- Invasive Species Management
- Current Topics
Invasive Species Highlights
Orange hawkweed is a List A noxious weed that requires complete elimination. This perennial member of the sunflower family (Asteraceae) grows in large mats of basal hairy deep green leaves. It prefers shady meadows and can be found in certain areas in the foothills of Jeffco.
We are asking landowners to inspect their land and if found, that they control the Orange hawkweed before August 1.
Invasive Species Highlights
With the heavier than normal moisture this spring, there have been more sightings of Hemlock (Conium maculatum). Hemlock can be poisonous, and in rare cases deadly if humans or livestock eat it. Studies have shown that it takes approximately 2.5 to 5 lb. of material per 1000 lb. animal for poisoning to occur which will sometimes result in death.
Some people with sensitive skin may develop dermatitis from contact. Hemlock is not normally poisonous if you just come into contact with it. However, it is always recommended that you wear gloves and a long-sleeved shirt if you are handling. You should also avoid contact with cuts, and mucus membranes (your eyes and mouth).
The best time to control Hemlock is in the first year or early second year when plants are young and before they flower. Either dig out the plants including the top few inches of the taproot or treat with herbicide before it flowers in the spring or to rosettes in the fall. If you are removing older plants, make sure you bag them and put them into the trash. Never place Hemlock in a compost pile or feed to livestock.
Our factsheet provides identification and control information.
Karakasi MV, Tologkos S, Papadatou V, Raikos N, Lambropoulou M, Pavlidis P. Conium maculatum intoxication: Literature review and case report on hemlock poisoning. Forensic Sci Rev. 2019 Jan;31(1):23-36. PMID: 30594904
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.