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I'm growing some plants from seeds indoors and they're coming up well. They sway toward the brightest light source, sometimes swaying so far they almost tip over. Some species do this more than others - e.g. 2" tall parsley is swaying much more than 2" tall chives. The brightest light, which they sway towards, is the window they're nearby when it's bright and sunny. They all enjoy a grow lamp directly above them, but they still seem to prefer the light. It could also be due to water mist being applied to them predominantly from one direction but we try to be gentle with misting and do it mostly from directly above them.

I have a habit of rotating their pot/planting trays so that they lean the other way. I see this as a kind of 'exercise' for them, and helping them form well-rounded growth. They do rotate and over the course of a few days will go back upright and then lean in the new direction of the window. Does anyone know, though, is it good to rotate young plants that are bending toward a light source?

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Do they prefer it? No, not really. Plants have evolved to grow and orient themselves to get the most light in any given conditions. It could even be argued that turning them away from this orientation reduces their overall light exposure to some degree (while they re-orient again), but the overall effect is likely negligible.

But there is another reason to rotate your houseplants.

Rotating Houseplants For Aesthetic Appeal

Folks typically rotate their houseplants and indoor vegetables to keep them growing somewhat symmetrically. Failing to turn your plants receiving natural light from an adjacent window can cause them to grow lopsided, and could even create thin or dead spots on the side opposite the window. Unfortunately, that's the side you see the most.

As you noted, a heavily lopsided plant can also fall over, making them difficult to care for.

In the outdoors, plants will naturally grow to one side and then the other as the sun passes overhead, so there is certainly no harm to this change of direction in their growth habit. There's no "exercise" in it per se, but rotating your plants regularly will help assure they grow at least somewhat upright and get enough sunlight to all parts of the plant.

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What is actually happening when a plant bends towards the light, is that the cells on the darker side develop in an elongated manner.

If you turn the plant round and round as it grows, it will certainly develop a more upright habit than if it was left in one orientation the whole time, but in my anecdotal experience, it will be more "leggy" (i.e. etiolated) which is (depending on the plant) equally and probably more undesirable for the future health of the plant.

I have the same problem - I live in an old cottage with small windows facing east and west, which only get direct sun for a few hours each day. In the past I've had long windowsill propagators with my tomato seedlings in which I've turned round occasionally to try and keep the plants straight. This year I grew them in rectangular seed trays, which I tilted towards the light by elevating the darker side, and anecdotally, the plants grew perfectly straight.

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