Invasive Plants of California's Wildland

Tansy ragwort (Senecio jacobaea) is a perennial herb in the dandelion family that sometimes reaches more than four feet in height. Numerous inch-wide, daisy-like yellow flowerheads with golden or light brown centers form at the tip of each branch from mid-summer to fall. The plant has a basal rosette of leaves, and the upper parts are branched. Leaves are deeply pinnately dissected into irregular segments, giving the plant a ragged appearance. Leaves or segments are wider than long. Ray flowers distinguish this plant from common tansy (Tanacetum vulgare).

In California tansy ragwort is found on the North Coast and into the Klamath and Cascade ranges, also occurring in the Sacramento Valley and San Francisco Bay region. It is commonly found in pastures, on roadsides, and in disturbed places. It grows best in light, well drained soils, but can become established in heavier soils, particularly soils broken up by trampling or frequent cultivation.

Tansy ragwort is endemic to parts of western Europe and western Asia, where it is a weed of minor importance on roadsides and grasslands. It is considered by some authors to be native to the dunes of Holland (van der Meijden 1971). From Europe it has been introduced into Argentina, New Zealand, Australia, Canada, and the western United States (Schmidl 1972a).

Tansy ragwort seeds, like those of other members of the Asteraceae, are assumed to be primarily wind-dispersed (van der Meijden 1971). Plants are commonly found along roadsides in otherwise uninfested areas (Holden 1989).

Animal transport of ragwort achenes seems likely. The pappus and bristle hairs of disk achenes would allow attachment to fur and feathers. Plants have been commonly observed along deer and elk trails through otherwise uninfested regions (Holden 1989).

Tansy ragwort has been transported in infested hay. This mode of dispersal was postulated to explain several isolated infestations observed in eastern Oregon when hunting parties carried hay into previously uninfected areas (Cox and McEvoy 1983).

The plant can also spread via regeneration of root fragments contained in mud or soil adhering to vehicles.

Tansy ragwort is highly toxic to livestock. It contains pyrrolizidine alkaloids that cause liver damage in horses and cattle (Cheeke 1979). Cattle and horses are affected seriously; goats may suffer poisoning; sheep are generally not poisoned by this plant. Tansy ragwort easily outcompetes native and naturalized grasses and forbs (Harper 1958). It is estimated to occur on three million acres in western Oregon, where two out of every five acres of pasture are infested. More than 112,000 acres of pasture in western Washington contain tansy ragwort. It has been reported in Idaho and is there considered a serious potential weed problem that has not yet reached a level of economic importance. Most ragwort is on forested and clearcut lands (Bedell et al. 1995).

Tansy ragwort is a biennial species. The first-year plant is a rosette of basal leaves that are raggedly lobed and to nine inches (22.5 cm) long. In the second year the first-year leaves die back and the plant develops tall, leafy flowering stems in summer. It is ordinarily about three and one-third feet (1 m) in height, but plants to ten feet (3 m) are not uncommon, with many branches near the top (Holden 1989).

Tansy ragwort is capable of regeneration from pieces of rootstock. Roots without any portion of the root crown are able to produce new plants, and even severed roots of cotyledons can grow new shoots (Holden 1989). Poole and Cairns (1940) found that first-year plants, or rosettes, buried five inches (12.5 cm) in the soil sprouted three months later, even in heavy soil.

In addition to vegetative propagation, ragwort also reproduces by seed. Two studies at different sites recorded 4,760 to 174,230 seeds per plant (Camoron 1935) and 6,480 to 137,500 seeds per plant (Poole and Cairns 1940).

Ragwort inflorescences contain two types of flowers, an outer row of radiate (ray) female flowers and many central tubular (disk) perfect flowers. Each flower type produces distinctive single-seeded fruits (achenes) that have different dispersal mechanisms, germination requirements, and germination rates (Harper 1965, Burtt 1977). Seeds are tipped by hair-like plumes that carry seeds in the wind for long distances. Although both fruit types are primarily dispersed by wind, disk achenes are dispersed earlier and farther than ray achenes (Green 1937). Poole and Cairns (1940) found that 60 percent of the total seed shed landed within fourteen feet (4.6 m) of the base of the plant; an additional 39 percent landed between fourteen and twenty feet (4.6 and 9 m) from the plant.

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Tansy ragwort seeds remain viable in the soil for several years. In this way the species can wait for favorable conditions to occur. McEvoy (unpubl. data) found 0.75 viable ragwort seeds/cc in the upper 1.2 inches (3 cm) of soil underneath a pasture heavily infested with ragwort. Viable seeds were encountered in samples taken as deep as ten inches (25 cm) below the surface. On the Oregon coast, tansy ragwort seeds mature in late summer and early fall. Seeds reach maximum germination potential before dispersal; they do not possess an innate

Soil moisture, soil surface humidity, and light are important factors in ragwort germination. Van der Meijden and van der Waals-kooi (1979) found germination of ragwort achenes to be greater at soil moisture levels between 15 and 29 percent and a relative humidity at the soil surface of 100 percent.

