The issue: Tansymustard and flixweed

Tansymustard (Descurainia pinnata) and flixweed (Descurainia sophia) are winter annual mustards that are very similar in appearance. Flixweed is slightly taller (3.5 feet) than tansymustard (2.5 feet). The seed pod of the flixweed is longer (1 – 1.5”) than the tansymustard (0.5”). Tansymustard is native and flixweed is from Eurasia. Both plants are poisonous to cattle when eaten in large quantities.

Otherwise, the seedlings and rosettes are difficult to distinguish. The seeds germinate late-summer through early-spring. Rosette leaves are bi- or tri-pinnately lobed, and the plants bolt and flower May through August. The yellow, four-petaled flowers give rise to a silique seed head that is held mostly upright. A silique is a long pod that is divided in two by an internal membrane.

Both mustards reproduce only by seed. Seeds may remain viable up to ten years in field conditions. Control efforts must focus on eliminating seed production and spread.

Integrated pest management options:

· Prevention: Feed weed-free hay; keep ditch banks free of seed-producing weeds; clean vehicles and equipment.

· Mechanical: Hand dig or cultivate rosettes or bolting plants before seed production; flame weed young seedlings in the fall or early spring.

· Cultural: Provide growing conditions for healthy competition from desirable plants; soil solarization; organic mulch at least 3 inches deep.

· Biological: None.

· Chemical: Pre-emergent herbicides applied in the fall; post-emergent broadleaf herbicides with 2,4-D in late fall or early spring—don’t apply when temperatures will reach above 85 F for three days. Be sure the target weed and crop, or landscape situations are listed on the product label. Always read and follow herbicide label directions.

Combine them:

These control options are all more effective when combined with the other efforts described.

For more information, contact Ron Patterson, University of Idaho Extension horticulture/agriculture educator in Bonneville County, at 208-529-1390 or rpatterson@uidaho.edu.

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