Second-year wild parsnip (Pastinaca sativa, family Apiaceae (= Umbelliferae)) plants are producing deeply grooved flower stalks topped by characteristic bright yellow blooms in southern Ohio. Landscape managers and gardeners should exercise extreme caution around this non-native invasive biennial plant.
Wild parsnip sap contains psoralens which are naturally occurring phytochemicals grouped in a family of organic compounds known as linear furanocoumarins. Psoralens kill epithelial skin cells by inserting themselves into the DNA in the cell's nucleus. These cells are responsible for protecting us from long-wave ultraviolet radiation (LWUVR) that bombards us from the sun.
Severe blistering occurs when skin affected by the psoralens is exposed to LWUVR. The synergistic effect is called phytophotodermatitis (a.k.a. Berloque dermatitis) and the burn-like symptoms, as well as skin discoloration, may last for several months. However, connecting skin blistering to exposure to wild parsnip sap can be a challenge. The cause and effect are muddled by time because symptoms do not appear for around 24 hours after exposure to LWUVR and severe blistering doesn't peak for another 48 to 72 hours.
Another challenge with connecting the dots is that wild parsnip commonly grows in and around other weeds, particularly poison hemlock (Conium maculatum) which is another member of the Apiaceae family. This deadly non-native biennial weed contains highly toxic piperidine alkaloid compounds which cause respiratory failure and death in mammals.
The poison hemlock toxins have a completely different mode of action and must be ingested or enter through the eyes or nasal passages to induce poisoning; they do not cause skin rashes or blistering. However, gardeners exposed to wild parsnip growing among poison hemlock may mistakenly blame the poison hemlock for their ultimate misery.
Psoralens are found in a number of other members of Apiaceae family including the notorious giant hogweed (Heracleum mantegazzianum) which has captured national attention in the past. However, giant hogweed has only been confirmed in Ohio growing in the extreme northeastern part of the state primarily in and around Ashtabula County. Wild parsnip is found throughout the state and is equally damaging. Of course, giant hogweed has a more threatening sounding common name; wild parsnip just sounds like a vegetable gone wild.
Wild parsnip is native to Eurasia and grows as a biennial in Ohio requiring two seasons to complete its life cycle. Plants spend the first year as rosettes with leaves confined to growing from a short stem only a few inches above the ground. While in this stage, the plant produces a long, thick taproot. Flower stalks are produced during the second year.
Second-year plants can grow to impressive heights topping 8'; however, most mature plants range in size from 4 - 6'. Leaves are alternate, pinnately compound, branched, and have saw-toothed edges. Each leaf has 5 -15 ovate to oblong leaflets with variable toothed edges and deep lobes.
Mature plants will produce a single, thick, deeply grooved, greenish-yellow stem that sprouts lateral branches topped with hundreds of clusters of the umbellate flowers. Plants are prolific seed producers meaning that small patches of this weed can develop into large patches in just a few years.
Keep in mind that sap in both the first year rosettes and second-year flower stalks contains damaging concentrations of psoralens. Always wear gloves and protective clothing if you find yourself working around any biennial growth stage of this malevolent weed!
Mechanical and Chemical Control
The toxic nature of the sap makes mechanical control of wild parsnip problematic. Hand-pulling is a high-risk endeavor and not recommended. There have been reports of sap spattered by mowers and string trimmers onto equipment operators producing phytophotodermatitis on exposed arms and legs.
The safest approach to controlling this invasive weed is to use herbicides. Of course, as always, read and follow label directions paying close attention to recommended rates and whether or not surfactants are recommended to enhance herbicide efficacy.
Wild parsnip plants are susceptible to postemergent herbicides such as the non-selective systemic herbicide glyphosate (e.g. Roundup) and the contact "burndown" herbicide pelargonic acid (e.g. Scythe). However, keep in mind that multiple applications of a burndown herbicide may be required to exhaust the energy stored in second-year tap roots. Effective selective postemergent herbicides include 2, 4-D, clorpyralid (e.g. Transline), and metsulfuron (e.g. Escort XP).