Question

why in the summer is it better to water potted plants during the evening than during the day?
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Answer 1

Im guessing you are talking about outside potted plants ? If that's the case, watering at night will avoid spilling water on the leaves and potential marks on leaves caused by sun shining through the watcer drops and magnifying/burning spots. Organisms like black spot enjoy wet rose leaves, perfect situation for it to set up housekeeping.

If you mean indoor plants, unless they sit in direct sunshine, I dont think it matters much when you water them.