I would think the answer to that might depend on what your goals are. If you are very particular about size and shape, you probably would start as soon as they put out new growth. If you are more like me, lazy and easy going, then I would do one trim in late June to keep them slightly more compact and leave it at that. Don't know if that helps much. Hopefully others will chime in.

Martha

When mine are about a foot high I pinch back about half. If I don't watch out they get too big and fall over. I have just pinched mine, and will no doubt have to pinch at least one more time. Al

For both asters and mums I pinch (actually just chop off the top inch or two with scissors!) once a month until the first week of July. Leave them alone after that to set flower buds. That keeps them sturdy and bushy with lots of flowers.

Some time around mid-late June (about 1-2 feet tall) I go through the gardens and give a lot of perennials a trim. Generally this makes them shorter and bushier, and less likely to flop. Also, they bloom a couple weeks later and the blooms can be more numerous but smaller. This includes the Asters, Phlox, Helianthus, Monarda, Solidago, Agastache, etc. For some, like the big patches of Monarda didyma, I will trim more selectively - about 1/2 the stalks. This staggers the bloom a little.

Most perennials only get trimmed once, but I have one type of hardy Mum, and they get trimmed multiple times because they grow so fast and get floppy.

I also pinch many of the annuals when they are about 6-12 inches and make them bushier and shorter.

Just don't trim the Filipendula rubra. I cut about half of the stalks one year and lost the blooms on those stalks. Apparently they don't send out side blooms. Yikes!

Thanks, everyone! This helps a lot. I think I may take the Terrene approach and wack them in June. We'll see!

In the UK, we call it the 'Chelsea Chop' in that we do it during the week of the Chelsea Flower Show (final week in May) - things like asters, helenium, monarda, sedum, get cut back by half to avoid later flopping and stimulate side shoots.

Campanula, love that "Chelsea Chop" too cute

I do my Perovskia too