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Sweeping up Scotch Broom

June 8, 2012

The management of invasive species is always a balancing act between what is possible and what is practical.  For instance, it is technically possible for a state or country to perform one comprehensive inch-by-inch sweep of all infested areas and exterminate each individual pest of a particular species (easier when the pest is as large as a shrub), but it is not necessarily practical.  In the case of the invasive shrub Scotch Broom (Cytisus scoparius), even this method borders on the impossible when one takes into consideration the fact that Scotch Broom seeds can be dormant for more than half a century (Bossard and Rejmánek 1994).  For this reason and others, C. scoparius offers itself as an excellent case study for the balancing act that is invasive species management.

One of Many: a Scotch Broom flower. Photo credit: pfly, Flickr Creative Commons

As a recap of previous posts, the following is a brief list of facts about C. scoparius that should provide some background.  If you want to learn a little more about Scotch Broom, click here or here or on any of the linked resources found in the references section.

  • As the name implies, Scotch Broom originates from Scotland.  Many forms of broom are found from the United Kingdom to mainland Europe and the Mediterranean countries.  Partly because of this distance (both in space and in evolutionary time), Scotch Broom faces little predation throughout most of its new range at any stage of its multi-year life (Hoshovsky 1986; Bossard and Rejmánek 1994).
  • Its vegetative growth rate is tremendous, enabling it to bounce back quickly from virtually any disturbance (Waloff 1968).
  • Scotch Broom grows very well in disturbed soils, such as along ditches, construction sites, edges of human habitation, and logging areas (Leblanc 2001).
  • C. scoparius is a legume, which means that it can fix its own nitrogen and thrive in nutrient-poor soils (Leblanc 2001).

These traits make Scotch Broom a formidable opponent of pest managers.  It has many supports, so any attempt to knock out just one of them will not necessarily loosen its grasp.  Additionally, some of these traits cannot be altered for the population at all, such as its ability to fix its own nitrogen.  However, C. scoparius is not unassailable.  Integrated pest management (IPM) is a strategy the focuses on using a combination of techniques such as biological control, pesticide use, mechanical control, and fire control, and it can be an effective approach to managing an invasive species like Scotch Broom (Leblanc 2001).

Map of Scotch Broom in North America. Photo Credit: USDA

Biological control seems like a very attractive option, as it can be more pervasive, long lasting, and occasionally less expensive than constant human intervention.  Scotch Broom is known to be very intolerant of shading, and so by promoting the growth of other plant species in infested or likely-to-be-infested areas, Scotch Broom is suppressed (Harrington 2011).  Another form of biological control is to introduce and encourage an invasive species’ native predators, but this can have mixed results.  It’s possible that the introduced “solution” could be a bigger problem than “the problem.”  For another, it could be ineffective, as the U.S. government discovered when it introduced a seed weevil, Apion fuscirostre, to California in 1964 to control Scotch Broom (Parker 1997).

When using pesticides, it is important that they be used as judiciously and narrowly as possible in order to lower the danger of killing non-target species or harming the ecology of the area.  After many trials, several pesticides have been found that can control Scotch Broom in its early stages or after mechanical controls and that can be applied in spot-treatment fashion (Petersen and Prasad 1998).  Mechanical control (cutting with hand or power tools) is always expensive and labor-intensive, and is particularly tricky with C. scoparius, as it should be done during the dry season so as to avoid prolific regrowth (Leblanc 2001).

The adage “Like burning down a house to kill a flea” should be in the forefront of the pest manager’s mind when it comes to using fire control.  A management plan that includes fire must consider the ecology of the infested region, the extent of the infestation, and the potential for property damage and life endangerment.  As well, fires can have mixed results with Scotch Broom: cooler fires can encourage seed germination, while fires hot enough to kill large numbers of dormant seeds pose a danger to biological controls (Leblanc 2001).

The Burninating. Photo credit: H Dragon, Flickr Creative Commons

While there are other means of pest control, these are the best researched and well-funded that the author is aware of.  Hopefully they paint a picture not only of the options that are available to pest managers, but also of the costs and benefits of each of those options.  And, of course, it helps to remember how C. scoparius was introduced to the Americas: as an ornamental shrub.  The best pest management is rooted in preventing invasions in the first place, and every gardener or pet owner has a share in the responsibility of good environmental stewardship.

Insert Theme To “Jaws” Here. Photo credit: brxo, Flickr Creative Commons

References

Bossard, C.C. and M. Rejmánek. 1994. Herbivory, growth, seed production, and resprouting of an exotic invasive shrub, Cytisus scoparius. Biological Conservation. 67:193-200.

Harrington, T.B. 2011.  Quantifying competitive ability of perennial grasses to inhibit scotch broom. United States Department of Agriculture, Forest Service.  Research paper PNW-RP-587.

Hoshovsky, M. 1986. Element Stewardship Abstract for Cytisus scopairus and Genista monspessulanus. The Nature Conservancy. Arlington, VA, USA.

Leblanc, J.W. Getting a handle on broom: Scotch, French, Spanish, and Portuguese brooms in California.  University of California Agriculture and Natural Resources, Publication 8049, 2001.

Parker, I.M. 1997. Pollinator limitation of Cytisus scoparius (scotch broom), an invasive exotic shrub. Ecology. 78(5): 1457-1470.

Peterson, D.J. and R. Prasad. 1998. The biology of Canadian weeds, 109, Cytisus scoparius (L.) Link. Canadian Journal of Plant Science. 78: 497-504.

Waloff, N. 1968. Studies on the insect fauna on Scotch broom, Sarothamnus scoparuis. Advances in Ecological Research. 5: 88-208.

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One Comment leave one →
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