Quaking Aspen Problems in Montana Yards

Why do aspens struggle in valley yards?

They are planted outside the conditions they prefer. The Montana DNRC nursery lists quaking aspen's natural habitat as streamsides, wetlands and cool slopes where the soil stays somewhat moist. It rates the tree's tolerance of alkaline or saline soil as poor. Valley yards here tend to be the reverse: hot and dry in summer, alkaline, and often compacted.

Colorado State University Extension, writing about the same mismatch in Colorado, says aspen prefer light, acidic, well-drained soils that are rarely found in urban areas and do best on the north and east sides of buildings. Montana State University Extension lists poplars among the trees more prone to leaf chlorosis, the yellowing between green veins that shows up where soil pH limits iron. See the guide to iron chlorosis if that describes your tree.

Why does an aspen send up suckers everywhere?

Because that is how aspen reproduces. The Forest Service species review explains that quaking aspen forms clones joined by a common root system and that root sprouting is its most common method of regeneration. The root system is shallow and wide, and the review reports studies in which most suckers came from roots within a few inches of the soil surface.

In a grove this is the tree renewing itself. In a yard it means shoots in the lawn, the flower beds and sometimes next door. Sprouting is strongest after the parent stem is killed or cut down. Mowing or cutting them is the safe control. Because suckers are attached to the parent, be careful with any herbicide. The sibling guide on herbicide damage to trees explains why.

How long does a yard aspen live?

Individual aspen stems are short-lived compared with most shade trees, even though the root system that produces them can persist far longer. The Montana DNRC nursery gives a life span of 30 years or more. Colorado State University Extension is blunter about landscape trees in its state, giving aspen a life expectancy of about 25 years in the home landscape even with careful preparation and care.

Those are Colorado and nursery figures, not a deadline. They do set expectations. An aspen planted when a subdivision was built two or three decades ago and now thinning from the top is behaving normally for the species.

What are the cankers and oozing on the trunk?

Most often cytospora canker. MSU Extension describes it as a disease that mainly affects stressed trees and enters through fresh wounds. On deciduous trees the cankers are patches of bark discolored yellowish, brown, reddish-brown, gray or black, sometimes with small black pimple-like structures, and aspen may emit a liquid ooze. In wet weather orange threads of spores emerge.

There is no spray that cures a canker. MSU's management is to reduce stress with adequate water and nutrients, avoid wounding the trunk with mowers and string trimmers, and remove affected branches in dry weather, disinfecting tools after each cut. A canker that girdles the main trunk kills everything above it. Thin aspen bark is easy to wound, which is part of why yard trees get this disease so often.

What causes spotted and black leaves?

Usually Marssonina leaf spot. MSU Extension describes small dark-brown spots, often with yellow margins, that merge into larger dead areas and can cause early leaf drop. It is worst in wet springs. MSU's verdict is that the disease may be unsightly but typically does not cause severe damage, although infections several years running will weaken a tree.

The fungus overwinters in fallen leaves, so the main control is raking and removing them in fall. That makes fall leaf cleanup under an aspen more than a matter of tidiness. Keeping sprinklers off the foliage helps too. MSU says fungicides are not usually recommended unless the disease is a continuing problem. Black, wilted shoot tips bent into a shepherd's crook in late spring are a different disease, Venturia shoot blight, which MSU says is most severe on young trees and is managed by pruning out infected shoots in dry weather.

Borers, scale and leafminers

Poplar borer is the serious one. MSU Extension says the larvae bore into the sapwood, that heavily infested trees show a blackish stain on the bark below the attack, and that weakened stems can break in high winds. The life cycle can take up to three years. MSU's advice is to keep trees adequately watered, plant aspen in groups so the trunks are shaded, and remove a heavily infested tree because it can affect others nearby.

Oystershell scale looks like tiny gray-brown shells crusted on the bark and can cause branch dieback when heavy. Aspen blotch leafminer leaves pale blotches on the leaves. MSU notes that it attacks stressed trees and normally needs no treatment. MSU lists insecticide options and timing for borers and scale. With any product, the label directions are the law, and restricted-use products need a licensed applicator.

How do deer and elk damage aspen?

In two ways: rubbing and eating. MSU Extension explains that bucks rub their antlers on trees in fall and early winter and prefer immature trees with smooth bark and no low limbs, which describes a young aspen exactly. The shredded bark is an open wound on a tree already prone to canker.

Browsing is the other half. The DNRC nursery rates aspen's deer resistance as low and lists it as browse for deer, rabbits, elk and grouse. The Forest Service review documents elk and deer browsing aspen sprouts heavily enough in parts of the Rockies to prevent stands from regenerating. On valley acreages that winter elk and deer, unprotected young aspen and suckers are eaten back every year. MSU recommends trunk guards and fencing. More on this is in the guide to deer and elk damage.

Can a declining aspen be saved?

Sometimes it can be slowed. The useful steps are the ones that reduce stress: deep, infrequent watering through hot, dry spells, mulch instead of turf over the roots, protection for the trunk, removal of dead and cankered branches, and fall leaf cleanup. They help most on a tree with a mostly full crown and problems confined to branches.

They do not reverse a trunk canker, a stem riddled by borers or a crown that is more than half dead. Colorado State's guidance for groves is that once an individual tree starts developing deadwood and a thin crown, it should be removed to allow new shoots to develop.

When is it time to replace an aspen?

When the main stem is failing, when dead wood hangs over a walk, drive or roof, or when the tree has become a yearly cycle of treatment with less to show each time. The DNRC nursery lists stem rot among aspen's usual problems, so dead and dying stems near targets should not be left standing. Expect suckers after removal. Grinding the stump and mowing the shoots for a season or two is the usual approach, and stump grinding is something we do.

If you want the look of aspen on a hot valley lot, consider a species suited to the site from our list of the best trees to plant in western Montana. A tree health and risk assessment will tell you whether your aspen is worth nursing. It is part of our tree services, and the other species guides are in the Montana Tree Guide.

Call HJ Property Care & Tree Service at (406) 215-7319 or request a tree assessment and an ISA Certified Arborist will look at the tree with you.

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