Noxious Weeds as Wildfire Fuel: What Matters on Montana Land

Which weeds count as noxious in Montana?

The Montana Noxious Weed List, effective June 21, 2019, sorts plants by priority. Priority 1A weeds are not present or have very limited presence, and include yellow starthistle, dyer's woad and medusahead. Priority 1B weeds have limited presence, such as rush skeletonweed and blueweed. Priority 2A weeds are common in isolated areas, including orange hawkweed and ventenata. Priority 2B weeds are abundant and widespread, and include spotted knapweed, leafy spurge, Canada thistle, Dalmatian toadflax, St. Johnswort, sulfur cinquefoil, oxeye daisy and houndstongue.

Priority 3 plants, including cheatgrass and Russian olive, are regulated but are expressly not listed noxious weeds. Counties add their own priorities. Ravalli County's list names blueweed, common bugloss, orange hawkweed, salt cedar, Japanese knotweed, purple loosestrife, rush skeletonweed and yellow flag iris. Our Ravalli County noxious weeds guide goes through those species, so this page stays on the fire side of the subject.

Are noxious weeds actually a wildfire fuel?

Some are, to very different degrees, and it is worth being accurate about it. The fuel most clearly tied to fire behavior is the annual grass group. The Forest Service brief on ventenata says it dries earlier than associated perennial grasses, stays highly flammable through the fire season, and increases fine fuel load and continuity by establishing in the spaces between shrubs and bunchgrasses. In some invaded scablands, fine fuel loads were about 50 times higher than in uninvaded communities. Its dry stalks and litter also decay more slowly than those of most associated grasses, so the fuel stays around.

Broadleaf weeds are a weaker story. A property thick with knapweed is not necessarily a bigger fire risk than the grass it replaced, and with some species there is little fire research at all. We did not find sources tying most Priority 2B species to fire behavior, so we are not going to claim it. Cheatgrass, which is regulated rather than listed, has its own guide: cheatgrass and dry grass.

Does spotted knapweed burn?

Yes, once it dries, but less readily than grass. The Forest Service's 2021 species review states that spotted knapweed does not carry fire as readily as grasses in invaded communities. Green plants are too moist to carry fire, but dried plants can provide fuel. Measurements from Missoula in 1986, reported in the review, show moisture falling from 235 percent of dry weight on July 1 to 30 percent on August 27, which shows how much a stand changes across the summer.

The review also records that intense fires in spotted knapweed stands in western Montana grasslands have been observed under some conditions, and it recommends grass fuel models over knapweed models where associated grass cover exceeds 40 percent. It says dense populations may change the fuel characteristics of a site formerly dominated by grasses, and that how this affects fire regimes is unclear. The fair summary for a landowner is that a late-summer knapweed patch can burn, and the grass mixed into it usually matters more.

Why do weeds move in after a fire?

Spotted knapweed is well built for it. The Forest Service review says plants sprout after fire and that low-severity fire leaving the root crown intact is unlikely to kill them, although severe fire can. Fire usually does not kill soil-stored seed, and it can create conditions favorable to seedlings. The review says the species grows in nearly every habitat type west of the Continental Divide in Montana, and that on National Forests in western Montana it was most common in burned areas in Douglas-fir and ponderosa pine habitat types, the same forest types found around Missoula and the Bitterroot.

So a fire that passes through a Bitterroot hillside may leave more knapweed behind than it found if no one follows up. Plan for that in any recovery work. Our guides on replanting after wildfire and erosion after a fire cover the rest of what to do on burned ground.

Can thinning and fuel work spread weeds?

It can, and it is the main reason to plan weed control and fuel reduction together. The Forest Service review reports a western Montana Douglas-fir and ponderosa pine study in which spotted knapweed cover increased after both high and low fuel consumption spring prescribed fires following shelterwood cuts: from 0.3 percent on the unburned control to 5.9 percent at the low-consumption site, and from 1.9 to 14.1 percent at the high-consumption site, across four postfire years. It also reports a lodgepole pine study where knapweed was rare along new roads and did not spread into burned or thinned units in the first 9 years, which the authors suggest may reflect its low overall abundance.

The takeaway is that existing weed pressure matters a great deal. Thinning next to a knapweed patch is a different project from thinning in clean ground. PNW 618, the Pacific Northwest Extension fuel reduction guide, also warns that bare mineral soil creates conditions for invasive weeds and suggests rock mulch over landscape fabric where a firebreak protects a structure. Keep the soil disturbance low, treat existing weed patches before the work, and look again the next spring and the one after.

What does Montana law require of landowners?

State law takes weeds seriously, whatever their fire behavior. MCA 7-22-2115 declares noxious weeds and their seed a common nuisance. MCA 7-22-2116 makes it unlawful to permit any noxious weed to propagate or go to seed on your land, except that a person who follows the weed management program of their district, or who is in compliance with a noxious weed management agreement, is considered to comply. The same section requires an owner offering property for sale to notify the agent and purchaser of weed infestations and of any program or agreement in place.

Enforcement runs through the county. Under the same part of the code, if a complaint is made or a district weed board has reason to believe noxious weeds are present, it notifies the landowner by certified mail and the process moves through inspection and compliance. This is general information, not legal advice; ask your county weed coordinator what your district's program requires. Fire mitigation does not suspend any of this, so a thinning job that leaves a knapweed patch going to seed is still your obligation.

Who do you call about weeds on your land?

Your county weed district. MSU Extension Forestry notes that each Montana county is statutorily mandated to have a weed district, and that each has a coordinator in charge of helping citizens manage noxious weeds. DNRC's west-central Montana landowner guide directs people to their local weed district for treatment recommendations and to the Montana Weed Control Association, (406) 925-0708 or mtweed.org, for education and a district directory. The association itself does not give identification or treatment recommendations.

DNRC lists the tools that are usually combined: herbicides, biological control using insects, fungus, or sheep and goats, mowing and cultivation, hand-pulling and digging, revegetation after eradication, and prevention. It also stresses working with neighbors, because invasive species ignore property lines. Your weed coordinator can tell you whether any landowner grant programs are currently available.

How should weed control fit into a fire mitigation plan?

Walk the property before any cutting and map which weeds are there and where. Treat before the crew arrives where you can, so equipment is not working through weeds in seed. Time the fuel work outside seed set where possible; fall and winter projects help here, as covered in why fall and winter suit fuel reduction. Ask any contractor, including us, how equipment will be kept clean between sites, and put the answer in the scope.

Then follow up. Reseed disturbed areas with something competitive, spot-treat the following spring, and add it to your defensible space maintenance schedule. Wood chips left from the thinning are not a weed program, and they should stay away from the first few feet of a building, as our mulch guide explains.

How can HJ help?

We are licensed for fertilizer, weed and pesticide application and treat noxious weeds such as knapweed, thistle and leafy spurge on lawns and larger properties, by the label. See our fertilizer and weed control page. Because we also do the fuel reduction, one crew can plan both: our fire mitigation work on the trees and ladder fuels, then weed treatment and reseeding on the ground it opens up.

Parcels where you need a formal weed management agreement or a grazing plan are the weed district's and a range specialist's territory, and we will say so. The rest of this series is in the Montana Wildfire Guide.

Call HJ Property Care & Tree Service at (406) 215-7319 or request a free estimate and we will walk the property with you.

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