Weeds winning? It's time for a new plan

Ellen Nelson
Larimer County Weed District

Northern Larimer County received luxurious amounts of rain in May and June this year. What a gift for our gardens and pastures! The weeds, opportunists that they are, have also responded to the additional moisture.

At this point, you may be wondering if you are destined to be forever locked into the expensive and time-consuming pastime of applying volumes of herbicides to your land, year after year. If you have reached a stalemate with your weeds, perhaps you should reassess your weed management strategy.

Albert Einstein defined insanity as “doing the same thing over and over again and expecting different results.” Maybe it’s time to stop the insanity and try a new approach. A very common weed control program consists of a yearly application of a broad-spectrum, non-specific broadleaf herbicide to every inch of a pasture “whether it needs it or not.”

The desired result is generally described as “I just want anything that’s not grass out of my pasture.” If the same weeds, or an assortment of even more difficult plants to control, continue to assert their right to live in your pasture, maybe your pasture is trying to tell you that a monoculture of grass is not sustainable at this time.


A non-specific broadleaf herbicide is designed to kill broadleaf plants without harming grasses. One significant drawback to this management approach is that the non-specific herbicide may “burn down” but not actually kill the roots of very persistent noxious weeds such as Canada thistle and leafy spurge. Another problem with the annual application of broad-spectrum herbicides is that in addition to affecting undesirable plants, any native forbs present are also eliminated.

While the more problematic noxious weeds are not actually killed, native plants are usually more sensitive and are eliminated. Removing the broadleaf plants from an area creates an unbalanced ecosystem, a disruption of the natural diversity usually found in the landscape. A monoculture of grasses is a less diverse and hence a less stable system that requires continual inputs of energy, chemicals and time to maintain. Your pasture wants to return to a more stable state, so the vacancies for forbs created by the broad-spectrum herbicide will be quickly filled by the most opportunistic plants, usually undesirable invasive and noxious weeds.

The presence of native forbs in your pasture keeps undesirable weeds out, by providing fewer opportunities for non-native plants to move in. The opportunistic weeds that move in to any vacancies created are familiar unwanted guests such as mustards, kochia, cheatgrass, knapweeds, mullein, various thistles and leafy spurge. For this reason, wholesale annual spraying with a non-specific broadleaf herbicide may not move your pasture any closer to your perceived “ideal.”

Is it time to rethink your ideal? Might a more diverse assortment of plants in your pasture be not only more sustainable but also easier and less costly to maintain? The competition provided by a healthy community of native forbs may prevent incursions of invasive plants. Less frequent herbicide use avoids development of herbicide-resistant weeds and unanticipated side effects on non-target plants and insects.

Targeted herbicides

Traditional broad-spectrum herbicides are often applied at rates approaching 2 quarts per acre. The newer, more specific or ”targeted” herbicides are applied at far lower rates, sometimes as low as 1 to 7 ounces per acre. In addition, the newer herbicides are much more effective at actually killing the weeds.

The Weed District has used the newer herbicides on leafy spurge throughout the county and we are hearing from pleased landowners that the spurge has been killed and is not reappearing in the same patches, as it had for so many years. The net effect, if we modify our herbicide application habits to apply smaller amounts of specific herbicides targeted at eliminating a specific problem weed, is that we benefit the environment and ourselves in many ways:

1) Less chemical applied to the landscape year after year.
2) Targeted herbicides should eliminate 80 to 90% of the problem weed the first year. You will need to follow-up for several years to insure that you catch any new plants germinating from the soil seed bank, but that will involve spot-spraying a limited number of plants.
3) Less collateral damage to native broadleaf plants. These plants will occupy the niche left by the weeds you removed, and hopefully your pasture will no longer be posting a “Vacancy Available – Move in Today” sign for any passing broadleaf plant, and therefore be less vulnerable to invasive weeds.
4) Less time spent spraying weeds.
5) Less money spent purchasing herbicides.
Pasture management

The key to this happy ending is always proper pasture management. Over use of pastures causes plant vigor to decline and is an open invitation for invasive plants. Hay from unknown sources brings noxious weed seed. Pasture grasses should be rested after grazing to allow them to replenish carbohydrate stores in their roots and replace leaves removed by grazing.

If you have weeds that you find yourself treating over and over with the same chemical year after year, maybe it’s time to rethink your management plan.

For recommendations on specific herbicide treatments targeted at some of Larimer County’s worst invasive weeds, contact the Larimer County Weed District at 970 498-5768.

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