Stinkwort (Dittrichia graveolens) is an annual herb that will grow to 91 cm tall. It blooms in late summer with yellow, radial shaped flowers. It has become invasive in California where it threatens the wine cultivation. Considered a noxious weed in many regions.
I. Appearance and Characteristics
Europe, North Africa, and western Asia as far east as Pakistan. It has become naturalized in California, Asia, Africa, Australia, and other places and is regarded as a noxious weed in some regions. It is classified as an invasive species in California, and a potential threat to wine production in the state.
The plant is a branching subshrub growing up to 130 centimeters (51 inches) tall, with an aromatic, camphor smell. Leaves are long and narrow, pointed at each end, with small teeth along the edges, glandular hairs on the surfaces, and a sticky resin. One plant can produce numerous yellow flower heads with as many as 16 ray florets and 40 disc florets.
Barbs on the fluffy-tipped seeds, which help it spread, can fatally damage the digestive systems of grazing animals. Oils in the plant also taint the flavor of meat and milk of animals that have consumed them. The sticky resin has been known to cause allergic reactions and severe dermatitis in humans. For these reasons, it is advisable to wear protective gloves when handling the plant.
II. How to Grow and Care
Stinkwort is generally considered an invasive weed rather than a plant requiring deliberate care. Its ability to thrive in poor soil conditions and resist drought makes it notably low-maintenance. Special care points for managing stinkwort include regular monitoring to prevent uncontrolled spread and ensuring it does not outcompete native plants in the area. Proper disposal of plant material is critical to avoid further propagation.
Sunlight
Stinkwort thrives best in full sun conditions, where it receives unfiltered, direct sunlight for at least 6 to 8 hours per day. This exposure to bright light is crucial for stinkwort’s optimum growth, ensuring robust health and vitality. Despite its preference for intense light, stinkwort exhibits a notable tolerance for partial sun. While it can adapt to less intense light environments, its growth may be less vigorous, potentially leading to reduced blooming and weaker development. Stinkwort does not typically undergo significant leaf adjustments or color changes in response to varying light levels, and it is primarily an outdoor plant. Planting stinkwort in open areas that receive ample sunlight is recommended to support its growth needs.
Watering
Stinkwort is a plant that has adapted to dry conditions in its native habitat. It exhibits drought tolerance, indicating a preference for less frequent watering. This plant should be watered every week to maintain optimal health. Being an evergreen herb, stinkwort can retain water throughout the year, making it suitable for arid climates.
Fertilizing
To optimize stinkwort’s growth and vitality, employ balanced nutrition fertilizers. Include high nitrogen content early in its growth cycle for robust development. Fertilize monthly during the growing season, adjusting to quarterly in dormancy. Use fertilizers sparingly, following package instructions to avoid overfeeding. Extra care is required; excessive fertilization can harm stinkwort. Fertilization enhances plant vigor and, by extension, resistance to pests and diseases. Apply granular fertilizers evenly around the base, and water thoroughly post-application to aid absorption. Monitor stinkwort for signs of nutrient deficiency or excess, and adjust practices accordingly for this resilient species.
Propagation
Stinkwort propagates effectively by sowing in Spring. Considered easy to propagate, successful propagation signals include emerging new shoots. No additional propagation-related tips are required for this low-maintenance species.
Transplanting
Transplanting stinkwort thrives best in the golden hues of early fall—optimal due to milder temperatures and less intense sun. Position it in well-drained soil, and if you’re moving stinkwort, consider its love for sunny spots to ensure success.
III. How to Control
Biological or cultural practices
There are currently two biological or cultural practices that can be employed to limit the ability of stinkwort to invade an area. One is to minimize disturbances such as overgrazing and soil manipulation in natural andrangeland sites. Second, pastures should be managed for dense, competitive stands of desirable perennial or annual grasses that maximize ground cover in spring, when stinkwort seedlings are beginning to establish.
