Have you noticed that the hillsides in Routt County have turned white again? No, Old Man Winter has not returned. What you’re seeing is the bloom of hoary cress — or “whitetop” — one of Routt County’s most aggressive noxious weeds.

I’ve written several times about this nasty actor, but given the unbelievable amount of it I continue to see, it seems necessary to remind everyone why it’s so important to control this plant.

Whitetop is a deep-rooted perennial that grows as much as two feet tall and reproduces from root segments and seed. The leaves are a blue-green or dull, gray-green color and are lance-shaped.

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When blooming, the plants have white flowers with four petals arranged in an umbrel shape, giving the plant a white, flat-topped appearance. Plants emerge, bloom, and set seed quickly, as evidenced this year.

A perennial weed, whitetop is common on alkaline, disturbed soils and is highly competitive with other species once it becomes established. It often is confused with field pennycress, or fanweed, which is an annual and has a strong odor. Whitetop has little to no smell.

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Each whitetop plant can produce as many as 850 seeds per stem and 4,800 seeds per plant. Seeds can remain viable in the soil for as long as three years, meaning that once you start to control, you’ll have to keep up with it for a few years in order to get total control.

Keeping your property in good shape is the best way to keep whitetop from getting a foothold on your property. Keep good plants growing on soil and do not introduce soil from other locations as it is often filled with seed.

If whitetop does become established, it should be contained as soon as possible. This can be achieved by managing the outside perimeter to prevent the spread of whitetop.

Pulling whitetop in the early spring is effective only on seedling plants. If the plants are established, mechanical means are not effective. In fact, many people unwittingly cause more harm than good by tilling or mowing whitetop. These activities actually stimulate the rhizomes and cause more plants to grow.

Cultural controls are limited because of whitetop’s competitive nature. Also, there are no biological controls available at this time. This means that, while sometimes difficult, spraying is really the only option when it comes to controlling whitetop.

Sprays recommended to control whitetop in rangeland situations include 2, 4-D, chlorsulfuron, and metsulfuron methyl. Pay close attention to the application rates and remember, the label is the law.

In yard situations, such as lawns, make sure to use an herbicide that is labeled specifically for turf. Combination herbicides that contain 2, 4-D, Dicamba, and MCPA or MCPP are most effective. In a garden situation, the safest spray to use would be one containing glyphosate. Make sure when using glyphosate in these situations that you are targeting individual plants of whitetop, because glyphosate will kill anything it is sprayed on.

Because whitetop season came on so quickly this year, we are nearly to the point where even chemical control will provide unsatisfactory results in areas where the blooms are beginning to fade.  That being the case, go out NOW, and cut and gather the tops off the plant to stop seed production and spread. 

To learn more about whitetop and other local weeds, including how to identify and control them, join the Routt County Master Gardeners and the Routt County Weed Program this Sunday, June 11th, from 8 a.m. to noon for Routt County Noxious Weed Days, held at 2300 County Shop Road in Steamboat Springs. Premixed herbicide designed to spray whitetop, houndstongue, and other prevalent noxious weeds will be available on a first-come, first-served basis, providing you bring a hand-held or backpack sprayer. Call the weed program at 970-870-2246 for more information.

Todd Hagenbuch is the CSU Extension agriculture agent.