When to Prune Oaks, Elms and Maples in Palatine

Oaks: nothing between mid-April and mid-October

The firmest date on the local pruning calendar belongs to oaks. The Morton Arboretum's oak wilt guidance says oaks should not be pruned between mid-April and mid-October, and it gives the safest window as early November to very early April. The reason is an insect. Sap beetles are drawn to fresh wounds during the growing season, and they carry spores of the oak wilt fungus from infected trees to the cut. Oak wilt occurs in every Illinois county, so there is no local exemption.

The stakes differ by oak group. Red-group oaks, including red, black, Hill's, pin and scarlet oak, usually die quickly once infected, often within weeks or months, and may drop all their leaves by midsummer. White-group oaks, such as white, bur and swamp white oak, show symptoms more slowly and may decline with branch dieback for years. Oaks of the same species within about 50 feet of each other can also graft roots and share the fungus underground, which the Arboretum says causes most new infections, usually within one to six years.

Palatine has a real stake in getting this right. Deer Grove, the county's first forest preserve, holds nearly 2,000 acres of upland forest, ravines, savanna and oak woodland near Palatine and Barrington, and an oak in a yard nearby faces the same beetles. When a storm tears an oak limb in July, the Arboretum's instruction is to sterilize tools between cuts and paint the wound with a nontoxic dressing, then leave any other pruning for winter.

Elms: act on the first flag

Elms have a different enemy with a similar pattern. Dutch elm disease is carried by two beetles, the native elm bark beetle and the European elm bark beetle, and it also passes through root grafts, which the Morton Arboretum says are especially common where trees are packed into narrow urban and suburban parkways. The first sign is flagging: leaves on one upper branch curl and turn gray-green or yellow, then brown, and brown streaks show in the wood under the bark.

Speed is what saves an elm. The Arboretum says an infected tree may be saved by pruning out the diseased branch promptly after the first flag appears, and that the odds are good only while no more than 5 percent of the tree shows symptoms. Systemic fungicide injections are available for valuable trees and protect them for about three years. Infected elms that are too far gone should be removed and the wood destroyed, never kept as firewood, since moving diseased elm wood can spread the disease.

For elm owners, then, the calendar matters less than vigilance through the summer. A walk around the tree every couple of weeks from June on, looking up at the upper crown, catches a flag while pruning can still help. Resistant American and hybrid elm selections are an option for replanting, though the Arboretum notes that they resist the disease without being immune to it.

Maples: timing for wounds, restraint for size

Maples raise a different set of questions, mostly about size rather than disease. For maples and most other species without a disease window, Purdue Extension's pruning bulletin notes that wounds close fastest when cuts are made in late spring and early summer, and it advises against pruning in late summer and early fall. Trees that bloom in spring are pruned right after flowering. Dead, broken or hanging limbs are the exception: they come off whenever they are found.

Silver maple needs the most attention. The Morton Arboretum describes it as fast-growing, reaching 50 to 70 feet, with weak wood and branch structure, and notes that without proper and frequent pruning, high winds and ice can break its limbs. Many authorities now recommend against planting it. Norway maple is a separate case: the Arboretum does not recommend planting it in the region at all, because it has proved invasive and its shade crowds out native plants. Neither is a reason to remove a healthy tree, but both argue for regular, light maintenance.

Arboretum research on red and Norway maples adds a structural point. Unions without a branch collar, without a single branch bark ridge, or shaped like a narrow V proved weaker in testing. Those are the forks to correct early on a young tree and to have inspected on an old one, where cabling and bracing may be the answer.

How much to cut in one visit

Timing is half the question; quantity is the other half. Purdue Extension caps live foliage removal at about 50 percent for young, newly established trees, 25 percent for medium-aged trees and 10 percent for mature ones. A 50-year-old maple is mature, so its tree trimming should be light, focused on deadwood, clearance and a few reduction cuts, and repeated every few years rather than done all at once.

Topping is the opposite of that approach. The ISA's brochure on topping says it removes 50 to 100 percent of a tree's leaf-bearing crown, forces fast, weakly attached sprouts that break in wind and ice, and opens large wounds to decay. Where a tree has to be made smaller, the ISA's alternative is to cut limbs back to laterals at least one-third the diameter of the limb being removed, or to remove a tree that has outgrown its space and replant.

The cut itself matters as much. Each one should sit just outside the branch bark ridge and the collar, never flush with the trunk, and large limbs should come off in three cuts so the bark does not tear. Purdue notes that wound dressings are not needed on pruning cuts and do the tree no good, with oaks in the growing season as the local exception.

The O'Hare calendar and the village schedules

NOAA's 1991-2020 normals for O'Hare, the nearest first-order station to Palatine, put the median last spring freeze on April 22 and the median first fall freeze on October 19, with a 177-day season between them. In one year out of ten, a freeze comes after May 7 or before October 3. The Arboretum's oak window, early November to very early April, sits inside the cold half of that year, when normal lows at O'Hare run from 32.4 degrees in November to 16.5 in January.

Municipal crews plan around similar seasons. Hoffman Estates prunes its parkway trees on a four- to seven-year cycle, mostly in late fall and winter, and Arlington Heights prunes its 36,000 parkway trees on a four- to five-year cycle. Palatine's Forestry Division handles its own parkway trees, and new ones are left unpruned for their first three years. Owners arranging tree service in Palatine for a yard oak should book it for the months between November and early April, and can fit maples and most other species into late spring.

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