- Invasive Species Management
- Current Topics
Noxious Weed Treatments
We receive many calls concerning weed treatment in July and August as the plants begin to go to seed. Knowing the correct methods and timing for treatments will save you time, energy, and resources.
Biennial Noxious Weeds
- Thistles (Musk, Scotch, and Bull thistle)
- Diffuse and Spotted Knapweed
- Common Mullein
Unfortunately, by the time biennial noxious weeds are seeding, it is too late to treat with herbicide. The only option is to cut the seedheads, place them into a plastic bag, secure the top of the bag, and place the bag into the trash. Sever the root a couple inches below the soil surface. If the plant has no buds, it can be left on the ground but if it has any buds, it needs to be bagged and put into the trash too.
Herbicide treatment of biennial noxious weeds needs to be focused towards fall rosettes and to spring rosettes and young plants (before they reach 12 inches tall). Once the flower bud begins to form, it will produce viable seed that are not affected by the herbicide.
Plants of biennial weeds die after they have flowered so treating flowering plants with herbicide is not providing any benefit. Some folks think they need to spray the root crown but this is not necessary if you get the first couple inches below the ground.
Perennial Noxious Weeds
If you chose to remove perennial weeds, you need to understand how the plant spreads. Is it by seed? Or by spreading roots that re-sprout when severed? Or by plant fragments?
Timing for treating perennial noxious weeds is specific to the weed and to the herbicide you choose.
Shallow fibrous rooted species
- Oxeye daisy
Shallow fibrous rooted plants may be removed. Pull the plant or cut the seedheads, place them into a plastic bag, secure the top of the bag, and place the bag into the trash. If the plant has no buds, it can be left on the ground but if it has any buds, it needs to be bagged and put into the trash too.
Species with spreading roots
- Canada thistle
- Dalmatian and Yellow toadflax
- Leafy spurge
Species that spread by rhizomes and spreading roots should not be pulled. They can be clipped and any plants with flower buds will need to be bagged and put into the trash. Plants without buds can be mowed. This won't control the plants but will make them more susceptible to fall herbicide applications.
Time spring herbicide treatments to before flowering to early flowering. Fall treatments should be timed to later in the season as the plants are moving resources to their roots.
Good results using Milestone on Canada thistle in late summer has also been reported.
Always refer to the herbicide label for specific instructions.
Species that spread by fragments
- Purple loosestrife
For those weeds that spread by plant fragments, all the parts cut from the plant need to be bagged and put into the trash. Any fragments left on the ground will form new plants.
The remaining plant needs to be treated with an appropriate herbicide.
Poison hemlock (Conium maculatum) is a very common plant in North America. In Colorado, it is a List C weed which means it is recommended for control but is not required to be controlled. Due to the increased moisture, populations are higher than normal this year.
If you have it on your property, it is best to control it when it is small. Cutting or removing before it flowers in early summer works well but may need to be repeated. If you are dealing with it once it flowers, you should clip and bag the flowers and dispose of them in the trash. Be sure to wear gloves, long sleeves, and eye protection when handling.
Treating with herbicide at the rosette stage will control it. Visit our Jeffco ISM Poison Hemlock Information page for more information.
University of California IPM - Poison Hemlock
Purdue Extension Poison Hemlock Fact Sheet (PDF)