Hoary Cress is a devilish weed with bad intentions

By Bonnie Orr
WSU Chelan/Douglas County Master Gardener
June 15, 2022

Hillside infested with hoary cress noxious weed.
This spread of Hoary Cress began with a single, uncontrolled plant growing in a vacant lot three blocks away. Now the entire side of the street is covered with the noxious weed. – Provided photo/Bonnie Orr

Bonnie Orr
Bonnie Orr – WSU Extension Chelan/Douglas County Master Gardener – photo by Don Seabrook, Wenatchee World

Hoary Cress, also called White Top, Cardaria draba, is another pernicious weed from Europe that probably came in animal feed. It has been found in Washington state since the early 1900s.

The plant is 2 feet tall and the roots can be up to 10 feet deep.

It is common on waste land and disturbed soil — such as adjacent to building sites or fill-soil from an area there.

The plant grows rampantly. It can out-compete nearly everything not only because of its prolific seeds and determined roots but also because the plant produces a chemical that prevents other plants from growing near it. This is not a weed you want in your neighborhood.

Hoary Cress is a perennial — and that is the rub. It is very difficult to eradicate because of its fast-spreading, thick, rhizome roots. The roots can spread from 2 feet to 5 feet per year.

This devilish weed can spread by seed as well. Each plant produces from 1,200 seeds to 5,000 seeds that can lie in the soil for up to three years before sprouting. Seeds are ripe and blown about by midsummer. Seeds can be transported also by hiking boots, car tires, animal fur, and contaminated hay and crop seed.

So how do you prevent your landscape being overrun with Hoary Cress?

Cutting off the plant or attempting to pull it is not effective. The roots reacts to the plant being mowed by putting up additional shoots.

If you attempt to dig it out and leave even a slip of a root, the plant feels it has permission to take off again.

The truth is you must control this plant when you first see the young roseate that sprout in the early spring. It is one of the first plants that appears when the soil begins to warm.

Herbicides are the control. They are applied to the roseate and before the plant blooms. Email the Douglas County Weed Task Force at dwhaley@wsu.edu. or the Chelan County Weed Board at noxious.weeds@co.chelan.wa.us for information for chemical controls. You can visit the Chelan County Weed Board website at wwrld.us/noxweeds.

I know from experience that it may take two to four years to totally get rid of the infestation on your property.

I ordered some top soil, and the next year white top appeared in my flower bed. I cut it off the first year. The second year there was more, so I began the process of digging it. It took another year of digging for me to totally eradicate it. I also worked to out-compete it by planting many perennial grasses and spreading flowering perennials so that there was no light getting to the soil. I succeeded in eradicating it because I attacked it early in the infestation.

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