Managing Weeds

Hand pulling is most effective on small or scattered infestations and on annual or biennial weeds that do not regrow from root fragments. It is often the right first response when you catch a new patch early.

• Pull weeds before they flower and set seed. Once seeds form, pulling still prevents spread but the plant's reproductive work is already done.
• Remove as much of the root as possible. Weeds with a taproot — like spotted knapweed — can resprout if the crown is left in the ground. Use a hand tool to dig the root out rather than snapping the stem.
• Do not hand pull weeds that spread by root fragments, such as Canada thistle or leafy spurge. Breaking the root stimulates new growth from each fragment left in the soil.
• Bag and dispose of pulled plants — do not leave them on the ground. Even a pulled plant can continue to ripen seeds already formed.
• Wear gloves. Some weeds, including spotted knapweed, contain compounds that can irritate skin. Canada thistle spines are sharp.
• Reseed the pulled area promptly to prevent bare soil from being recolonized.

Mowing can weaken weeds by forcing them to draw on root reserves at critical growth stages, but timing matters greatly and varies by species. Mowing at the wrong time can actually stimulate regrowth or allow a plant to re-flower below the mower blade.

• The most effective window for most weeds is the bolting stage — when the stem is elongating rapidly, just before the plant flowers. Mowing at this point forces the plant to expend stored root energy on regrowth.
• Repeated mowing over multiple seasons can gradually deplete root reserves and reduce a population, but rarely eliminates it on its own.
• Mowing does not work well for weeds that spread by underground rhizomes — such as Canada thistle — because it does nothing to limit root spread and can actually stimulate vegetative growth.
• Do not mow spotted knapweed or houndstongue. These plants adapt by continuing to bloom at shorter and shorter heights with each mowing, producing seeds close to the ground where the mower cannot reach them.
• Weeds that respond poorly to mowing include: yellow toadflax, leafy spurge, meadow hawkweed, orange hawkweed, noble yarrow, and oxeye daisy.
• Clean mowing equipment thoroughly after use in infested areas to prevent seed spread to new locations.

Burning is a targeted tool, not a broad-area solution. Its effectiveness depends on the heat intensity, the maturity of the seeds, and careful application. It should be used sparingly and with caution.

• The most appropriate use of burning is targeting mature seed heads on individual plants using a propane torch — directing the flame at the flower head to destroy seeds before they drop, while minimizing damage to surrounding vegetation.
• Burning off dead material from the previous season can be useful as preparation for an herbicide application in spring — clearing the old growth so the herbicide reaches actively growing new plants.
• Burning destroys seeds on the plant but does not destroy seeds already in the soil. A weed population's seed bank in the soil can persist for years regardless of burning.
• Dry, mature seeds are more heat resistant than green seeds. High-intensity sustained heat is required to reliably destroy them.

Biological control uses insects or other organisms that feed specifically on a target weed to reduce its population over time. It is a long-term maintenance tool, not a quick fix, and works best on large acreage properties as part of a broader integrated management program.

• Biocontrol is generally not practical for small residential properties. It is most useful for large rangeland, timber, or agricultural parcels.
• Results vary by year and by site conditions. Biocontrol will not eradicate a weed — it reduces population pressure over time when combined with other methods.
• The Flathead County Weed District purchases a limited supply of biocontrol agents each year for tansy ragwort, spotted knapweed, and leafy spurge. Landowners can request these agents at no cost — call the office to check availability.
• Using several insect species together, combined with herbicide or other controls, produces the best results.
• The same agents available through the county can also be purchased through local suppliers if county supply is exhausted.

Herbicides are one of the most effective tools available, particularly for established infestations or weeds that cannot be controlled by mechanical methods alone. Choosing the right product, applying it at the right time, and following label instructions closely are all essential to getting results and minimizing unintended impacts.

• Spring application is most effective for most weeds — when plants are actively growing and readily absorbing what they take in through their leaves and roots.
• Fall application is also effective for spotted knapweed, Canada thistle, and oxeye daisy. Around the first hard overnight frost, these plants begin moving energy down to their roots in preparation for dormancy. Herbicide applied at this time travels with that energy deep into the root system, producing excellent results.
• Selective herbicides target only specific weed species and will not harm nearby grasses. These are the appropriate choice for pastures, roadsides, and anywhere vegetation is desired.
• Non-selective herbicides kill any plant they contact. Use these only in areas where all vegetation is to be removed, such as driveways, gravel lots, or fallow cropland.
• A surfactant added to the spray mix helps the herbicide stick to and penetrate waxy or hairy leaf surfaces. This significantly improves results on hawkweeds and toadflax.
• The county offers spray equipment rentals for landowners. See below.
• Staff at the Weed District can advise you on which herbicide to use, how much to apply, application method, and protective equipment. Call before you buy.
• Always read and follow the product label. The label is the law.

Grazing removes weed top growth and, over time, can weaken root systems and prevent seed formation — similar in effect to mowing. However, it must be managed carefully. Overgrazing creates exactly the conditions weeds need to thrive.

• Do not turn livestock out too early in spring. Early grazing damages desirable pasture grasses before they establish competitive growth, leaving bare soil for weeds to fill.
• Never overgraze. Overstocked pastures and compacted soil from too many animals are among the most common causes of persistent noxious weed problems in Flathead County.
• Goats are an exception — they can graze some weeds that are toxic to horses and cattle, and are used effectively on certain problem species. Contact the Weed District for guidance on goat grazing programs.
• Horses can reduce seed production of Canada thistle, but will not reduce a thistle patch because the plant's underground rhizome system expands regardless of grazing pressure.
• Weeds that cannot be effectively controlled by grazing include hawkweeds, houndstongue, and yellow toadflax — due to toxicity concerns or plant structure.
• After grazing in weed-infested areas, hold animals in a clean pen long enough to pass any consumed seeds before moving them to uninfested ground.