Dittrichia Graveolens) Is a Mediterranean Native That Has Become a Weed in Areas of Europe As Well As in Joseph Ditomaso Australia
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NEW PESTS AND DISEASES ▼ Stinkwort is rapidly expanding its range in California by Rachel Brownsey, Guy B. Kyser and Joseph M. DiTomaso Stinkwort (Dittrichia graveolens) is a Mediterranean native that has become a weed in areas of Europe as well as in Joseph DiTomaso Australia. This strongly aromatic weed was first reported in California in 1984 in Santa Clara County, and it had spread to 36 of the 58 California counties by 2012. Stinkwort is not palatable to animals, and can be poisonous to livestock and cause contact allergic dermatitis in hu- mans. In California, this weed is found primarily along roadsides. However, the biology of this annual plant suggests that it could also invade open riparian areas and overgrazed rangelands. Stink- wort has an unusual life cycle among annual plants: Unlike most summer or late-season winter annuals, stinkwort flowers and produces seeds from Sep- tember to December. Such basic biologi- Stinkwort is related to fleabanes and goldenasters and grows to about 2.5 feet tall. In California, this rapidly invading weed most often occurs in disturbed and wasteland sites. cal information is critical to developing timely and effective control strategies for this rapidly expanding weed. resemble Russian-thistle (Salsola tragus a variety of soil types and survives under L.), also called tumbleweed. Because it a range of soil conditions, temperatures is fairly unattractive and nondescript in and precipitation regimes (Preston 1997). appearance, stinkwort initially passed When adequate moisture is available, ittrichia graveolens (L.) Grueter, com- unnoticed by many botanists and weed stinkwort can even survive on serpentine Dmonly known as stinkwort, is a managers, and it was not included in the or saline soils. In Europe, this plant was member of the Asteraceae, or sunflower, 1993 edition of The Jepson Manual of Cali- shown to tolerate and to possibly hyper- family. This plant is native to the Mediter- fornia flora (Hickman 1993). accumulate heavy metals, including ranean region of Europe, occurring as far In its native range and some intro- mercury, zinc and copper (Higueras et al. east as Turkey, Afghanistan and Pakistan duced regions, stinkwort inhabits ripar- 2003; Shallari et al. 1998). (Brullo and de Marco 2000; Qaiser and ian woodlands, margins of tidal marshes, Worldwide invasion Abid 2005). Stinkwort is an erect, fall- vernal pools and alluvial floodplains, al- flowering annual that can grow about 2.5 though it has not yet invaded these wild- While stinkwort is native to the feet tall. Its foliage has sticky glandular land areas in California. In California and Mediterranean region, including Egypt hairs covered in resin. The resin emits a other introduced areas of the world, stink- and other areas of North Africa, this spe- strong aromatic odor that resembles the wort is most often found in disturbed cies has also been introduced to several smell of tarweeds. The flowerheads are 0.2 places, such as overgrazed rangelands, European countries where it is not na- to 0.3 inch (5 to 7 millimeters) in diameter roadsides, pastures, wastelands, vineyard tive. Within the last two decades, this and consist of short yellow ray flowers edges, gravel mines, levees, washes and weed has been spreading rapidly along on the outer edge and yellow to reddish mining sites, although in California it is the highways of Central Europe. In sum- disk flowers in the center. Stinkwort is seldom found in rangelands or pastures mer 2008, stinkwort was detected for closely related to fleabanes, horseweed (DiTomaso and Healy 2007; Higueras et the first time in Slovenia and Austria (Erigeron; formerly Conyza), goldenasters al. 2003). Stinkwort grows best on well- (Frajman and Kaligaric 2009). Outside of and telegraphweed (Heterotheca), but it drained, sandy or gravelly soils and also closely resembles the tarweeds (Cen- thrives in areas with hot, dry summers Online: http://californiaagriculture.ucanr.edu/ tromadia spp., Hemizonia spp. and Holocar- but can also do well along the margins of landingpage.cfm?article=ca.v067n02p110&fulltext=yes pha spp.). From a distance, stinkwort can wetlands. In addition, this plant tolerates doi: 10.3733/ca.v067n02p110 110 CALIFORNIA AGRICULTURE • VOLUME 67, NUMBER 2 Europe, stinkwort has been reported as 40 an invasive species in Australia (Parsons and Cuthbertson 2001) and South Africa 35 (South African National Biodiversity 30 Institute 2009). Stinkwort is not considered as a pal- 25 atable species to animals. In fact, it is reported to cause poisoning in livestock 20 (Philbey and Morton 2000). Although livestock mortality is rare, it appears to be 15 due to enteritis caused by the barbed pap- California counties 10 pus bristles on the seed, which can punc- ture the small intestine wall (Philbey and 5 Morton 2000). Stinkwort can also cause contact allergic dermatitis in humans 0 (Thong et al. 2008). However, impacts to 1980 1985 1990 1995 2000 2005 2010 2015 wildlife, natural ecosystems