Glen Buschmann Slightly Wild Garden Service P.O. Box 11464 Olympia, Washington 98508 Weeds of South Puget Sound Prairies by Glen Buschmann Abstract While Scotch Broom may be the most apparent weed threat to the prairies of South Puget Sound, it is far from the only meance. Several non-native forbs and grasses have invaded the prairies, or threaten to do so, in numbers large enough to constitute an ecological threat. This paper looks at five common weeds well established in the prairies of South Puget Sound and at four weeds common elsewhere, but presently limited in distribution in the Puget prairies. Consideration is given to growth habits, tolerances, propagative methods, pests, and other characteristics which both makes these weeds plats of concern and which potentially limits their spread. The paper first discusses two common invasive grasses, colonial bentgrass (Agrostis tenuis), and velvet-grass (Holcus lanatus), and three common invasive forbs, St. Johnswort (Hypericum perforatum), hairy cat’s ear (Hypochaeris radicata), and ox-eye daisy (Chrysanthemum leucanthemum). This is followed with discussion of four threatening wees, (gorse (Ulex europaeus), leafy spurge (Euphorbia esula), spotted knapweed (Centurea maculosa), and mouse-eared hawkweed (Hieracium pilosella), and some near relatives. The paper also looks at government noxious weed programs and how their work can affect prairie conservation, and concludes with a brief look at some issues prairie managers may face. Introduction presented is based on research and interviews, some of the material is This paper is an examination of a few anecdotal in nature, based on my pest plants of the Puget Prairie. It is observations. Many of those undertaken with the belief that a better observations were made during 1995 and understanding of pest plants can assist in 1996 while employed by The Nature their control. This paper looks at both Conservancy as a member of its prairie weed already common to the Puget restoration team. Other observations are Prairies, and at invasive plants common based upon many years experience as a elsewhere which have the capacity to professional gardener. become pests on the Puget Prairies. While weeds often share common The prairies of Puget Sound are a characteristics, and their control may relatively small ecosystem, facing share common solutions, each weed here double threats from invading human is examined independently. Presenting development and invasive plant species. consistent comments about each plant It is generally agreed that the most has proven difficult, and research gaps blatant prairie invaders are Douglas-fir, are present. While much of the material (Psuedotsuga menziesii), a very large Prairie Weeds G. Buschmann 163 native conifer historically excluded from widespread, but is uncontrolled and Puget prairies by grass fires, and Scotch looks to be rapidly increasing. broom (Cytisus scoparius) and similar European relatives (C. striatus, C. Colonial Bentgrass- Argostis tenuis mospessulanus). But Douglas-fir and Sibth. Scotch broom are only the two most dramatically observable weeds of the One of the most outstanding nuisances to prairie. This paper examines some of the the prairie is Agrostis tenuis, commonly other prairie weeds whose presence pose known as Colonial bentgrass. The significant – possibly even greater – common and scientific names are both ecological threats to the Puget prairies. descriptive of this plant’s nature. “Tenuis” shares the same root as First of all, what is a prairie weed? In “tenacious”, a very apt description, and native landscapes a weed is a non-native Colonial refers to the (large) colonies it plant which is able to thrive in a foreign forms as it grows. Pojar and MacKinnon ecosystem. In this paper attention is (1994) contend that A. capillaries is the given to those weeds which either proper name for this weed, but that name overwhelm native plants and /or animals is ignored by Hitchcock and Cronquist of the existing prairie landscape, or (1973) and rejected by Hortus III (1976). appear to have that potential. The weeds These disagreements greatly confuse the discussed in this paper are but a few of research. The thin leaves and relatively the many weeds which grow in the short size of bentgraass cloaks its prairies. Most of the prairie weeds grow aggressive nature. Bentgrass is a sod comfortably alongside their established forming grass. In a bunch-grass prairie native cousins in a tame and unobtrusive such as southern Puget Prairies, sod manner: they will not be discussed in excludes many plants which depend on this paper. open soil for reproduction and survival, making bentgrass a very unwelcome Five Well-Established Invading pest. Weeds Also present on the Puget prairies is The five plant species singled out in this Agrostis diegoensis, a native Agrostis. section of this paper are among the most Similar in appearance to and easy to significant of the many weeds found on confuse with Colonial bentgrass, A. the Puget prairies. The two