Birch trees (Betula spp.) naturally grow with a single trunk, but odd numbers of single-stemmed trees can be planted together in a clump in which the roots weave together to form what appears to be a single specimen. Alternatively, a single young birch tree can be pruned to grow multiple trunks. The trunks grow outward, leaving an open center, to make room for the tree canopy. Common clump birch species that will thrive in U.S. Department of Agriculture plant hardiness zones 8a to 10b include river birch (B. nigra) and gray birch (B. populifolia). Birch trees require minimal pruning.
Clean and disinfect all pruning tools with a disinfectant such as isopropyl alcohol, chlorine bleach or a household disinfectant. You will need bypass pruners for small branches, lopping shears for medium branches, a pruning saw for large branches and a pole pruner to reach the higher branches.
Plant a single-trunk birch tree in spring and allow it to grow for one year or until the trunk is at least 1 inch in diameter. Cut back the entire tree in the following spring, leaving only about a 2-inch stub in the ground. Given plenty of water, the tree will grow several shoots that become the trunks. Allow another year for the shoots to grow. Alternatively, you can plant three to five single trees together in one hole, using a rock or block of wood to keep the spacing between the trunks.
Wait until midsummer to early fall, when the birch is dormant, to prune to reduce the amount of sap that drains from the cuts. This helps reduce disease and infestation that are attracted to the oozing cuts.
Remove any sucker sprouts that grow from the base of the tree after the new trunks are well established. Cut these as close to the ground as possible so they don't steal moisture from the clump.
Cut or pinch off any branch shoots that develop low on the trunks so the colorful gray or cinnamon bark and the clump of trunks are on display.
Eliminate any branches with weak-angled crotches, allowing only branches with 70- to 90-degree angle branch crotches to grow, which leads to strong, healthy branches in the tree. You must remove these branches in young, developing trees to train the main scaffold branches, but you should also remove any weak-angled, smaller branches throughout the tree's life.
Cut all rubbing, crossing or inward-growing branches from the tree canopy to allow open airflow within the canopy and encourage an open framework in the upper branches. It is okay for branches to grow on the insides of the trunks, but you want to avoid branches that interfere with other branches from other trunks within the clump.
Cut diseased branches from the tree as they occur. Make the cut about 6 inches outside the diseased area to ensure all the disease is removed. Disinfect any pruning tools used to cut diseased branches immediately.
Remove dead or broken branches as they occur, cutting the branches back to just above the branch union with a healthy branch or just outside the branch collar if you must remove an entire branch back to one of the trunks. Dead, dry wood will not bleed when cut like green wood, so you can remove these at any time.
Things You Will Need
- Isopropyl alcohol, chlorine bleach or household disinfectant
- Bypass pruners
- Lopping shears
- Pruning saw
- Pole pruner
- Gray birch grows in U.S. Department of Agriculture plant hardiness zones 3 through 8, while river birch grows in zones 4 through 9.
- If one of the trunks spreads out so far that it appears in danger of splitting, you can tie it to the other trunks with soft twine to hold it up until it grows strong. As the trunk grows and is trained in this position, you can untie the tree and allow it to grow on its own.
- Clemson Cooperative Extension: Birch -- Betula Spp.
- Arbor Day Foundation: Birch, River Betula Nigra
- USDA Natural Resources Conservation Service: Gray Birch
- University of Minnesota Master Gardener: Avoiding the Clump White Birch Curse
- North Dakota State University Extension: Questions On: Birch
- Pat Welsh: Planting a Clump of Three Birch Trees. European White Birch (Betula Pendula)
- The Arbor Day Foundation Blog: How to Create Clumps of Birch Trees
- North Carolina State University Extension: Pruning Trees
- Ryan McVay/Lifesize/Getty Images