When it comes to pruning, plants aren’t fussy, either. If you live where winter temps fall below 0 degrees F, butterfly bush will die back to the ground. Simply cut off any remaining dead growth in spring. Where winters are mild, prune all branches back to 2 or 3 in. from the ground in early spring to keep new growth even and compact. Avoid cutting plants back in the fall, as it leaves the crown of the plant exposed to harsh wind and damage from the freeze-thaw cycle.
Pruning butterfly bush
In areas where winter temps dip below 0 degrees F, butterfly bush dies back to the ground like an her-baceous perennial. Remove the winter-killed growth by cutting plants back in the spring. Fall pruning weakens the plant by taking away stored energy and leaves the crown exposed. Even if you live in an area where it doesn’t die back in the winter, plants will grow more evenly with some hard pruning. Deadheading spent blooms keeps plants looking tidy and encourages reblooming. Learn more about how and when to prune in the illustrations below.
Pruning in early spring
In early spring, cut back all of the growth to 2 or 3 in. from the ground. New shoots emerge from the crown late in the season, once the soil warms up.
Pinching for more flowers
Pinching encourages more branching and more flowers. Prune all of the branch tips back a few inches in early spring, and then again a few weeks later for midsummer blooms.
How to training a butterfly bush into a standard
You can train a butterfly bush (Buddleja davidii) as a standard, or single stem, so it looks like a small tree. This form is perfect for a container or small garden. Here’s how to do it:
Step 1: Choose a leader
Start with a young plant, preferably one that’s less than a foot tall. Select the straightest upright stem as your leader — often the tallest one in the center of the plant is the best pick. You'll train this to be the trunk of your butterfly tree.
Insert a sturdy stake next to the leader, as the illustration above shows. This keeps it growing straight and tall. Tie the leader to the stake with something soft and pliable, such as strips of fabric or nylon stocking. Leave the side stems for now to help the plant gather energy for growth. But keep an eye out for suckers that may form at the base of the plant and remove them.
Step 2: Pinch and prune butterfly bush for shape
As the butterfly tree grows up to 4 ft. tall strip off all any side shoots that form along the leader. A proportion of two-thirds trunk to one-third head looks good. Continue to attach the leader to the stake spacing the ties about a foot apart.
Step 3: Trim for flowers
Once the head begins to round out, pinch back the tips of the branches, as in the illustration, to encourage more growth and so it forms a thick, full display of blooms.
As your tree grows, you might have to put in a taller stake to support it. Insert the new stake in the same spot as the old one to avoid damaging roots. Continue to tie the trunk to the stake and adjust the ties occasionally so they don’t strangle the tree. After the trunk thickens and becomes stronger, you can remove the support stake. Or if your tree is in a very windy spot leave the stake in place for extra support.
Step 4: Prune to maintain
As the head grows and fills in like the one above, prune stems just as you would any other butterfly bush: Remove a few older branches every year, and do a bit of shaping after the “tree” flowers. Trim off any sprouts from the ground or from along the trunk. This is a fast-growing plant, so you may need to do quite a bit of trimming to keep it looking neat and tidy.