Ragwort needs an opening through some form of disturbance to become established (van der Meijden and van der Waals-kooi 1979). Several disturbance agents have been cited, including moles and gophers, ants, rabbits, livestock, and humans (Harper and Wood 1957).

Mechanical methods: Deep plowing has been unsuccessful in controlling tansy ragwort (Holden 1989). This technique severs roots and distributes them over a wide area. Plowing also unearths buried seeds and can contribute to a more severe infestation (Holden 1989).

Mowing has been widely used in attempts to prevent the spread of ragwort, but has provided only cosmetic control. Repeated mowing could deplete reserves and eventually exhaust the population. Fields would have to be mowed every six weeks during spring and summer months and be accompanied by moisture stress (Cox and McEvoy 1983). Single mowings during flowering might intensify local infestations. Given that ragwort seeds ripen over a range of time and that severed capitula are able to produce seed, mowing might increase local seedling density while freeing the parent plant from reproductive burdens and engendering basal sprouting (Holden 1989).

Hand pulling has been the most common technique used on small pastures in the early stages of infestation. Soil moisture is critical, as drier soils allow root breakage and pulling in wet soils removes large soil clumps. Partially opened flowers continue to set viable seed if sufficient moisture is available in the cut plant. Therefore, plants must be removed from the treated site and buried or burned (Holden 1989).

Clipping or otherwise deflowering plants eliminates seed set and prevents further spread if flowerheads are collected and burned (Holden 1989). The original plant, however, continues to sprout, grow, and reflower in subsequent seasons.

Prescribed burning: Burning is traditionally used in agricultural croplands as a preemergence weed treatment. In ragwort control, Poole and Cairns (1940) experimented with a flame thrower, but their results were inconclusive and have not been reproduced by others. Mastroguiseppe et al. (1982) conducted several control burns at an infested site in Redwood National Park, California, but the results were inconclusive.

Insects and fungi: Biological control has proven to be effective for long-term control of extensive infestations of tansy ragwort. Several USDA certified biocontrol agents have been released for control of this weed. The cinnabar moth, Tyria jacobaeae, is a day-flying moth indigenous to Europe and Asia. Cinnabar larvae feed primarily on the flower buds, but readily consume leaves and stems as well. Unfortunately, the cinnabar moth is subject to disease, predation, and parasitism.

Tansy seedfly (Pegohylemia seneciella) is a small muscid fly common in tansy flowers in France and Italy. The seedfly consumes the seeds of tansy, but does not kill or inhibit growth of the host plant.

Tansy flea beetle, Longitarsus jacobaeae, is another host-specific pest of tansy that is indigenous to Mediterranean Europe. Flea beetle larvae feed throughout the root crown and sometimes feed externally on lateral roots. Larvae bore into the stem and leaf petioles for two to five inches (5-10 cm), causing wilt and death of the plant. Plants seldom recover from a heavy infestation of the beetle. Adult flea beetles feed on the leaves, causing a characteristic “shot-hole” pattern.

Holden (1989) obtained dramatic results from introduction of the flea beetle; percent absolute cover of ragwort declined from a mean of 22.7 in the fall 1982 to zero in summer 1984. At the beginning of this study (October 1982) ragwort cover in the circular vegetation plots ranged from less than 1 percent to 38 percent. By July 1984, all measured plots showed zero cover.

Vegetative competition: In New Zealand many hectares of land were converted to forests by planting commercial trees over infested sites (Holden 1989).

Grazing: Sheep grazing reduced the density of tansy ragwort, but the effect appears to be temporary. Ragwort reappeared as soon as the sheep were removed (Holden 1989).

The first chemical used to combat tansy ragwort was the soil sterilant, sodium chlorate, a non-selective inorganic compound. The high cost of sodium chlorate prevented its use as a widespread control agent. Whitson et al. (1985) recommends dicamba (as Banvel®) for ragwort control at label concentrations. Ragwort plants treated with herbicides have been found to be more readily eaten by livestock, so grazing must be discontinued. Cattle may reenter the field only after the plants have completely dried and been replaced by other forage species (Holden 1989). Often herbicides have not been effective in killing this plant because the herbicide leached out of the roots without being transported throughout the plant (Poole and Cairns 1940).

Removal of individual ragwort plants or small populations requires immediate attention. To maintain control of tansy ragwort, a combination of control efforts must be diligently applied, and every opportunity must be taken to eliminate outlying individuals and populations.