and working Year landscapes have not been broadly charac- Fig. 1. Approximate rate of spread of Dittrichia graveolens in California as represented by the number terized. This is likely due to its very recent of California counties where plant collections have been made between 1984 and 2012 (Consortium introduction and expansion in California of California Herbaria 2012). and to the lack of published information on the species elsewhere in the world. original infesta- Rapid spread in California tion in Santa Clara 1984 The first record of Dittrichia graveolens County, uncon- 1995–1996 in California is a collection made in 1984 nected populations 1997–1998 near Milpitas in Santa Clara County (HOT have also been 1999–2000 Harvey s.n., University of California discovered in San 2001–2002 Jepson Herbarium/JEPS). Although the Diego and Riverside 2003–2004 2005–2006 initial mechanism and time of introduc- counties (fig. 2). This 2007–2008 tion of stinkwort in the state are not is likely due to either 2009–2010 documented, many of the earliest collec- separate introduc- 2011–2012 tions were made in the south and east tions or long-distance San Francisco Bay Area (Preston 1997). movement on vehicles. Stinkwort has since spread to numerous Unusual life cycle counties in California, and many addi- tional herbarium collections have been Stinkwort has very made throughout the state (Consortium of high seed viability, with an California Herbaria 2012). average of about 90% of the Using the Consortium of California seeds capable of germination Herbaria records, we determined the at the time they disperse from rate of stinkwort’s spread since the first the plant. There does not appear discovery in Santa Clara County. Based to be primary dormancy in the on collection date and location data from seeds, which is defined as a seed herbarium records, this weed invasion ap- that is dormant at the time it disperses pears to have had only a brief lag period from the plant (Brownsey 2012). These and to have expanded at an exponential traits, combined with the small seed rate over the past 18 years (fig. 1). This has size, suggest that seed longevity in the caused increased concern among resource soil should be relatively short, perhaps 2 managers across the state. Although it is to 3 years. Seeds are capable of germinat- still uncommon in many places where it is ing at nearly any time of year in the field, Fig. 2. Chronological spread of stinkwort in California counties from 1984 to 2012. found, stinkwort has been reported in 36 but they typically germinate throughout of the 58 California counties (figs. 1 and 2). winter and early spring following peri- Stinkwort seeds are likely spread by ods of precipitation. We have shown that wind, on the fur and feathers of mam- germination is limited by soil moisture, spring and summer, they develop into mals and birds and on motor vehicles rather than soil temperature or low light pyramid- or sphere-shaped plants that su- and equipment, thus moving along trans- conditions (Brownsey 2012). When seeds perficially resemble Russian-thistle. portation corridors. While the primary germinate in winter, the plants remain as What makes stinkwort’s life cycle expansion has moved radially from the small rosettes until mid-May. During late rather unusual is that it matures much http://californiaagriculture.ucanr.edu • APRIL–JUNE 2013 111 later in the season than most annuals, A. Stinkwort Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec even other late-season winter annuals (fig. 3). For example, yellow starthistle Germination Germination Germination begins to send up a flowering shoot (bolt, Exponential Moderate Growth Rosette canopy referred to as moderate growth in figure growth growth 3C) in April, begins flowering in late June, Flowering and — like most late-winter or summer Reproduction annuals — has completed its life cycle by Seed production September or October. In contrast, stink- Dispersal Dispersal wort begins to bolt in mid-May (moderate growth in figure 3A), grows most of its B. Wild mustard Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec branches and leaves between June and September and flowers and produces Germination Germination Germination seeds from September to December. Growth Rosette Rapid growth Flowering in stinkwort appears to be Flowering controlled by photoperiod (day length), Reproduction Seed production as all plants initiate flowering at the same time regardless of when they germinated Dispersal Dispersal (Brownsey 2012). C. Yellow Aside from the tarweeds, there are starthistle Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec few other late-season winter annual spe- Germination Germination Germination cies with a similar life cycle in the native California flora. Some other weedy spe- Exponential Moderate Growth Rosette canopy cies, such as Russian-thistle, horseweed growth growth (Conyza canadensis (L.) Cronquist) and Flowering yellow starthistle (Centaurea solstitialis Reproduction L.), have similar life history strategies, Seed production with only Russian-thistle and horseweed Dispersal Dispersal flowering within the same time frame as stinkwort.