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Generally, trim in the spring for size & shape when you see them growing in the spring. They look fine as in healthy, but if you think they're too big, at this time of year, I would cut about half of the largest trunks to the ground. That should cause a lot of new growth from the ground this year & next. Once that's off to a good start, cut off the remaining trunks next spring, and shorten any parts that look like they'll get too tall. Then proceed to chop each spring as desired for size & shape. Incidentally, unless you're after a different flower, you can stick your trimmings in the ground in the spring to make new bushes. Keep moist, not soggy until they get going. Very easy to start. They're just naturally kind of scraggly and I'm constantly trimming little dead bits when I'm deadheading the flowers. Perfectly normal. Keeping them low enough that it's mostly new growth every year gives a much neater appearance, in most people's opinion but it's just a matter of personal preference.
Trust me, it'll survive. They are very tolerant of hard pruning. In colder zones, they can actually die all the way back and resprout in the spring. You definitely won't hurt it by chopping it way back.
Perfect timing! I'm in zone 5a and my butterfly bush dies all the way down every year and this year it's already over 5 feet tall.
I'm wondering about yellow leaves near the base. Each day - especially with our warmer weather, the plant tops wilt and the new flowers (no color yet!) wilt over. Yet the bottom leaves near the base are yellowing and I'm confused if it's because of too much water or not enough water or if it doesn't have anything to do with the water.
The plant perks up each evening once it cools down, but it does look pretty awful during the day. I think of if like a restaurant that I wouldn't want to visit.
If it perks up when the sun goes down in the evening then it likely doesn't need more water--that is typically a signal that there is enough water in the soil, but the plant's roots for whatever reason can't take up water fast enough during the heat of the day, so the plant wilts as a defense mechanism. But you can check by sticking your finger down a few inches in the soil to see how wet it feels.
We live on the east side of Michigan about half way up the state and usually, except this year, get really cold weather and sometimes lots of snow[which acts as insulation on plants] and only the first year did we prune our butterfly bush. We discovered that the new years growth comes out on the old stems and yes some do die and those are pruned out. Our plant started off at about 5 ft tall this year and has leaved out all along the stem, even sending up new shoots from the base.
I would suggest that if you have had success in the past keep up what you are doing. But I agree with ecrane3 that hard pruning does not harm the plants and it can give you a more uniform looking plant.
ecrane3 - thanks so much - if I can figure out how to do it, I'd like to send a photo once the flowers bloom because I don't know what butterfly bush it is and I'm curious. It's a flower that starts out purple and graduates to a pinkish orange color. It's stunning - almost looks like my kniphophia (sp?) with the variegated coloring.
I've had these for many years in 5b and now 8b. In 5b they usually do die back to ground level, so pruning is not such a big deal. But unless you want to need a ladder to deadhead, I like to keep them shorter. It's pretty useless if it stops blooming, this is not an attractive plant, IMO, especially if its' gone wild, but I keep several for the butterflies, the most reliably-blooming and longest-blooming nectar plant I know. Fall pruning can be fatal, especially in the colder zones, best done in spring (which also allows you to see which parts are alive/dead.)