grasses, diegoensis is less competitive, and tends colonial bentgrass (Agrostis tenuis) and to grow in distinct and contained velvet grass (Holcus lanatus), are both patches. On Puget prairies, with practice very invasive but have distinctly the two species can be readily different growth habits. Of the three distinguished in the field, even at a forbs, hairy cat’s ear (Hypochaeris distance. A good field guide and a good radicata) is already very wide-spread, hand lens will aid in differentiating the and uncontrolled; St. Johnswort two. (Hypericum perforatum) is also very widespread, but is contained with Bentgrass’s running root system is biological controls; and ox-eye daisy supplemented by its prodigious (Chrysanthemum leucanthemum) is less production of small, light-weight seed Prairie Weeds G. Buschmann 164 which can be wind-blown considerable stunt bentgrass; conclusions on this distances. The seed of bentgrass appears dilemma are left to the reader. to have an extended ripening season which continues into late summer. Some Velvet Grass- Holcus lanatus L. seed germinates almost immediately with the fall rains, while other seed does Holcus lanatus, (velvet grass, or in its not germinate until the following spring, native England, Yorkshire fog), is a or even later. My research did not turn large, coarse, easily recognized perennial up any information on long-term seed grass, with bulky seeds, a loosely viability, although grass seed in general clumping habit, and a large fast growing is short-lived. fibrous root system. The seed is light enough to disperse on the wind and also An effective means of controlling Scotch spreads by adhering to fur and clothing. broom may encourage the spread of Velvet grass is generally a grass of poor bentgrass when bentgrass is producing and infertile soil, and tolerant of widely seed. When chopping down Scotch ranging environmental conditions. It has broom with a tractor and heavy duty a remarkable ability to tolerate salts and mowing deck, at Fort Lewis the first heavy metals and to alter its root system week of October 1996, I observed that a in the presence or absence of water, thick layer of grass chaff and seed had nutrients, salts and metals (Thompson accumulated on the mower deck at the and Turkington 1988; Pilcher and Russo end of the day. In all likelihood seed 1988). It is a vigorous competitor and its tumbled onto and was broadcast from ability to absorb nutrients in essence the mower deck all day. Roads through robs other weaker plants. Furthermore, the prairies appear to have denser stands in lab studies Newman and Rovira of bentgrass, presumably because of (1975) found that velvet grass increased seed movement. In October alleopathically inhibits competition. 1995 at Thurston County’s Glacial While velvet grass does spread by Heritage Park a wide swath of dense vegetative means, it mostly spreads by Scotch broom was mowed, including an seed, which it produces prodigiously. old railway bed. The following spring Thompson and Turkington (1988) write the old rail bed showed up distinctly in that Watt (1976) found that plants grown the cleared prairie, a thriving straight from seed sown the previous spring line of bentgrass. produced as many as 240,000 seed for each plant. Some herbicides will damage bentgrass, as is recurrently mentioned in manuals While velvet grass is an inarguably on lawn management and confirmed by important graminoid pest of Puget Richard Robohm (1996) in his recent prairies, its roles as an invader in studies on Idaho fescue. However, it is undisturbed prairie remains unclear. unclear as to how much herbicide would Based on two years of undocumented be needed to successfully and observations made while undertaking significantly reduce bentgrass over the prairie restoration at Fort Lewis, velvet long-term. Ho (1964, in Aarssen 1981) grass seems to grow most strongly in found Hypochaeris radicata (hairy cat’s prairie soils altered by Scotch broom and ear) to have alleopathic qualities which Douglas-fir infestations. It does not Prairie Weeds G. Buschmann 165 appear to spread as easily through “ …reduce[ed] the relative abundance of undisturbed prairie as does bentgrass. H. lanatus in pastures” (Thompson and Studies on Scotch broom and associated Turkington, 1988). In barren restoration plants found that bentgrass was a sites potentially infested with velvet common invader everywhere and velvet grass, it may be possible to manipulate grass quite uncommon. In broom condition and stimulate germination, and infested prairies the likelihood of finding then treat the germinated area with velvet grass increased, but not herbicide or heat treatments such as significantly (Parker, unpub. data). flaming or steam. While these techniques may have been tried on Regardless of the role velvet grass plays velvet grass, I did not uncover any in disrupting pristine prairies, the published research. Velvet grass has a disruptive role it plays in restoration number of biological pests and merits equal or greater concern.