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Colorado Master Gardeners: Managing whitetop

Hoary cress, or ‘whitetop,’ one of Routt County’s most aggressive noxious weeds is seen here covering a hillside.
Courtesy Photo





Hoary cress, or ‘whitetop,’ one of Routt County’s most aggressive noxious weeds is seen here covering a hillside.

Editor’s note: This article was originally printed in the Community Agricultural Alliance column June 5, 2014.

Have you noticed 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.

Two years ago, I wrote an article about this nasty actor, but given the unbelievable amount of it I continue to see, it seems necessary to remind everyone why it’s important to control this plant.



Whitetop is a deep-rooted perennial that grows up to 2-feet-high and reproduces from root segments and seed. The leaves are a blue-green or dull, gray-green color and are lance-shaped.

When blooming, the plants have white flowers with four petals arranged in an umbel shape, giving the plant a white, flat-topped appearance. Plants emerge in very early spring and 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.

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 its growth, you’ll have to keep up with it for a few years to gain complete control.

Keeping your property in good shape is the best way to keep whitetop from establishing a foothold. If whitetop does become established, it should be contained as quickly as possible. This can be achieved by managing the outside perimeter to prevent the spread of whitetop.

Pulling whitetop in 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 due to whitetop’s competitive nature. Also, there are no biological controls available at this time. This means that, while sometimes difficult, spraying is 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. Make sure you carefully read the label for all products to figure out if they will interfere with any seeding application you may make in the area and to see how long you need to wait to harvest hay in the area. Also, pay close attention to the application rates. Remember, the label is the law.

In yard situations, such as lawns, make sure to use a herbicide that is labeled specifically for turf. 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. Make sure to spray on a day that is free of breeze, and if you’re worried about overspray hitting desirable plants, place a paper towel or toilet paper roll on top of the whitetop plant and spray the herbicide into it.

Because whitetop season came so quickly this year, we are nearly to the point at which even chemical control will provide unsatisfactory results. That being the case, go out now and cut and gather the tops off the plant to stop seed production and spread.

Make note of the area that has the whitetop by taking photos, marking a map or taking GPS coordinates so you can spray the young plants next year, even before they bloom, increasing your chances of getting good control.

Controlling whitetop is mandatory, by law. The longer we wait to get this weed under control, the more difficult control will be, because of the aggressive nature of the plant.

Todd Hagenbuch is the CSU Extension agriculture agent.


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