The short version
| Tree | Prune | Avoid | Why |
|---|---|---|---|
| Ash | Labor Day to Memorial Day | Memorial Day to Labor Day | Adult emerald ash borers are flying and drawn to fresh cuts; the City says don't prune, remove or move ash then |
| Oak | July through March | April, May, June | The state advises against oak pruning then because of oak wilt |
| Elm | Dormant season | Spring and summer | Fresh wood and wounds attract the beetles that carry Dutch elm disease |
| Spruce | Dry weather | Wet periods | Cytospora canker spreads in wet conditions; tools cleaned between cuts |
| Most other trees | Late fall to early spring | Leaf-out | Structure is easy to see, and wounds close as growth starts |
Ash
The City of Sioux Falls says not to prune or remove ash from Memorial Day to Labor Day, because adult beetles are drawn to fresh wounds and can spread in moved wood. Moving ash wood is prohibited in that window. From Labor Day to the next Memorial Day, ash work is fine, and the state's 2018 alert advised that ash cut in fall and winter be burned or chipped before spring. The trunk-injection window for treatment is different: SDSU Extension puts it at leaf-out to mid-June.
Oak
The South Dakota Department of Agriculture and Natural Resources advises against pruning oaks in April, May or June, when fresh wounds attract the beetles that carry oak wilt. It also says oaks within 50 feet of each other can be assumed root-grafted, and advises avoiding grade or drainage changes within 100 feet of oak stands. Red-group oaks are hit hardest; bur oaks can survive for years.
Elm
Elm bark beetles breed in dead and dying elm wood and carry Dutch elm disease to healthy trees. We prune elms in the dormant season and chip or dispose of the wood promptly. Preventive injections are repeated every three years. DANR says therapeutic use is limited to elms with less than 10 percent yellowing, so the time to protect a valuable elm is before symptoms show.
Spruce and pine
Cytospora canker on Colorado spruce spreads more in wet weather, so we prune cankered branches back to the trunk in dry weather and clean tools between cuts. Scotch pines that turn brown all at once in late summer should be checked for pine wilt and removed promptly.
Frost and the season
At Sioux Falls, the National Weather Service puts the average last spring freeze at May 6 and the first fall freeze at September 30, about 147 freeze-free days over the period of record. That's the planting season, and it's when trees are actively growing and moving water. The 2023 USDA hardiness map put much of southern South Dakota in zone 5a, with typical winter lows of −15 to −20°F.
Common questions
Can I trim my ash in June if it's just a small branch?
Leave it until after Labor Day. The City says not to prune ash from Memorial Day to Labor Day, because fresh wounds attract adult beetles.
What's the best month to prune most trees?
Late winter, February and March, for most species: the tree is dormant, you can see its structure, and wounds close quickly when growth starts. Not oaks in April, though.
When should EAB treatment be done?
SDSU Extension puts the ideal trunk-injection window at leaf-out to mid-June.
Is fall a good time to plant?
Yes. Early fall gives roots time to grow before winter. The average first fall freeze at Sioux Falls is September 30.
Related: Tree trimming · Elm and oak care · Spruce care · EAB guide