Wild Parsnip (Pastinaca sativa L., family Apiaceae (previously Umbelliferae)) is in full bloom in Ohio with recent hot temperatures accelerating seed development, particularly in the southern part of the state. This means it’s too late to reduce the seed bank of this highly dangerous non-native invasive weed by mechanical removal (e.g, mowing with caution!) or using herbicides.
The slow but relentless spread of this good vegetable gone bad was on display along I-71 as I drove from Cincinnati to Columbus last Thursday. Masses of 3 – 4 ft. tall plants capped by clusters of bright yellow “flat-topped” flowers were a common sight on each side of the interstate right-of-way for the entire trip. There was a time not long ago when I would go out of the way to take pictures of wild parsnip.
It's also common to see another dangerous non-native invasive, Poison hemlock (Conium maculatum), growing near or among the wild parsnip. The two growing together creates a high-risk scenario for the uninformed. I reported in an Alert last week that it’s also too late to control poison hemlock in Ohio [see BYGL Alert, “Poison Hemlock: TOO LATE!,” June 17, 2022].
Most consider poison hemlock to be one of the most dangerous plants in North America owing to the highly toxic piperidine alkaloid compounds it produces for chemical defense. Indeed, history tells us poison hemlock “tea” was used to kill Socrates as well as the Greek statemen Theramenes and Phocion. Accidental poisonings have occurred in the U.S.
The toxins in poison hemlock must be ingested or enter through the eyes, nasal passages, and cuts in the skin to induce poisoning. There’s also anecdotal evidence that mechanical removal may cause the sap containing the toxins to become aerosolized and inhaled. However, poison hemlock toxins do not cause skin rashes or blistering.
The defense chemicals produced by wild parsnip are very different and have a vastly different mode of action. Skin contact with wild parsnip sap can produce painful severe blistering requiring medical attention. Various online reports describe the skin damage as being comparable to a 2nd-degree chemical burn and hospital treatment commonly involves burn units.
It's important to note that it is not unusual to find poison hemlock and wild parsnip growing together which can create misinterpretations of exposure symptomology. This may account for some online resources incorrectly attributing skin blistering to contact with poison hemlock.
The goal of this BYGL Alert and the previous posting on poison hemlock is not to scare readers into avoiding the outdoors. It’s to raise awareness of plants we should avoid. The same goal is exemplified by, “leaves (leaflets) of three, leave it be.” Many of us were taught this saying as a way to learn how to avoid contact with Poison Ivy (Toxicodendron spp.).
Word of the Week: Phytophotodermatitis
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 because of its ability to crosslink DNA to interfere with transcription 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 the 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. When you say the effect out loud, it sounds like “fido,” although dogs have nothing to do with it.
Skin blistering 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.
All in the Family
Psoralens are also found in several other members of the Apiaceae (carrot family). They are used as chemical warfare agents against herbivores intent on consuming parts of the plants.
The most notorious member of Apiaceae with high concentrations of psoralens is Giant Hogweed (Heracleum mantegazzianum) which has captured national attention in the past. However, giant hogweed has not become widespread in Ohio with confirmations confined to the northeast part of the state.
Wild parsnip is found throughout the state and is equally damaging. Of course, the name giant hogweed sounds more threatening. Wild parsnip just sounds like a vegetable gone wild, which it actually is!
Parsnips have been cultivated as a root crop in Europe for thousands of years. 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 between the two types.
It is theorized that the wild parsnip plants in Ohio represent escapes from cultivated types brought to North America from Europe and a reversion back to a wild type. The wild genes were always there but remained suppressed until revealed through natural selection shaped by herbivory.
As I noted in my BYGL Alert on poison hemlock, plant defense chemicals are most commonly secondary metabolites meaning that they don’t play a role in any primary plant physiological processes. However, secondary metabolites are not synthesized at no cost to the plant. Plants must dedicate resources to their production at the expense of other enterprises such as growth and reproduction.
In other words, if there is no need for parsnip plants to crank up their secondary metabolite synthesis machinery to defend against herbivores, the biennial plants can shift resources during their first growing season to form large taproots to store carbohydrates that will support second-year flowering plants with more flowers and seeds. The side benefit for us is large taproots make a tasty side dish if roasted to caramelize all of that extra stored sugar. There are many other recipes.
The non-native Parsnip Webworm (Depressaria pastinacella, family Depressariidae) was not originally imported into North America along with its vegetable host. The webworm focuses its entire attention on its namesake host; it doesn’t eat anything else. The webworms feed within silk webbing encasing the flowers and seed. Late instar caterpillars bore into and feed on the stems.
The webworm thwarts the plant's chemical warfare by excreting most of the plant toxins 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 protect against predators and parasitoids given that the webbing surrounds the caterpillars as they feed on the flower parts.
The parsnip webworm arrived in North America in the 1850s. Research has revealed that its arrival exerted selective pressure on wild parsnip to invoke its chemical arsenal.
Plants stored in herbariums before the arrival of parsnip webworm showed a significantly lower concentration of psoralens compared to plants collected after webworms had become established. Instead of behaving like a classical biocontrol agent by devastating its host, the webworms had increased the noxious nature of an already noxious non-native weed.
