Light conditions during transition

[Grim Tuesday]

I've read that you should reduce a plant's light while it is transitioning to S/H. What is the logic behind this? I thought plants need light to produce mass (e.g. new roots) and will often invest more in their foliage if they are receiving insufficient light (but we we want roots, not leaves during the transition!) I am particularly thinking of the case of plants that like medium high light conditions, and received them at a nursery, so reducing light would be a change for the plant while keeping it at medium high light would be maintaining the status quo.

[Ray]

When repotting plants to a drastically different medium, the existing roots are immediately rendered “sub-optimal” for the plant, so cannot function as well as they had - and in extreme cases, will die in short order. The plant, however, can still lose water through transpiration. Reducing light levels reduces leaf temperature, so may reduce transpiration losses. I usually recommend inverting a clear plastic bag over the plant and pot to trap humidity, further slowing those losses, and moving it to lower light prevents that from becoming a “broil in bag” scenario.

I also believe that while your assessment that lower light may cause the plant to create more leaves has some validity, I don’t think it’s that straightforward of a relationship.

When light is reduced, the production of additional chlorophyll happens a great deal more easily and rapidly than the addition of all tissues associated with growth to increase the surface area. If the reduction is temporary, that may be all that happens, and the chlorophyll concentration will decline once the light levels are restored.

Plus there’s the fact that a plant with “so-so” roots “knows” it needs new roots to survive, so will redirect its efforts into doing that first.

[Grim Tuesday]

I see, that makes sense! Do you ever worry that putting the plant in the bag could suffocate the plant? Or are the amounts of gas exchange associated with respiration and photosynthesis on the order of 1/2 gallon of air per week (i.e. there's enough air in the bag)

[SADE2020]

OMG ...I have no idea of this with SH. I follow this rule with organic medium but never came across these comments for SH before. Good 2 now for next time.

Well..in my case I don't have humidity problems and even not providing less light the transition has been good!

[estación seca]

I've never bothered changing the light.

[Ray]

Originally Posted by Grim Tuesday

I see, that makes sense! Do you ever worry that putting the plant in the bag could suffocate the plant? Or are the amounts of gas exchange associated with respiration and photosynthesis on the order of 1/2 gallon of air per week (i.e. there's enough air in the bag)

The bag isn’t sealed, it’s just slipped over the top to trap humidity from the water evaporating from the medium.