A New Menace for Containerized Plants

A New Menace for Containerized Plants
by Molly Murphy

A new naturalization record promises to add even more burden to the potted plant and landscape industry in Hawaii. Fatoua villosa commonly called hairy crabweed, was found naturalizing near recently planted pikake plants this past fall in Hilo. The potting mix contained copious amounts of viable seeds as well as numerous offspring. New recruits were detected in the vicinity and pulled out on subsequent trips to the Hilo garden. The herbarium at Bishop Museum documented reports of local naturalization in Oahu. Both Foster Botanical Garden and Lyon Arboretum were plagued with this nursery contaminant. It is unclear at this time if eradication was successful on Hawaii Island.  Hairy crabweed is a small, annual forb that is easy to overlook. In fact, it has a history of growing unnoticed only to be discovered after it is thoroughly established.

The winter of 1962/1963 was particularly harsh in Louisiana, many plants died when temperatures dropped to 15 degrees F. Suddenly conspicuous against the barren landscape, the exotic species F. villosa was noticed and documented by Thieret, J.W. Apparently, hairy crabweed had been proliferating, growing but overlooked, around New Orleans for at least fifteen years prior to 1963. Helped along by regular fertilizer and irrigation, hairy crabweed outcompeted the frozen flower beds and shrubs of Southwestern Louisiana, forming mats of green that were impossible to overlook.

Native to Southeast Asia, Fatoua villosa, likely came to Louisiana as a botanical contaminant on imported live plants. It continues to contaminate containerized plants, rapidly spreading from Louisiana to Michigan, North Carolina to California, Washington State to New England and now Puerto Rico to Hawai’i. Hairy crabweed is problematic for irrigated areas, containerized nurseries, horticultural stock, greenhouses and other landscaped areas. Damp, shaded forests and wetland areas are also known to suffer the invasion from hairy crabweed.

Hairy crabweed implements multiple mechanisms for dispersal. Propagules move short distances by ballistically dehiscing up to 4 feet from the mother plant, by wind, and by birds. Like other problematic hitchhikers (ie little fire ants), hairy crabweed travels long distances by being inconspicuous. The minute seed size allows them to taint seed packets, mulch, soil and muddy tools. The small seed size makes both long and short dispersal a breeze.

Hairy crabweed can produce five generations in one year on the US Mainland, reaching reproductive maturity in as little as twelve days or as soon as the emergence of two true leaves. Some joke that hairy crabweed is born pregnant; it is no laughing matter when a self-fertile, monoecious plant produces prolific seeds that remain dormant yet viable while waiting for optimal conditions. The shade tolerant weed colonizes landscaped areas and forms dense carpets that leave little room for more desired plants.

This horticultural invasion is not inevitable! Prevent and sanitize, look at all incoming potted plants, destroy if propagules are found. In the absence of prevention, multiple methods can be deployed to stave off the problem. Cultural control can be achieved by spreading a thick layer of mulch, 3 inches at least, in your irrigated areas or by limiting or stopping irrigation to infested areas. Manual labor by physically pulling the weeds out is effective. Both preemergence and postemergence herbicides are recommended when other methods fail or in combination with other methods.

Hairy crabweed scored high risk on the Hawaii-Pacific Weed Risk Assessment. The key words associated with the assessment are: Annual Herb, Nursery Weed, Shade-Tolerant, Contaminant, Ballistic Dispersal, similar to the keywords for Pilea microphylla.

Terms such as “invasive” and “weed” can be interpreted differently by various people, hairy crabweed is one plant that all of us can call an invasive weed, a highly effective invasive weed that has no place in Hawaii. Let’s join together and nip this problem in the bud before it’s as common as Oxalis or Pilea microphylla in our potted plants. Prevention is preferable, early detection and vigilance is the crucial next step to stop this weed before it spreads.

The Big Island Invasive Species Committee is working with the Hilo garden manager and with the source of contaminated pikake to help prevent further invasion. If you suspect this weed call your local invasive species committee.

Molly Murphy is the Plant Pono Specialist for the Big island Invasive Species Committee

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About the author  ⁄ Garrett Webb

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