Wild parsnip (Pastinaca sativa) is an invasive plant species and noxious weed across North America, posing a significant threat to ecological habitats and human health. This non-native plant aggressively colonizes disturbed areas like roadsides, fields, and pastures. Pastinaca sativa is characterized by its tall, grooved stem and umbrella-shaped clusters of yellow flowers, typically blooming in the second year of its biennial life cycle. The plant’s toxic sap necessitates a cautious, multi-year strategy for its safe and effective eradication.
Essential Safety Precautions
The sap of wild parsnip contains furanocoumarins, chemicals responsible for phytophotodermatitis. This is a phototoxic effect where the compounds penetrate the skin and are activated by ultraviolet (UV) light, even on overcast days. The reaction results in skin inflammation, often manifesting as painful, severe burns and blistering that can resemble second-degree burns.
Contact with the sap followed by sun exposure can lead to hyperpigmentation, with discoloration and sensitivity sometimes lasting for up to two years. To prevent injury, impervious personal protective equipment (PPE) must be worn when handling the plant. This includes long-sleeved shirts, long pants, and closed-toe shoes to cover all exposed skin.
Gloves should be made of nitrile or thick rubber, as the sap can penetrate standard cloth. Eye protection, such as safety glasses or a face shield, is also necessary, especially when working near eye level. Avoid using string trimmers or brush cutters, as these tools can atomize the sap and spray it over a wide area, increasing the risk of exposure.
If skin contact occurs, immediate action is necessary to minimize the severity of the reaction. The affected area must be covered immediately to shield it from sunlight exposure, preventing the furanocoumarins from activating. Once out of the sun, the skin should be thoroughly washed with mild soap and cool water to remove the oily sap residues. Note that the effectiveness of washing decreases rapidly, often becoming ineffective within 30 minutes of contact.
Mechanical and Cultural Control Methods
Non-chemical control strategies rely on physically removing the plant or suppressing its growth through competition, with the goal of preventing seed production. For smaller infestations, mechanical removal is a highly effective, though labor-intensive, option. The plant’s biennial nature means first-year plants are low-lying rosettes, while second-year plants send up a tall flower stalk.
First-year rosettes can be removed by hand-pulling or digging, which is easiest when the soil is moist. The taproot must be severed at least one to two inches below the soil surface, as the plant can regrow if the root crown remains intact. When removing larger, bolted second-year plants, a sharp spade or shovel can slice the taproot below the crown, minimizing handling of the mature stem.
Mowing manages larger populations but must be precisely timed. Mowing too early can encourage the plant to produce new, lower flower stalks. The most effective time to mow is after the plant has bolted and begun to flower, but before the seeds have matured and become viable. Since the plant does not flower uniformly, multiple passes may be necessary throughout the early to mid-summer period. Mowing prevents current year’s seed production, and repeated mowing over time can exhaust the existing seed bank.
Cultural control methods focus on establishing dense, competitive vegetation to outcompete wild parsnip seedlings. Wild parsnip thrives in open, disturbed areas with full sunlight. Establishing a healthy, dense ground cover, such as turfgrass or native perennial species, suppresses the growth of rosettes by limiting light availability. The long-term success of any removal method is enhanced when followed by the establishment of competitive plantings to fill the ecological niche.
Chemical Treatment Strategies
For large infestations or areas where mechanical removal is not feasible, the application of herbicides provides an efficient control strategy. Wild parsnip is susceptible to several types of broadleaf herbicides, including those containing active ingredients such as 2,4-D, glyphosate, or triclopyr. The systemic nature of these chemicals allows them to be absorbed by the foliage and translocated down to the taproot, ensuring a complete kill of the plant.
Timing is crucial for maximizing efficacy and minimizing impact on surrounding vegetation. Herbicides are most effective when applied to the first-year basal rosette stage. This stage is best targeted in early spring, as soon as rosettes green up, or in late fall before the first hard freeze, when the plant moves nutrients to the root for winter storage.
Applying herbicides during the rosette stage ensures the chemical is absorbed and translocated, preventing the plant from bolting and flowering the following year. Applications can be made to bolted plants, but they are less effective, often requiring higher rates, with the goal shifting to seed production suppression rather than complete plant kill.
Spot treatment, applying herbicide directly to individual rosettes, is the preferred method in areas with desirable non-target plants. This protects beneficial species. Broadcast application, spraying over an entire area, is reserved for large-scale infestations in non-sensitive areas like roadsides. Always follow the manufacturer’s label instructions for mixing ratios, application rates, and environmental conditions to ensure safety and optimal performance.
Safe Disposal and Long-Term Site Management
Proper disposal of removed plant material is necessary to prevent re-infestation and continued exposure to the toxic sap. Plant parts, especially mature seed heads, should never be placed in a home compost pile. The seeds remain viable for several years, and backyard composters typically do not reach temperatures high enough to kill them.
All plant material, including roots, stems, and flower heads, should be bagged securely in heavy-duty plastic trash bags. The safest disposal method is to send the sealed bags directly to a municipal landfill. Alternatively, the bagged material can be left in direct sunlight for several weeks, a process called solarization, which uses heat to kill the seeds and desiccate the toxic sap.
For long-term control, site management must continue for multiple seasons following initial removal. Wild parsnip seeds can remain viable in the soil’s seed bank for approximately four years. Therefore, the treated area must be monitored each spring and fall for several subsequent years to spot and remove emerging rosettes. Controlling new seedlings before they reach maturity and produce new seeds is necessary to deplete the seed bank and achieve lasting eradication.