I have no answer to your question - sorry! I would guess that, if the frogs have no alternative, they will return to your pond regardless of when you choose to thin plants. But that's just a guess.

I do feel for you all out there who are suffering for water - it's a bad situation.

I assume the frogs would never want the plants disturbed. More plants = more cover and food. If you had to pick a time I suppose when food was most abundant.

However there is a harshness to nature. I wouldn't personally be overly differential to the frogs and would divide the plants or do whatever when I needed to. Trying to do too much for any species seems to always have the opposite effect long term.

Thank you lisak, waterbug guy and also FHFchrish (whose reply didn't show up here for some reason). I appreciate your thoughtful input. Guess I will just thin out the water plants a little at a time over the summer. We won't any get rain until about October and we all hope it goes on until our reservoirs are full again. Stay cool! Min