don’t NEED light (so the subheading was a lie – apologies) but it’s a bit of a missed opportunity – if you root a cutting in the dark you may as well chop the leaf off, because it’s using energy and not bringing anything to the table. It’s more of a hindrance than a help.
However, cuttings don’t have a root system (yet) so leaves are more likely to burn or dry out in hot weather or direct sunlight. Therefore you need to keep them protected from extremes of both light and temperature.
If your spot is generally pretty bright but the light is only direct for a couple of hours a day (this common in south-facing windows) you can use sheer curtains to diffuse the light.
Consistent light is preferable
It might be worth investing in grow lights if you propagate a lot – preferably dimmable ones – because that’s the easiest way to ensure consistent light.
If you’re happy to spend time moving your plants closer and further away from the windows, then you can maximise the amount of bright indirect light to root your cuttings faster, but I’ve found that a few (three/four/five) feet away from a ground floor, south-facing window is fine.
I specify the ground floor because the light can be stronger in upper-level windows. I know it doesn’t seem like it would make much of a difference, but my Ficus do WAY better upstairs than downstairs, whereas my Hoya prefer to be downstairs.
Wet sticks don’t need light but seem to prefer it
Wet sticks don’t need light, because roots prefer darkness, and there are no leaves to photosynthesize.
(There’s always a however)
It seems that new growth appears quicker when the wet stick gets light. It’s again recommended that you don’t expose the stick to direct sunlight because it can burn or dry up, but diffused bright light can aid in activating the growth point.
Roots prefer to grow in darkness
Roots are more than happy to grow in the light. We tend to propagate cuttings in glass jars when we root cuttings in water because it looks cute and we can see the roots forming.
However, roots prefer to be kept in the dark. I’m not sure how much of a difference it makes, but a lot of people like to either root cuttings in opaque vessels or wrap glass in black paper to block out the light.
I just use glass jars, because it works fine. Perhaps I should try rooting two cuttings next to each other and black out the roots of one. Hmm.
I root my cuttings in a south-facing window (more on that later) and my cuttings root pretty quickly depending on the plant. However, on hot sunny days, the algae growth is wild. Whilst it doesn’t damage the plants or affect how fast roots form, it looks a bit grim.
Don’t waste any time or money trying to stop algae growing. My boyfriend has fish tanks and he’s tried EVERYTHING. If water has access to light, algae will grow unless you crowd it out with other plants.
It won’t harm your cutting. It just looks grim.
Be aware of geographical variances
The sun in the UK is rarely out, and when it is, it’s not that hot. I have to worry about my plants getting too hot MAX a couple of days a year. If that.
For those of you living somewhere super hot, dry, and sunny, then you may be better off keeping your cuttings in a north or east-facing window.
If you plan on propagating valuable plants, then it might be worth buying a cheap plant, taking a bunch of cuttings and trying them out in different spots.
I root my cuttings in my south-facing window – it gets pretty hot (again, for the UK) but not too hot so roots form fairly quickly.
I almost always lose the leaves on the cuttings just after the roots form. This doesn't really bother me - I'd rather root cuttings quickly so I can get them potted up before I lose interest or forget about them entirely (ADHD for the win). However, if you want to keep hold of your leaves, you might want to keep your cuttings in an east-facing window, or a few feet away from the window.
Too little light is bad, too much light is bad. A large amount of diffused light is best, but it can be a pain to come by. Grow lights are a great investment for avid propagators, especially if you plan on propagating plants in winter.