Pruning an ash tree comes down to timing, technique, and knowing how much to remove. The best window is late fall through early spring, when the tree is dormant and wood-boring insects like the emerald ash borer aren’t active. With the right approach, you can improve your tree’s structure, remove hazardous branches, and keep it healthy for decades.
When to Prune an Ash Tree
Prune your ash tree during dormancy, roughly October through early March. During this period the tree isn’t actively growing, so cuts heal more efficiently and cause less stress. Equally important, insect pests that target ash trees are inactive in cold weather.
The emerald ash borer, the most destructive pest of North American ash trees, typically begins emerging in mid-June and peaks in mid-to-late July. Fresh pruning wounds release scents that attract these beetles. By pruning before March or after September, you avoid the flight season entirely and reduce the risk of infestation. If you live in an area where emerald ash borer has been detected, this timing isn’t optional. Avoid pruning from April through September.
Dormant pruning also gives you a clearer view of the tree’s branch structure, since there are no leaves blocking your sightlines. You can spot dead wood, crossing branches, and structural problems much more easily.
How Much to Remove
On a mature ash tree, remove no more than 15 to 20 percent of the live canopy in a single season. Often 5 to 10 percent is enough. Taking too much triggers a stress response where the tree pushes out clusters of weak, fast-growing sprouts (called water sprouts) that undermine the structure you’re trying to improve. Young ash trees can tolerate slightly more, up to about 25 percent, because they recover faster and are still being shaped.
If your tree needs significant work, spread it over two or three years rather than doing everything at once.
What to Cut and What to Keep
Start by identifying the branches that need to go. Work through these priorities in order:
- Dead, diseased, or broken branches. These are the most urgent. Dead wood is brittle and can fall unpredictably. Look for branches with no buds, peeling bark, or visible decay.
- Crossing or rubbing branches. When two branches grow into each other, friction wounds the bark and creates entry points for disease. Remove the weaker or less well-positioned branch.
- Co-dominant stems. These are two leaders of roughly equal size growing from the same point, forming a tight V-shaped crotch. The narrow angle traps bark between them and creates a weak attachment that’s prone to splitting in storms. Remove or shorten one stem while the tree is young enough to redirect growth.
- Low-hanging or obstructing branches. Branches that block sidewalks, driveways, or sightlines can be raised by removing the lowest limbs.
- Inward-growing branches. Branches that grow toward the center of the canopy crowd the interior, reduce airflow, and compete for light. Removing them opens up the tree’s structure.
Leave the branch collar intact on every cut. The collar is the slightly swollen ring of tissue where the branch meets the trunk or parent limb. It contains specialized cells that seal over the wound. Cutting flush with the trunk removes this tissue and dramatically slows healing.
The Three-Cut Method
For any branch thicker than about an inch and a half, use the three-cut method. Skipping this step risks tearing a strip of bark down the trunk as the heavy branch falls, leaving a large wound that’s slow to close.
Cut 1: the undercut. About 12 to 18 inches out from the trunk, saw upward from the bottom of the branch, going roughly a third of the way through. This cut acts as a stop. When the branch breaks free, the tear hits this notch and stops rather than ripping down the trunk.
Cut 2: the top cut. A few inches further out from the undercut, saw down from the top of the branch all the way through. The branch will snap off cleanly, leaving a short stub.
Cut 3: the final cut. Now remove the remaining stub by cutting just outside the branch collar. Angle your saw so it follows the collar’s natural contour. This clean, properly placed wound gives the tree the best chance of sealing over completely.
Don’t apply wound sealer or paint to the cut. Research has consistently shown these products don’t help and can actually trap moisture against the wood, encouraging decay.
Choosing the Right Tools
Match your tool to the branch diameter:
- Hand pruners (pruning shears): branches up to 3/4 inch in diameter. Good for small twigs and minor shaping.
- Lopping shears: branches from 3/4 inch to about 1-1/2 inches. The long handles give you more leverage and reach.
- Pruning saw: anything larger than 1-3/4 inches. A curved folding saw handles most residential ash tree work.
- Pole saw or pole pruner: branches up to 2 inches in diameter that are too high to reach safely from the ground.
Keep cutting edges sharp. Dull tools crush and tear bark instead of cutting cleanly, which slows healing. Clean your blades with rubbing alcohol or a diluted bleach solution between trees (or between diseased and healthy branches on the same tree) to avoid spreading pathogens.
For any branch you can’t reach from the ground with a pole saw, or for any limb thick enough to cause damage if it falls, hire a certified arborist. Working above your head with a chainsaw on a ladder is one of the most dangerous tasks in home maintenance.
Aftercare for a Pruned Ash Tree
A pruned tree benefits from consistent watering, especially if you’ve removed a significant portion of the canopy. Most of an ash tree’s absorbing roots sit in the top 12 inches of soil, so water slowly and deeply rather than giving the area a quick soak. A soaker hose or slow-drip setup left running for 30 to 60 minutes ensures moisture reaches the root zone instead of running off the surface.
Hold off on fertilizer, particularly nitrogen-heavy products. Nitrogen pushes leafy canopy growth at the expense of root development. After pruning, you want the tree investing energy in closing wounds and strengthening its root system, not rushing to replace foliage. If the tree appears healthy and is growing in reasonably fertile soil, it likely doesn’t need supplemental feeding at all.
Disposing of Ash Wood Safely
Even though the federal emerald ash borer quarantine was lifted in January 2021, moving ash wood remains a concern because the beetle has spread across much of the eastern and central United States and into Canada. Many states and municipalities still have their own transport restrictions.
The simplest rule: don’t move ash firewood or branches off your property unless you know your local regulations. Chipping pruned branches on-site reduces the wood to pieces too small for beetle larvae to survive in. If you burn ash wood, use it locally rather than transporting it to a campsite or cabin. Visit DontMoveFirewood.org to check area-specific rules before hauling anything.