Wild parsnip (Pastinaca sativa L., family Apiaceae (= Umbelliferae)) plants are now large enough to be readily identified in southwest Ohio. Parsnips have been cultivated as a root crop in Europe for centuries. However, wild parsnip is a cultivated plant that’s returned to its dark side.
Both the cultivated type we grow in our vegetable gardens and the escaped wild type which is the focus of this Alert share the same scientific name. However, it is clear that there are significant differences in the toxic biochemical properties between the two types.
Both types contain a plant defense chemical called psoralen in the sap, a naturally occurring phytochemical grouped in a family of organic compounds known as linear furanocoumarins. However, the wild type of parsnip contains much higher concentrations of psoralen to defend against herbivores. Indeed, herbivory applies selective pressure that rewards plants that produce more psoralen.
Psoralen acts as a photosensitizing compound by inhibiting DNA synthesis in epidermal cells, killing 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 and skin discoloration may last for several months.
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.
Psoralens are also found in 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. 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!
Life Cycle and Identification
Wild parsnip development from seeds to flowers usually follows a biennial life cycle. Seeds most commonly germinate in the spring and the first year is spent in the vegetative stage as a low-growing basal rosette. The rosettes use carbohydrates acquired through photosynthesis to produce a robust root system.
Plants “bolt” during the second-year reproductive stage to produce a single, thick, deeply grooved, greenish-yellow stem that sprouts lateral branches topped with hundreds of clusters of the umbellate flowers. Mature wild parsnip plants can rise as high as 8 ft.: however, most mature plants only grow to 5-6 ft. Plants die after producing seeds.
Occasionally, wild parsnip behaves as a monocarpic perennial with plants remaining in the rosette stage for 2 – 3 years. This helps to explain why the growth stages within a wild parsnip infestation are seldom synchronous. It’s common for vegetative rosettes to be mixed with reproductive plants.
Wild parsnip is a member of the carrot family, Apiaceae. The old name for the family was Umbelliferae which refers to the umbel flowers. The flowers are a key family feature with short flower stalks rising from a common point like the ribs on an umbrella. Wild parsnip produces yellow flowers on a flat-topped flower structure.
Plants are prolific seed producers; however, seeds only remain viable for around 4 years and germination rates are relatively low. Regardless, small patches of this weed can develop into large patches in just a few years.
All stages of the wild parsnip plants have light-green pinnately compound leaves that strongly resemble celery leaves. Each leaf has 5 -15 ovate to oblong leaflets with variable toothed edges and deep lobes. The leaf structure is even evident on newly germinated seedlings.
The acute 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. Landscape managers and gardeners should also exercise extreme caution around this non-native invasive plant.
Hand-pulling is a high-risk endeavor and not recommended. Likewise, tilling would release 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.
The graphic below illustrates that the best time to make herbicide applications is just after last season’s rosettes are starting to bolt but before flowers are produced. Seeds have also germinated by this time. Killing the seedlings will reduce next year's rosettes and killing the bolting rosettes will prevent seed production later this season. Eliminating these plants can significantly reduce infestations. Note that mowing is not included in the graphic as a viable management option.
Fortunately, wild parsnip is susceptible to a wide range of selective and non-selective postemergent herbicides. Non-selective herbicides with the active ingredients glyphosate (e.g., Roundup) or pelargonic acid (e.g., Scythe) are effective but can also eliminate plants that compete with wild parsnip.
Herbicidal openings produced by non-selective herbicides provide perfect opportunities for wild parsnip to spring forth from previously deposited seed. Thus, it's important to have a plan for establishing competitive plants such as over-seeding with grasses (family Poaceae).
Grasses are effective competitors against wild parsnip and a range of selective post-emergent herbicides can be used that will preserve grasses but kill the poison hemlock. These include clopyralid (e.g., Transline), metsulfuron (e.g., Escort XP), triclopyr (e.g., Triclopyr 4), and products that contain a combination of 2,4-D, dicamba, mecoprop, and dichlorprop.
Of course, as with using any pesticide, it’s important to closely read and follow label directions. Some post-emergent herbicides can seriously damage trees if applied over the root zone.
Don’t Be Fooled
Wild parsnip is commonly found growing 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 the body 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.