Wild parsnip (Pastinaca sativa L., family Apiaceae (= Umbelliferae)) plants are now large enough to be readily identified in southwest Ohio. This Eurasian native 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 near the ground. While in this stage, the plant produces a long, thick taproot. Flower stalks rise from overwintered rosettes during the second year. 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.
Second-year plants can grow to impressive heights topping 8'; however, most mature plants range in size from 4 - 6'. 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.
Wild parsnip has become increasingly common in recent years in Ohio landscapes and fields. Reasons behind the ever expanding distribution and rapid expansion of infestations are not entirely understood. However, it is clear that the proliferation represents an increasing risk to both people and animals.
Landscape managers and gardeners should exercise extreme caution around this non-native invasive plant. Wild parsnip sap contains psoralen which is a naturally occurring phytochemical grouped in a family of organic compounds known as linear furanocoumarins. Psoralen acts as a photosensitizing compound by inhibiting DNA synthesis in epidermal cells which kills these light-shielding cells responsible for protecting us from long-wave ultraviolet radiation (LWUVR) bombarding us in sunlight.
Severe blistering occurs when affected skin 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. It takes around 24 hours for symptoms to first appear after exposure to LWURV and severe blistering typically doesn't peak until 48 -72 hours. The time required for symptoms to appear after exposure to the sap means the effect may be disconnected from the cause.
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 also found in a number of other members of the 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 northeast 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; which it actually is!
A Good Veg Gone Bad
Parsnips have been cultivated as a root crop in Europe for centuries; perhaps millennia. The "L." in the scientific name Pastinaca sativa L. means Linnaeus first described the species. Both the cultivated and wild types share the same scientific name; however, it is clear that there are significant differences in toxic biochemical properties, invasive behavior, and some morphological features between the two types.
Early taxonomic efforts treated the different types of parsnips as subspecies. More recent research has shown that while the subspecies designation may still be valid for some types of parsnips, the cultivated types have their roots in varieties which makes sense from both an evolutionary and agricultural perspective.
As with most of our agricultural crops, cultivated parsnips no doubt got their start through trial and error by our early ancestors. Indeed, wild types are still found in Europe and Asia just like wild potatoes are still found in South America.
It is now theorized that the wild parsnip plants in Ohio represent "escapes" from cultivated types brought to North American from Europe and a "reversion" back to a wild type. The wild genes were always there but remained suppressed until revealed through natural selection.
For example, research on wild parsnip has shown that psoralen behaves as a phytoalexin which are plant defense compounds aimed at thwarting attacks by macro and microorganisms. Although the concentration of psoralen is inherently higher in wild types compared to cultivated parsnips, researchers found that the production of the plant toxin increases in wild types in response to wounding by herbivores imparting an obvious plant survival advantage.
On the other hand, a selective advantage is awarded to herbivores that can handle phytoalexins. Heavy damage by fourlined plant bugs (Poecilocapsus lineatus) will be a common feature on wild parsnips later this spring in Ohio.
The parsnip webworm (Depressaria pastinacella) focuses its entire attention on its namesake host. The webworm thwarts the plant's chemical warfare by excreting most of the plant toxin in their feces. However, some of the toxins are incorporated into the caterpillar's silk webbing. The purpose is unknown, but it is speculated that the toxins provide protection against predators and parasitoids as the caterpillars feed on the flower parts. Unfortunately, wild parsnip is so prolific that damage by the caterpillars provides little relief from the continual spread of this vegetable gone wild.
Mechanical and Chemical Control
The extreme skin reaction to the wild parsnip sap means this non-native invasive weed should not be allowed to grow where it can be easily contacted by people. However, mechanical control is problematic. Hand-pulling is a high-risk endeavor and not recommended. Likewise, tilling would release a huge amount of harmful sap. There have been reports of sap spattered by mowers and string trimmers producing phytophotodermatitis on exposed arms and legs of equipment operators.
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 a number of selective and non-selective postemergent herbicides. However, keep in mind that non-selective herbicides such as glyphosate (e.g. Roundup) can also eliminate plants that compete with wild parsnip.
Herbicidal openings produced by non-selective herbicides provide perfect opportunities for more wild parsnip to spring forth from previously deposited seed. Thus, it's important to have a plan for establishing competitive plants after the wild parsnip dies off such as over-seeding with grasses.
Selective post-emergent herbicides will preserve competitive plants. Herbicides effective against wild parsnip include 2, 4-D, clopyralid (e.g. Transline), and metsulfuron (e.g. Escort XP).