Many invasive plants began as garden plants and escaped cultivation to wreak havoc on native ecosystems. Put into numbers, 21 species officially designated as noxious weeds in the state of Oregon cost around $125 million per year in control, fire damage, and production losses, according to a 2009 study — and the amount is probably higher today. As gardeners, we can take action by refusing to purchase invasive plants and removing those already growing in our landscapes. Learn about some of the worst and most common invasive plants in Oregon gardens, as well as native plant alternatives, in the list below.
Invasive Plants in Oregon Gardens
If you notice any of the following plants on your property, contact your local Extension Office for help on how to properly remove them. And if you see anyone selling or giving away these plants, kindly inform them about the invasiveness of the plant and ask them not to distribute it. Each of the descriptions in this list includes suggested native plants to grow instead.
1. Himalayan blackberry (Rubus armeniacus)
The invasive plant with the highest annual economic impact in Oregon is the Himalayan blackberry (also known as the Armenian blackberry). Well-known for its delicious berries, Himalayan blackberry forms dense, impenetrable thickets of long, arcing canes covered in sharp spines. Its compound leaves are typically composed of five ovate leaflets with dark green tops and gray-green undersides. Showy, white to pink flowers precede the juicy black berries.
There are several native plants with edible berries in the same genus, such as salmonberry (R. spectabilis) and thimbleberry (R. parviflorus).
2. Scotch broom (Cytisus scoparius)
Scotch broom is the second-most costly invasive plant in Oregon. This small to medium deciduous shrub is also easily recognizable, with its broom-like habit and yellow, pea-like blossoms. It has tiny trifoliate leaves on its green stems and hairy, flattened seedpods that burst open to release thousands of seeds per plant, each seed remaining viable for as long as 50 years. Not only is this shrub toxic to humans and livestock, but it is also highly flammable.
Golden current (Ribes aureum) is a lovely native shrub with a similar height and showy yellow to orange flowers. Though a bit taller, western mockorange (Philadelphus lewisii) is another multistemmed shrub with masses of fragrant, white flowers.
3. Gorse (Ulex europaeus)
Similar to Scotch broom, gorse has golden pea-like flowers and hairy seed pods that split open when ripe. It also poses a significant fire hazard. However, the two plants differ significantly in that gorse is a medium to large, spiny evergreen shrub that has a long spring bloom time and may continue blooming sporadically throughout the year. The flowers are said to smell like coconuts.
Plant instead tall Oregon grape (Berberis aquifolium) or mountain mahogany (Cercocarpus ledifolius), two native evergreen shrubs with yellow flowers.
4. Dalmatian and yellow toadflax (Linaria dalmatica, L. vulgaris)
Dalmatian and yellow toadflax, tall herbaceous perennials, can reach heights of up to 10 feet and form dense stands that crowd out native plants. They have waxy, blue-green leaves that clasp the stem and yellow, snapdragon-like flowers with a distinctive long spur. A mature plant can produce hundreds of thousands of seeds as well as spread via creeping roots.
Beautiful native yellow flowers include sulfur buckwheat (Eriogonum umbellatum) and Oregon sunshine (Eriophyllum lanatum).
5. Knapweeds (Centaurea cyanus, C. macrocephala, C. pratensis)
Bachelor’s button, bighead knapweed, and meadow knapweed are three- to five-foot herbaceous plants with showy, tufted flowers. An annual, bachelor’s button has white, purple, or blue flowers and foliage covered in short gray hairs. The other two are both perennials. The largest of the three, bighead knapweed has broad yellow flowers and large basal rosette leaves. Meadow knapweed is often highly branched and features white to rose-purple blossoms.
Bluehead gilia (Gilia capitata) has similar globe-shaped, purplish flowers, while showy fleabane (Erigeron speciosus) features daisy-like flowers with a yellow disc surrounded by thin, purplish petals for a somewhat tufted appearance.
6. Knotweeds (Polygonum cuspidatum, P. sachalinense, P. polystachyum)
Japanese, giant, and Himalayan knotweed form dense stands along riverbanks, streams, and ditches. Their hollow, bamboo-like stems grow up to 10 feet tall and have a reddish, papery sheath at each joint. In late summer, white flowers bloom in feathery, upright panicles. They primarily spread via stem fragments, which easily disperse in flowing water.
Oceanspray (Holodiscus discolor) is a native shrub with drooping clusters of white flowers, and goat’s beard (Aruncus dioicus), a tall herbaceous perennial, produces large, upright, feathery to fingerlike panicles of white flowers.
7. Leafy and myrtle spurge (Euphorbia esula, E. myrsinites)
Herbaceous perennials up to three feet tall, leafy and myrtle spurge have a distinctive appearance. Their blue-green leaves have a geometric arrangement on the stem, and both leaves and stem exude a milky sap that is toxic to both humans and livestock. Leafy spurge has lance-shaped leaves, while myrtle spurge has fleshy, ovate leaves. Both produce greenish-yellow terminal flower clusters.
Native species of stonecrop, such as broadleaf stonecrop (Sedum spathulifolium), make a great alternative for myrtle spurge, while silver pussytoes (Antennaria argentea) is a good option in place of leafy spurge.
8. Hawkweed (Hieracium floribundum, H. aurantiacum)
Yellow hawkweed and orange hawkweed grow up to two feet tall from a basal rosette of hairy, lance-shaped leaves. The flowers, either yellow or orange depending on the species, resemble small dandelion blossoms but bloom in clusters rather than singly. The seed heads that follow also resemble those of dandelions, and the plants can also spread via stoloniferous stems.
Lovely yellow and orange native flowers include orange sneezeweed (Helenium hoopesii) and California poppy (Eschscholzia californica).
9. English ivy (Hedera helix)
English ivy, as well as its cousin Irish ivy (H. hibernica), is an unfortunately popular woody climbing vine and groundcover. Its shiny, dark green leaves have three lobes in the juvenile form and are diamond-shaped on mature vines. English ivy produces tight clusters of white flowers, followed by purple berries. Its seeds are dispersed by birds, and it can also regenerate from stem fragments.
Creeping mahonia (Mahonia repens) is an excellent native groundcover, and woodbine (Parthenocissus vitacea) is a similar native vine.
From the coast to the mountains to the high desert, Oregon is a beautiful state rich in diversity. When you can recognize some of the worst and most common invasive plants in the state, you can help protect this beauty by planting native species instead of invasives in your garden. One plant at a time, we can make a difference.
Plant These Oregon Natives Instead
If you want a garden that requires little upkeep, this list of Oregon native plants can help you get started.