Use herbicides in a more friendly manner

Spring is here. Our wonderful spring moisture followed by warm temperatures brings wildflowers, acres of green grass and weeds! Everyone is ready to get out there and control their weeds. Before charging ahead, slow down, take a moment to assess your weeds and develop a plan to guide your weed control efforts. Do you know what weeds you are dealing with? Do you have any noxious weeds? Do you know what stage of their life cycle your weeds are in?

By Ellen Nelson
Larimer County Weed District

Annual weeds (examples: blue mustard, flixweed and many more) are already flowering and producing next year’s seeds. Their life cycle is almost complete. It is a poor use of time and money to apply herbicides to weeds in their seed production phase. Your best control method at this point is mowing, and adding a reminder to your calendar to assess your weeds earlier next year.

Biennial plants have a two-year life cycle, you have more time to plan and control these weeds, but once they produce seeds, your control methods become more limited. Mow or pull and remove the seed heads.

A perennial plant lives more than two years. Perennials generally have massive root systems. You can sometimes control their seed production by mowing or grazing, but to obtain effective control you may need to use herbicide. Two very prevalent noxious weeds in Larimer County, leafy spurge and Canada thistle, are examples of perennial weeds that are extremely difficult to control.

The Larimer County Weed District conducts trials to determine effective herbicides for these tough, deep-rooted noxious weeds. These herbicides can be purchased from the weed district, but to properly apply these herbicides, you need to know the output of your sprayer in gallons per acre (GPA). Instructions for calibrating your sprayer are available on the weed district’s website (, or in the Weed Management Reference Guide, available at the Weed District office.

It is common for people to spray year after year with 2,4-D in combination with other broad-leaf herbicides. The typical application rate for these herbicides is 1 to 2 quarts per acre. Compare this to rates for the newer, more targeted herbicides of 1 to 7 ounces per acre. If you are going to properly and safely apply herbicides at these lower rates, it is imperative that you know the output of your sprayer. Apply too little, you are wasting your money, apply too much, you will damage desirable plants, be in violation of the herbicide label, which is the law, and be harming the environment.

If you want to use less herbicide, to use herbicides in a more environmentally friendly manner, and to effectively control weeds, you must: Know what weeds you have; know their life-cycle and when it is most effective to treat them; understand how to calibrate your sprayer and calculate the amount of concentrated herbicide to add to your tank.

For assistance identifying weeds, calibrating your sprayer or calculating how much herbicide to use, contact the
weed district at 970-498-5768 or

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