Wild Parsnip Life Cycle
Wild parsnip has a biennial life cycle. The first year is spent in the “vegetative stage” as a low-growing basal rosette. While in this stage, the plant produces a long, thick taproot to store carbohydrates to support the following season's production of flowers and seeds.
Plants “bolt” during the second year “reproductive stage” to produce erect multi-branched stems topped with yellow umbrella-like flowers. Of course, plants bolted long ago in southwest Ohio.
It’s important to keep in mind that the graphic above provides a generalized view of a biennial life cycle. In reality, there can be considerable variability in the timing of events meaning that the growth stages within a group of wild parsnip plants are seldom synchronized. It’s common for first-season vegetive plants to be mixed with second-season reproductive plants.
The literature notes that some plants may occasionally behave as monocarpic perennials spending more than one year in the vegetative stage before flowering once and then dying. This would help to explain the rapid rise in asynchronous life cycles in developing wild parsnip infestations.
The timing of seed germination may also affect what we see. While most of the seeds germinate in the spring, some will also germinate in the fall. As a result, first-year rosettes commonly range in size from small plants if seeds germinated in the spring to larger plants if seeds germinated in the fall.
Finally, wild parsnip is a prolific seed producer. Although the literature notes that seeds may remain viable for only around 4 years, management tactics must account for new plants arising annually from the “seed bank” until there are no longer any viable seeds to contribute to infestations.
Wild parsnip plants have celery-like leaves. The leaves are pinnately compound, branched, and have saw-toothed edges. Each leaf has 5 – 15 ovate to oblong leaflets with variable toothed edges and deep lobes.
During the first-year rosette stage, the leaves are confined to growing from a short stem near the ground. Once plants bolt in the spring, the leaves alternate on the flower stalks.
The mature flowering plants have a single, thick, deeply grooved, greenish-yellow stem that sprouts lateral branches topped with hundreds of clusters of the yellow umbellate flowers. Mature wild parsnip plants are shorter in stature compared to poison hemlock but still impressive at up to 4 – 5 ft. tall.
Be aware that certain cultivated members of the Apiaceae family may be mistaken for wild parsnip and vice versa. Two of the most commonly reported look-a-likes are Fennel (Foeniculum vulgare) and Golden Alexanders (Golden Zizia) (Zizia aurea).
Wild Parsnip Management
Unfortunately, wild parsnip is becoming more common throughout Ohio. Worse, this dangerous non-native weed is increasingly being found growing near people which increases risks to human health. Wild parsnip is becoming too widespread in Ohio to be eradicated from the state. However, infestations that present a clear and present danger to the public should be targeted for elimination.
Various online reports note that wild parsnip can be managed by mechanic removal (e.g., mowing) or hand pulling. However, the benefits must be weighed against the risks.
Hand pulling presents a direct route for exposure to the photosensitizing chemicals in the sap. While chemical-resistant gloves will prevent direct contact with the skin, long-sleeved shirts may allow the sap to soak through.
String trimmers and brush saws should not be used. Both can fling sap.
Mowing just after plants begin to bolt but before they bloom can be highly effective. However, most mowers will pass over the low-growing rosettes and even if they are cut, the plants will produce new stems. But equipment operators should approach mowing large wild parsnip infestations with caution. Equipment with unshrouded blades should not be used. PPE should be considered even if brush or flail mowers are shrouded.
The graphic below shows the biennial life cycle with an arrow pointing to the best time of the year to mow or spray. Note that management strategies targeting the second-year plants after they start bolting may have a limited impact on first-year rosettes. This means wild parsnip infestations are not likely to be eliminated in a single season.
Mowing after seeds are produced is a good recipe for spreading wild parsnip. The seeds can easily hitchhike on mowing equipment, particularly atop mower decks. Heavy infestations along roadways and within other rights-of-ways provide anecdotal evidence that human-assisted spread has played a significant role in spreading this non-native invasive weed in Ohio. Mowers should be thoroughly cleaned before moving to new locations if mowing must be done after wild parsnip has produced seeds.
Given the problematic nature of controlling wild parsnip by physical removal, herbicides may be the safest option. Fortunately, the non-native weed is susceptible to several selective and non-selective postemergent herbicides.
As with mowing, herbicide applications should be made after plants begin to bolt in the spring but before flowering. The time window for making effective herbicide applications can vary from year to year depending on springtime temperatures. A warm spring means the window may open and close rapidly which emphasizes the need to identify wild parsnip and make management plans long before infestations reveal themselves by yellow blooms.
Keep in mind that non-selective herbicides such as glyphosate (e.g. Roundup) can also illuminate 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.
Selective post-emergent herbicides will preserve competitive plants. Herbicides effective against wild parsnip include clopyralid (e.g. Transline), metsulfuron (e.g. Escort XP), triclopyr (e.g. Triclopyr 4), and combination products such as those that contain 2,4-D, mecoprop, and dichlorprop (e.g. Triamine). However, herbicide timing is critical. It’s too late for herbicide applications to prevent seed production this season in Ohio.