I'm never sure whether my plants are best pruned back in late fall/early winter after the first frost, or late winter/early spring before everything starts growing/blooming again. In particular, I have a couple of shrubs (butterfly bush and St. Johns Wort) that are prolific and tend to get leggy, some vines like clematis and honeysuckle, a couple of ornamental grasses, and a bunch of perennial flowers. Can you give me some guidance?
You ask many good questions, and it can be confusing as the timing is often based on what plant it is, when it blooms and also if you are willing to leave some brown stalks to feed and shelter wildlife in the winter. (Some seed heads are beautiful with snow or ice on them as well and so offer winter interest.)
Let's start with your butterfly bush. They can easily get way overgrown. The good news is they can take a hard pruning, meaning you can even cut them to the ground and they will come back and bloom for you. The timing for this is later winter into early spring, maybe on a nice day in late February or March.
The timing is the same for St. John's Wort.
Ornamental grasses are usually left to enjoy during the winter and are cut back in early spring all the way to the ground.
For clematis, it depends on the variety that you have. Do you remember when it bloomed? For ones that bloom in June, the time to cut it all back to maybe 3-5 healthy buds is late winter into early spring.
This link from the Brooklyn Botanical Garden will help with your vine pruning:
Here is our pruning publication with more information as well:
thanks, that was very helpful! I have two follow-ups - first, the clematis I have are Josephine and Kilian Donahue. They flower early and that's about it, despite the tags saying "late spring until autumn" and "June through September". Neither gets as thick as I would like either. Second, I'm fine with leaving my plant remains for the wildlife, I was just concerned that leaving them as is through the winter could result in some sort of harm. But it seems like from what you sent me that most everything is best pruned or cut back late winter/early spring. Would that also apply to perennial flowers like the ones I planted this year for the monarchs (milkweek, aster, coneflower, liatris, coreopsis)?
Clematis Josephine and Kilian Donahue are in the pruning Group 2. See the clematis pruning link https://www.bbg.org/gardening/article/pruning_plants_that_ascend#clematis
Pruning depends upon upon the plant. In general, cutting plants to the ground tidies up the garden and removes debris that might harbor insects or disease but it can also remove the
winter weather protection that it provides.
Milkweed - cut back in the spring.
Aster - There are several types of asters. You did not mention what species you have. Most asters are not cut back for the winter to increase survivability. Plants are cut down in the spring after all danger of cold is past.
Coneflower & Liatris. - Many leave the seedheads for the birds over the winter. You can remove any unwanted seedlings in the spring. Prune in the spring.
Coreopsis - Prune in the spring.
For more information on pruning perennials see the link from Illinois http://web.extension.illinois.edu/ccdms/yg/090831.html
Also, if you have a lot of perennials in your garden take a look at the book by Tracy DiSabato-Aust 'The Well Tended Perennial Garden' for planting and pruning techniques. Check in your local library.