Backyard Forest Restoration

Seven Non-Native Deciduous Trees Invading the Puget Lowlands

The evergreen laurel and holly may be the most invasive trees in the Puget Lowlands, but we have invasive deciduous trees as well. The seven described in this post are all naturalized in the Pacific Northwest and at minimum take up space in our forests that would be better served with native trees. I advocate removing all of them from our backyard forests, ideally when they are still small enough to wiggle-and-pull by hand or pry-and-pop with a shovel.

European Mountain Ash (Sorbus aucuparia)

The European Mountain Ash is sufficiently invasive to make the Washington State Noxious Weed Control Board’s (NWCB’s) monitor list (here). It is easily spotted in late summer and fall by the drooping clusters of bitter red berries. Though they are not poisonous, Arthur Lee Jacobson quips, “If taste could kill, these fruits might do it.” (here, page 78)

Known in the British Isles as the Rowan tree, the European Mountain Ash has a long association with mythology and folklore.
European Mountain Ash should not be confused with the native Sitka Mountain Ash (Sorbus sitchensis) which is usually more shrub-like and uncommon in the Puget Lowlands.

Common Hawthorn (Crataegus monogyna)

The Common Hawthorn is sufficiently invasive to make the NWCB’s Class C list (here) and warrant a “must control” designation in Snohomish County (here). Once it’s bigger than a sapling it becomes very hard to remove, being extremely tough, deeply rooted, and thorny.

Common Hawthorn is so tough and thorny that it is sometimes used as a living fence for cattle.
The native Black Hawthorn (Crataegus douglassii) is a much friendlier tree and a pleasant surprise in urban forests.

Norway Maple (Acer platanoides)

The Norway Maple is sufficiently invasive to make the NWCB’s monitor list (here). Like the Sycamore Maple (below) it’s gradually replacing the Bigleaf Maples in our forests because the seedlings of the invasive maples can grow in full shade whereas those of the native maple cannot.

Kaufman and Kaufman say of the Norway Maple, “it casts dense shade, and its shallow, dense root system makes it difficult for other plants to establish.” (here, page 154)

Sycamore Maple (Acer pseudoplatanus)

Though not on any NWCB list, the Sycamore Maple is rightly recognized as invasive in some areas.

The Sycamore Maple, like the Norway Maple, seeds prolifically, sprouts easily, and spreads quickly.
These reddish-purple undersides of leaves are characteristic of one of the cultivars of Sycamore Maple.

Black Locust (Robinia pseudoacacia)

Turner and Gray find that Black Locust “suckers aggressively, rapidly forming broad, dense thickets that crowd out natives.” (here, page 70)
Be wary of the vicious thorns of the Black Locust!

White Poplar (Populus alba)

Like the Black Locust, the White Poplar can spread by suckers and form dense colonies that crowd out natives.” (here page 88 and here page 111)

Horse Chestnut (Aesculus hippocastanum)

The Horse Chestnut is easily recognized by its distinctive groups of 5-7 leaves radiating from a common point. Each leaflet is widest near the tip.
The increasing number of naturalized Horse Chestnut trees in the Puget Lowlands can live long, grow large, and compete for resources with native trees.

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