Mechanical practices
Mechanical control options can take advantage of the stinkwort root system, which is slow growing and initially relatively shallow. Plants may be controlled by hoeing or pulling. However, because stinkwort can cause dermatitis, it is important to wear appropriate protective clothing (long sleeves, long pants, gloves) to minimize exposure to the irritating oils. Once in flower, stinkwort plants should be bagged and removed from the site to prevent seeds from maturing and dispersing after the plants have been cut and left on the soil surface. Mowing can provide partial control when conducted late in the season.
However, buds remaining on branches below the level of the mower may regrow. Mowing a second time can give improved control, especially when conducted after the soil has dried out in mid- to late summer. In contrast, mowing too early, as is done on highways to reduce the threat of grass fires, will favor stinkwort by removing competing annuals while this weed is still small and lower than the mowing blades.
Postemergence herbicides
Postemergence herbicides are applied to small germinated seedlings or young plants. Thus, in contrast to preemergence herbicides that are generally applied to larger areas before seeds germinate, postemergence applications can directly target known infestations visible to the applicator. However, the sticky oils on the foliage, especially on mature plants, make it difficult to control stinkwort with postemergence herbicides. To overcome this, it may be necessary to use ester formulations of postemergence phenoxy-type herbicides (2,4-D, MCPA, triclopyr, etc.). However, these compounds are more volatile compared to salt formulations (commonly used in summer), and some should not be applied when ambient temperatures will reach or exceed 80°F.
In University of California experiments conducted for the postemergence control of stinkwort, it was found that the salt formulation of triclopyr at 24-ounce acid equivalent (a.e.) per acre (2 quarts Garlon 3A per acre) gave the most effective level of control following a postemergence application. Triclopyr is selective and relatively safe on grasses, but it must be used cautiously around vineyards, as grapevines are extremely sensitive to triclopyr drift. It is also important to note that control with postemergence herbicides is most effective when plants are young, actively growing and not exposed to stresses such as drought. For stinkwort, this is generally just before or at the time of bolting.
Glyphosate (Roundup Pro) at 1 quart product per acre also gave fairly good control, and anecdotal information from other land managers indicates that a rate of 2 quarts product per acre gives control similar totriclopyr at 2 quarts product per acre. Unfortunately, other herbicides, including aminopyralid (Milestone) and aminocyclopyrachlor (one of the active ingredients in herbicide Perspective), did not provide effective late-season post-emergence control of stinkwort. As previously discussed, plants also partially recovered from late-season mowing.
Pre- and early postemergence herbicides
Because stinkwort germinates throughout the rainy season, the most effective control options are likely to be broadleaf selective herbicides with both pre- and early postemergence activity, which can control both new germinants and young emerged seedlings. A fairly new group of foliar- and soil-active growth regulator herbicides have proven very effective in winter and spring applications for control of yellow starthistle and other members of the sunflower family (Asteraceae).
These herbicides have the ability to control both emerged young plants through foliar activity, as well as germinating seedlings through soil activity. These chemicals include clopyralid (Transline), aminopyralid (Milestone) and aminocyclopyrachlor, and they are generally safe on grasses. In preliminary research, UC found that winter applications of aminocyclopyrachlor and spring applications of Milestone VM+ (aminopyralid plus triclopyr) showed the greatest potential for controlling stinkwort.
Early-season application of glyphosate, however, controlled competing vegetation and so allowed late-germinating stinkwort to thrive. Thus, glyphosate is best used later in the season as a post-emergence application.
It will be critical to assess the impact that stinkwort is making on your rangelands. A rancher should inventory and map the impacted areas of the ranch; then develop a management strategy to control this invasive plant. Having invaded sites mapped allows ranchers to assess future expansion and can help prioritize management activities. This work also lays a foundation for future implementation of specific management methods. To stop or slow the spread of this newly invasive plant in California, we must quickly develop effective management tools and an informed management approach.