Weed of the Month

Dalmatian Toadflax

Dalmatian toadflax, Linaria genistifolia spp. dalmatica, was introduced in the 1890’s from southeastern Europe as an ornamental. It is aggressive and may be found in fields, pasture and rangeland, along roadsides, and in natural areas where it may outcompete natives or other desirable species. Toadflax infestation can dramatically reduce forage production. It is scattered throughout the western United States but is most common east of the Cascade Mountains including Washington State. An extensive and deep root system along with a waxy leaves make this an extremely difficult plant to manage.

Dalmatian toadflax is an erect perennial, up to 3 feet tall, reproducing by seed and underground root stalks. It has a large taproot that can penetrate to six feet or more. Lateral roots are produced two to eight inches beneath the soil surface and may extend up to twelve feet in all directions from the parent plant. New plants form at buds along these roots resulting in patches of the plants. A single mature Dalmatian toadflax plant can produce up to 500,000 seeds per year. Seeds germinate in both the fall and spring but can remain dormant in the soil for up to 10 years.

Early spring growth has waxy leaves with a blue-green color. Leaves clasp the stem and are pointed, heart-shaped, and alternate but may appear to be opposite because of their crowded condition. Vegetative stems die back once flower stem production begins in spring. Each plant produces up to 25 flower stems that are somewhat woody at the base and branched at the top. Yellow flowers, ¾ to 1 ½ inches long, with orange, bearded throats and long spurs appear from May to August. Flowers resemble snapdragons and are borne in axils of upper leaves. Irregular, angular seeds are formed in two-celled capsules.

Toadflax is very difficult to manage once it becomes established so prevention is the best method of control. Intensive clean cultivation can effectively control Dalmatian toadflax but requires at least a two year effort. Biological control has been effective. The stem mining weevil, Mecinus janthinus, was first intentionally released as a control agent in Washington in 2000. Adult beetles puncture and feed on the plant’s leaves and stems during early spring. When beetle populations are high, death of the terminal portions of the flower stems prevents flower development with seed formation. Calophasia lunula, a defoliating moth, is well-established in Washington and reportedly provides good control. Brachypterolus pulicarius is more widely found on yellow toadflax but larvae feeding on flower and fruit can reduce seed production by 75% or more.

Commonly used herbicides are picloram (Tordon), picloram + 2,4-D, and dicamba (Banvel or Clarity) although repeated applications of dicamba may be necessary to achieve complete control. Follow pesticide label instructions and precautions.

WEED ARCHIVE

Puncturevine

Purple Loosestrife

Buffalobur