Invasive Shrubs in the Puget Lowlands – Backyard Forest Restoration

On the tallgrass prairies of the American Great Plains, woody shrubs are replacing grasses, and in the aging deciduous forests of the Puget Lowlands, woody shrubs are replacing trees. In our region, the shrubs include native species, especially Salmonberry, but also various invasives including Feral holly, Cherry laurel, and Himalayan blackberry.

Because there are few seedlings to replace them, as the Red alders and Bigleaf maples reach the end of their natural life spans, dense thickets of shrubs take over.

In this post I briefly review our most invasive shrubs (and thicket-forming invasive plants) and general strategies to control them. I’ve included quotes from three excellent books:

Wild Plants of Greater Seattle by Arthur Lee Jacobson
Invasive Plants: Guide to Identification and the Impacts and Control of Common North American Species by Sylvan and Wallace Kauffman
Weeds of the Pacific Northwest by Mark Turner and Sami Gray

Invasive Shrubs and Thicket-Forming Invasive Plants

Feral Holly, Cherry Laurel, and Himalayan Blackberry

Within a few decades, Feral holly (Ilex aquifolium), Cherry laurel (Prunus laurocerasus), and Himalayan blackberry (Rubus armeniacus) may dominate many unmaintained natural areas in the Puget Lowlands.

Himalayan blackberry has “*astonishing vigor and thumbtack-like thorns.*” *(Jacobson page 134)*

Holly and laurel can grow into trees over 40’ high and form dense thickets to the exclusion of other plants.

Birds consume countless holly berries every winter and spread the seeds in nearby forests.

Broom (Various Species)

Four species of broom make the State of Washington’s noxious weed list.

“Most brooms tolerate baking sun and poor soil with easy nonchalance.” (Jacobson page 120)

*“Treating cut stems with triclopyr will kill plants. Once established, manual or mechanical control of broom is very difficult.” (Kaufman page 81)*

I hand-pulled the broom out of this infestation several times and it always came back; either from bits of roots left in the ground, and/or from the germination of dormant seeds triggered by the disturbance of the soil.

Knotweed (Various Species)

Four different species of knotweed have been listed as Class B noxious weeds in Washington. Though not really shrubs, knotweeds “form dense colonies that crowd out native vegetation and alter plant communities.” (Turner and Gray page 78)

Because knotweed is extraordinarily invasive and very difficult to control, early detection and removal is paramount.

Cutting knotweed to the ground every couple of weeks throughout its growing season can keep it from going to seed, but killing it this way (by exhausting the rhizomes) may take a decade (Kaufman page 161). I have had success using cut-stump treatments in very small patches, but clearing large patches without subsequent dense plantings may result in colonization by other invasive plants like Himalayan blackberry.

Butterfly Bush (Buddleja davidii)

“Butterfly bush runs wild in the Seattle areas as an industrial strength weed.” (Jacobson page 108)

A Class B noxious weed, Butterfly bush spreads by seed but can still form thickets.

Butterfly bush “*does not spread vegetatively, so it can be pulled or dug out.” (Kaufmann page 163)*

Spurge Laurel (Daphne Laureola)

Spurge laurel, a Class B noxious weed in the State of Washington, has spread in the Puget Lowlands by birds that eat its fruit. Its attractive appearance, fragrant flowers, and dark berries belie its toxicity and invasive habits.

Though small Spurge laurel seedlings are easy to pull by hand, it’s important to “*wear protective clothing to avoid contact with toxic compounds found in all plant parts.” (Turner and Gray page 97)*

Once established, Spurge laurel spreads vegetatively into large clumps. I’ve never tackled one of these, but I imagine it would require extensive digging or cut-stump treatments.

Privets (Various Ligustrum Cultivars)

“Privet forms dense stands that outcompete native plants for space, light, and water.” (Kaufman page 86)

Various privet cultivars are sold in retail nurseries, primarily for use in clipped hedges. Already a serious invasive in the southeastern United States, varieties that produce bird-friendly fruit are enabling privet to become naturalized in the Puget Lowlands.

This unmaintained privet hedge in Forest Park grew to be twenty feet high. An ambitious volunteer (not me) started to cut it down but was not able to finish.

About two years ago, I cut part of the same privet hedge down to the ground and covered it with thick landscape fabric and chips. So far, it hasn’t come back.

Cotoneasters (C. Simonsii, C. Rehderi, and Others)

EarthCorps includes cotoneaster in their two-page fact sheet, Invasive Shrubs of the Puget Sound Region.

C. simonsii “*has been bird-distributed here for decades,*” and C. rehderi has “*long been wild in Seattle.” (Jacobson page 102)*

*“Cotoneaster is a genus of widely planted non-native shrubs that can be aggressive in conquest of space in gardens or when escaping into wild landscapes.” (Turner and Gray page 80)*

Review of General Control Strategies

Preferably, remove invasive shrubs manually when they are still small, getting all or at least most of the roots with minimal disturbance to the ground.

Seedlings can be pulled by hand.
Shrubs with woody stems up to about 1 inch in diameter may give way to a pry-and-prop with a shovel.
Shrubs with woody stems up to about 2 inches in diameter can usually be removed with a weed wrench.

In most of King County, you can borrow a weed wrench for free.

Larger shrubs are problematic.

Digging out large shrubs severely disrupts the soil.
Repeatedly cutting shrubs to ground level will eventually kill them but can take years.
Cutting shrubs to the ground and covering the stumps with thick black landscape fabric for several years may work.
If you’re not opposed to the limited use of herbicides, the cut-stump treatment is usually effective. The U.S. Forest Service permits spot spray and selective applications of triclopyr. Cut-stump treatments can be effective at most times of the year except during heavy sap flow (typically during early Spring).