Hi kristinlp - I haven't done mixed baskets in awhile, although I have grown most of the plants you mention in window boxes... But from my experience, I think a good rule of thumb is to prune about 1/3rd to keep the flower mound shape. All of them will respond by eventually branching and if you don't wait too late, the basket should continually fill in again until frost... particularly if you are fertilizing. Some of the plants will take a break from bloom though if it gets too hot, but will bloom again as it cools (e.g., they alyssum). The begonia is an interesting one as it seems to be more versatile and tolerant of a wide range of conditions (blooming in sun or shade, and in dry or somewhat damp conditions, although too wet and they may become mush) and that is probably why it was in the 2 sets of baskets.

The thyme and alyssum are creepers so those will stay low. Of course you could use the thyme prunings for cooking!

Look forward to seeing some pics!

with all of my baskets i do regular pruning.... i never cut everything back at once... as i don't want to ruin the show...... however i am out there every weekend hacking back something.... usually i will chop 1/3 of the stems back then wait.... make sure it regrows.... then do more.... :) i wish i knew how to post pics on here.... otherwise i would show you what i have done.

good luck :)

How about cutting some of it back as soon as you plant?