Know Your Enemy: Noxious Weeds Threatening Spokane's Native Forests
Native conifers like ponderosa pine, western larch, and grand fir are the headline performers in eastern Washington's forests — but they are under pressure not just from climate change and bark beetles, but from an army of invasive plants that arrived from Europe and Asia over the last 150 years.
These "noxious weeds" — plants declared harmful to agriculture, ecosystems, or public health by state law — displace native vegetation, disrupt nutrient cycles, increase wildfire risk, and reduce the biodiversity that makes forests resilient. Understanding them is the first step to fighting back.
This post covers three of the most ecologically significant invasive plant threats in the Spokane region.
1. Cheatgrass (Bromus tectorum) — The Fire Amplifier
iNaturalist · USDA PLANTS (BRTE) · Wikipedia
Cheatgrass — also called downy brome — may be the single most ecologically destructive invasive plant in the western United States. A winter annual grass native to Eurasia, it was first recorded in the US in 1861 in New York and Pennsylvania. By 1928 it had spread to every state except Florida. Today it dominates vast areas of the Columbia Basin and Great Basin, and it is pervasive throughout Spokane's surrounding grasslands, roadsides, and disturbed forest edges.
What It Looks Like
Cheatgrass germinates in autumn, overwinters as a seedling, and flowers in spring — weeks ahead of most native plants. In late spring, as the seeds ripen, it transitions from green to purple, then to a dry straw color. At this point it becomes a dense mat of fine, highly flammable material. The hairlike "awns" on the seeds stick aggressively to socks, shoelaces, and animal fur — a feature that has helped spread it to every corner of the landscape.
Mature plants typically stand 40–90 cm, with drooping, feathery seed heads. Even a plant as small as 2.5 cm tall can produce seeds. One healthy plant can produce 300 or more seeds, and under optimal conditions a single hectare can yield 450 kg of seed — roughly 330,000 seeds per kilogram.
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Why It's So Dangerous
The fire cycle: Cheatgrass is a fire cycle amplifier. It dries out weeks before native bunchgrasses, creating a continuous mat of fine fuel across the landscape by early summer. Once a fire ignites, it spreads rapidly across cheatgrass-dominated areas. After the fire, cheatgrass is among the first species to recolonize — faster than native perennials — further increasing fuel loads for the next fire. This grass/fire cycle can convert fire-adapted native shrublands into monoculture cheatgrass fields within a few decades.
CO₂ response: Research has shown that cheatgrass grows more biomass as atmospheric CO₂ rises — approximately 1.5–2.7 grams more per plant for every 10 ppm increase above the preindustrial baseline. This means climate change will likely make cheatgrass even more aggressive in coming decades.
Soil disruption: Cheatgrass has a shallow, wide-spreading root system that efficiently absorbs moisture from light precipitation events, driving soil moisture down to the wilting point to a depth of 70 cm. This undercuts native perennial grasses that depend on that same moisture for late-spring growth.
Seed bank persistence: Cheatgrass seeds in bare soil lose viability in 2–5 years, but in dry storage they can remain viable for over 11 years. This makes eradication extremely difficult — you have to outlast the seed bank.
What's Being Done
Management options include targeted herbicide application (imazapic is the most widely used), prescribed burning (fall burns are more effective than spring burns for reducing cheatgrass and increasing native diversity), and seeding native perennial bunchgrasses like bluebunch wheatgrass (Pseudoroegneria spicata) to outcompete the invader. Ecosystems with healthy biological soil crusts — the community of cyanobacteria, mosses, and lichens that live on undisturbed soil surfaces — are naturally more resistant to cheatgrass invasion.
2. Spotted Knapweed (Centaurea stoebe) — The Allelopathic Invader
USDA PLANTS (CEST2) · WA Noxious Weed Control Board
Spotted knapweed, native to eastern Europe, is classified as a Class B noxious weed in Washington State — meaning it's already widespread, but control is required when feasible and especially to protect high-value lands. It is one of the most significant invasive plants in the Pacific Northwest grasslands and forest openings.
