Diffuse knapweed

This photo taken near Entiat in 2003 shows diffuse knapweed gone wild.

It’s weed season again. Weeds not only are unsightly in your garden, driveway or pastureland, but when they proliferate, they destroy hundreds of thousands of acres of rangeland and native plant habitat.

An annual weed sets seed and dies and grows the next season from seed — common examples being cheatgrass or goathead.

A perennial weed persists from year to year and re-grows from the same roots — think of white top.

A biennial weed grows from seed the first year and then the second year blooms, produces seed and dies.

Bonnie Orr

Bonnie Orr

Chelan County Master Gardener

Now is when the roseate — the group of leaves at the base of the plant close to the ground — can easily be pulled out or killed. It is important to control the weeds at the early stage before they bloom and set seeds.

Weeds grow so successfully because each plant can produce hundreds or even thousands of seeds. The seeds are spread by the wind, hidden in dog fur, stuck to the bottom of your walking shoes, in the wheels of your off-road vehicle, the tires of your bicycle. It is easy to create new weed infestations in previously clean areas because we help to distribute weeds seeds.

Everyone has heard of the efforts to stop the spread of some of the 400 types of knapweed. One of the most difficult knapweeds to control is diffuse knapweed. This plant is especially insidious. In 1907, it was accidentally introduced mixed into alfalfa seed in Eastern Washington. This plant can grow as a perennial or biennial depending on the soil type and the moisture available. It can be difficult to control — except at this time of year when all the plants have a roseate of leaves that can easily be removed.

Diffuse knapweed, Centaurea diffusa, easily invades roadsides, disturbed land or recently plowed land. A serious concern for our area is that it easily establishes itself in recently burned areas. It thrives in open land with very little water and eliminates competition from other weeds by producing a plant toxin that prevents other weeds from germinating.

This pest is one of the shorter knapweeds, growing only to 2 feet tall, and it can be very bushy. The white or pale pink flowers produce a seed capsule that is “spiny” and rough to the touch. The roseate leaves are a complete stem of finely cut parts. Each plant can produce 12,000 seeds, and the plant is only spread by seed, not plant parts.

Diffuse knapweed is listed as a class B Noxious by the Washington State Noxious Weed Control Board, and homeowners are required by law to control this plant on their property.

A WSU Master Gardeners of Chelan County column appears weekly in The Wenatchee World. Bonnie Orr is one of four columnists featured.