Troublemaker 2: orange hawkweed

Posted: Friday, May 09, 2008

Yes, yes, I know. It is such a gorgeous color - that bright red-orange against the dark hairy stem - and a rare hue in our environs. But, think carefully before considering the introduction of orange hawkweed (Hieracium aurantiacum) into your garden or elsewhere. You have probably noticed them along Egan Drive north of downtown and sprinkled out in the undisturbed wetland near the airport.

Courtesy Of Leslie Kerr
Courtesy Of Leslie Kerr

Hawkweeds are in the chicory tribe of the sunflower family and are closely related to dandelions, sowthistles, prickly lettuce, and chicory. There are 11 highly invasive species, including the orange one, that have been introduced into North America from Europe. Orange hawkweed was first reported in Washington in 1941 and has since spread rapidly into much of the Northwest.

Alaska has four native hawkweeds, but none of those have orange flowers. Unfortunately, none of the six other non-native hawkweeds also in Alaska have orange flowers, either. Distinguishing natives from non-natives is another story. For more information contact the author, USFS State and Private Forestry, or Cooperative Extension.

Invasive plants exhibit many strategies for getting a head start on, and out-competing native species. Among these are: early season sprouting and/or rapid growth that out- compete natives for water, nutrients, and light; prolific seed production: efficient seed dispersal, often by diverse methods; long-lived seed bank: deep penetrating taproots or creeping rhizomes (underground stems) that re-sprout if not completely removed; above ground stolons (like strawberries) or drooping canes (like blackberries) that root where they touch the ground; the ability to secrete chemicals that inhibit the growth of roots and important fungal connections of other plants (allelopathic). Disturbance is often a key factor in facilitating infestation by invasive plants.

Orange hawkweed and many of its relatives possess a variety of these strategies; actually, just about all of them. This plant has an AKNHP invasiveness ranking of 79. It spreads primarily by rhizomes and 4-12 stolons per plant, and will eventually exclude all other plants by covering the ground with rosettes of leaves. It may have from 5-35 flower heads with 12-30 seeds each, potentially more than 1,000 seeds per plant. Seeds are dispersed by wind, animals, in hay, mud in tires or boots, and on clothing, and can germinate immediately or lie dormant in the soil (seed bank) for up to seven years.

These plants don't necessarily need pollination to produce viable seeds. It is widely believed orange hawkweed is allelopathic. Orange hawkweed doesn't much care whether the site is rich or poor, and does well in low-productivity soils. It is usually seen along roads, riparian areas and beaches on well-drained, coarse textured soils. Scariest of all, it can establish itself in undisturbed areas, like remote mountain meadows and forest habitats. Camp Island in the Kodiak National Wildlife Refuge is a case in point.

As disheartening as all this may seem, there may be some weak points that could be exploited by a combination of management practices. Controlling small patches early, and working from the edges of the infestation inward, helps prevent expansion. Studies in British Columbia revealed that most orange hawkweed populations are in sites low or deficient in sulphur. By fertilizing light infestations with ammonium sulphate, grasses and forbs are encouraged to out-compete the hawkweed.

Fertilizing heavy infestations in sparse grass and forb cover is not effective. Careful pulling, being sure to get as much of the rhizome and stolons as possible, along with fertilization, will likely control the beast. Any divots should be replaced, and bare ground should be seeded with grass or other desired plants. Keep an eye out for new plants that will keep appearing until all seeds and roots are exhausted. They hide well among the other plants.

Flower heads should be removed and bagged, especially before they go to seed. If pulled and left at the site, the heads will continue to mature, adding seeds to the seed bank. Don't count on mowing to solve the problem. That misses the low-growing leaf rosettes and just encourages more growth and faster flowering on shorter stems. Growth-regulating herbicides are effective, especially when applied to the rosette stage.

Remember the label is the law.

If you are still hankering to have that luscious dark orange color in your garden, try the orange varieties of dwarf strawflower, Helichrysum bracteatum. Contact the Cooperative Extension and your local nurseries for more suggestions on treatment and alternatives.

• Not in My Backyard is a monthly feature about invasive species created by U.S. Forest Service botanist Ellen Anderson. She can be reached at ellenanderson@fs.fed.us.

An invasive species is one that is not native to an ecosystem, and is likely to cause harm to human health, the economy, or the environment. This column will address plant and nonplant invasives. AKNHP Weed Ranking Project: akweeds.uaa.alaska.edu/akweeds_ranking_page.htm.

Contributors to this column will include Juneau Invasive Plant Action, Juneau Watershed Partnership, Forest Service State & Private Forestry and the National Marine Fisheries Service. Others may be added in future columns.



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