What It Looks Like
Spotted knapweed is a biennial or short-lived perennial, typically 30–120 cm tall. The stems are wiry and branched, and the leaves are grayish-green with a rough texture. Flowers are pink-to-purple, thistle-like, blooming June through September. The key identifier is the black-tipped bracts below each flower head — those dark fringe markings give spotted knapweed its name and make it easy to distinguish from native wildflowers.
Why It Spreads So Effectively
Two mechanisms make spotted knapweed a formidable competitor:
Allelopathy: Spotted knapweed releases a compound called catechin through its roots that inhibits the germination and growth of many native grass and forb seedlings. It effectively poisons the soil around it for competitors.
Seed production: A single plant can produce 1,000 or more seeds per year, which remain viable in the soil for many years. Seeds disperse by wind and on animal fur and vehicle tires.
Spotted knapweed is also highly drought-tolerant. It thrives in disturbed roadsides, overgrazed rangelands, and the open forest floors created by logging — exactly the kind of habitat that forms around Spokane's suburban-wildland interface.
Control Options
Biological control has proven highly effective for spotted knapweed at large scales: several root- and seed-feeding insects (Cyphocleonus achates, Larinus minutus, Agapeta zoegena) have been approved for release in Washington and can dramatically suppress populations over time. Contact your local WSU Extension office for guidance on bio-control programs in your area. For smaller infestations, hand-pulling before seed set (wear gloves — knapweed can cause contact dermatitis) combined with native grass seeding provides good results.
3. Dalmatian Toadflax (Linaria dalmatica) — The Snapdragon Invader
USDA PLANTS (LIDA) · WA Noxious Weed Control Board
Dalmatian toadflax is a Class B noxious weed in Washington State, particularly widespread in eastern Washington's dryland areas, roadsides, and disturbed open forests. It's surprisingly attractive — which has made it a persistent garden escapee — with showy yellow flowers resembling a snapdragon, a waxy blue-green leaf, and stems that can reach 1.5 m.
Why It's a Problem
Toadflax spreads through both seed (up to 500,000 seeds per plant per season) and lateral root sprouting, making it very difficult to eradicate once established. It forms dense monocultures that exclude native forbs and grasses in forest openings and meadows. Because it thrives in disturbed soil, trail corridors, logging roads, and fire-reopened areas are particularly vulnerable.
The plant is unpalatable to most native grazing animals and insects — and unlike spotted knapweed, does not have a complete biological control program in place. Early detection and removal before seed set is the most effective approach for small infestations.
How to Help
Report What You Find
Washington State has excellent weed reporting tools:
Washington Noxious Weed Control Board — searchable weed database, county weed board contacts
WA Invasive Species Council — report invasive species online
iNaturalist — your observations become part of a statewide database that researchers and land managers use
Prevent Spread
The most common vector for noxious weed spread? People. Cheatgrass seeds in your boot laces, knapweed seeds on your dog's coat, toadflax in fill dirt from a landscaping project — every movement counts. Before leaving trailheads and work sites, brush off clothing and equipment. Use certified weed-free seed mixes for any restoration or erosion-control work.
Pull Weeds at Work Parties
ConiFriends volunteers regularly combine tree plantings with invasive plant removal — because keeping a young ponderosa pine alive means giving it a fighting chance against the cheatgrass and knapweed competing for the same soil. Check our events calendar for upcoming work parties where you can help directly.
The Big Picture
The native forests of eastern Washington are not static. They are systems shaped over thousands of years by fire, climate, Indigenous land management, and ecological relationships between plants, animals, and soil. Invasive plants disrupt every layer of that system — from the biological soil crusts that anchor grasslands, to the native bunchgrasses that outcompete annual invaders, to the open-canopy conditions that ponderosa pine and western larch need to regenerate.
Restoration isn't just about planting trees. It's about fighting for the whole ecosystem.
Further reading:
WA Noxious Weed Control Board — county-level weed control resources
USDA NRCS Invasive Plants — national and regional resources