Wild parsnip (Pastinaca sativa) plants are rising towards their full height and their yellow blooms are beginning to open in southern Ohio. Landscape managers and gardeners should exercise extreme caution around this non-native invasive plant.
Severe blistering can occur if chemicals in the plant juices (furanocoumarins (= furocoumarins)) come in contact with skin and the skin is then exposed to sunlight; specifically ultraviolet light. The effect is called phytophotodermatitis (a.k.a. Berloque dermatitis) and burn-like symptoms as well as skin discoloration may last for several months. Always wear gloves and protective clothing if you find yourself working around this weed!
This Eurasian native can grow to impressive heights topping 8'; however, most mature plants range in size from 4 - 6'. The umbellate flower arrangement looks like an upside-down umbrella; a characteristic shared by all members of the carrot family (Apiaceae (= Umbelliferae)). The umbels on wild parsnip are topped with tiny yellow flowers.
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.
Wild parsnip 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.
There are Enemies but No Natural Controls
Thus far, no bio-control agents have been found that will effectively control wild parsnip. Plants may be noticeably damaged by our native Fourlined Plant Bug (Poecilocapsus lineatus); however, the bug has a limited impact on the overall health of affected plants. Part of the challenge is that this sucking insect is a generalist and will feed on over 250 herbaceous plant species.
The non-native Parsnip Webworm (Depressaria pastinacella) has a more confined palate feeding on its namesake host as well as several species in the Heracleum genus. The moth caterpillars handle the furanocoumarins by simply excreting them in their feces (frass) or by incorporating the chemicals into their silk webbing. It is speculated the furanocoumarins in the webbing surrounding the caterpillars may provide protection against predators and parasitoids.
The caterpillars damage wild parsnip by feeding on the flowers and boring into plant stems. Early and middle instar caterpillars envelop the flower parts with ever-expanding silk nests and they feed on the plant tissue within their nests. As the caterpillars grow, the nests become larger. It is not unusual for all of the flowers to be consumed by the caterpillars.
Mature caterpillars feed as stalk borers. The large caterpillars migrate to lower portions of the stem where they bore into the stalk. Numerous webworms boring into the stalk can completely cutoff the vascular tissue connected to the upper portions of the plant causing premature plant death.
Despite the considerable damage done by the parsnip webworm, there is little evidence these moth caterpillars are slowing the spread of the non-native weed. Indeed, I'm seeing relatively small patches of wild parsnip becoming increasingly larger patches from year-to-year in southern Ohio.
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 producing dermatitis on the exposed arms and legs of equipment operators.
The safest approach to controlling this invasive weed is to use herbicides. Plants are susceptible to postemergent herbicides including both non-selective such as glyphosate (e.g. Roundup) or selective such as 2,4-D and metsulfuron (e.g. Escort XP). As always, read and follow label directions paying close attention to recommended rates the use of surfactants to enhance herbicide efficacy.