You can train a fountain butterfly bush as a standard, or single stem, so it looks like a small tree. This form is perfect for a smaller garden. Here’s how to do it:
Step 1 — First start with a young plant, preferably one that’s less than a foot tall. You can plant it where you eventually want the tree. Select a good, sturdy leader, the stem that will eventually become the trunk of your tree. Often you just have to pick the tallest one in the center of the plant. Cut away any other branches coming from the ground as close to the roots as you can.
Step 2 — Insert a sturdy stake next to the leader like in illustration A. Make sure the leader continues to grow straight and tall by tying it to the stake with strips of fabric or stretch ties about a foot apart. Don’t cut off any side branches yet — the young plant needs as many leaves as possible. If you cut them off now, you’ll be weakening the plant. But as the season progresses, keep trimming away any shoots that sprout from the ground.
Step 3 — By the following spring, the leader may be tall enough that you can pinch off the tip to make it branch out and form the ascading head. You can train it to any height you want, but 4 or 5 feet is a good place to start the head. This means the tree will be 8 to 10 feet tall.
Start at the ground and prune all of the side branches off halfway up the the trunk. Leave some branches so the leaves will produce food until you start to form the head.
Step 4 — Once the head begins to round out, you can prune off the rest of the branches along the trunk, up to where the head forms, as in illustration B. And trim the tips of the branches in the head to make a thick, full display when it blooms.
When you start forming the head, you might have to put in a taller or more substantial stake to support the young tree. Insert it in the same spot as the old stake 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 sturdier, you can remove the support stake. Or if your tree is in a very windy spot leave the stake in place for extra support.
As the head grows and fills in like the one in illustration C, prune it just as you would a shrub-form fountain 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.