Does mowing down the Golden Parsnips sustain them?

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by Fred Schueler & Aleta Karstad

Anyone driving Hwy 416 in early July would have wondered why its tourist name isn’t “The Golden Parsnip Highway,” and long stretches of county roads, such as County Road 20 between East Oxford and Oxford Station, have also been brilliant with the blooms of this escaped garden vegetable. Parsnips are biennials, growing for one year, and storing reserves in the carrot-like taproot. In their second summer, they bloom with yellow flowers in flat umbels on a tall stalk, and then die after setting seed.

The sap of Parsnips can cause what is termed “phyto-photo-dermatitis”, if it gets on tender skin which is subsequently exposed to the sun. This is a severe rash and blisters, quite painful, although not itchy. These blisters are much longer-lasting than those of Poison Ivy, and the scars last for years. Different people seem to have different degrees of sensitivity to this effect.

It seems that, over the past 30 years, the “Wild” or “Poison” Parsnip has become more widespread in Ontario, and it’s likely that the combined increase in populations of Parsnip, and the recent prevalence of vegetation-shredding machinery, have increased the rate at which sensitive skin simultaneously comes in contact with Parsnip sap and sunlight. Operators of hand-held rotary weed trimmers are especially likely to have painful and prolonged blistering when their skin is sprayed with Parsnip sap on a sunny day.

Parsnip flowers are a wonderful nectar source for flies and other small insects, and Parsnips are a host plant for gorgeous Black Swallowtail caterpillars, which mature as one of our handsomest butterflies.

The major “natural enemy” of Parsnips is the Parsnip Webworm Moth, Depressaria radiella, whose caterpillars blankets the flowerheads with webby silk, and feed on the developing seeds. The toxins that cause the phyto-photo-dermatitis are furanocoumarins, which serve the plant as a defence against the webworms.

Because of the alarm about the toxicity of Parsnips, counties spray herbicides to kill both roadside Parsnips and other broad-leaved “weeds”, while issuing warnings about the frightening danger that Parsnips pose. The Friends of Lanark County have extensively researched the problems of these herbicidings, which (google their name) is on their website.

Deliberately introduced biological control agents, like the tiny Galerucella beetles that have done a good job of knocking back Purple Loosestrife in our meadows and wetlands, often bring an introduced weed under control, but the Parsnip Webworm has been overlooked as a biocontrol agent, because it was already here. The word “webworm” does not appear on the Friends of Lanark County website, and the control the webworms could provide isn’t mentioned in the various Parsnip control efforts. The webworms have only one generation per year, and pupate in the stem of the plant, so control by mowing, with or without previous herbiciding, results in new leaves and flowers sprouting from the cut stems, producing a second, smaller but adequate, crop of seeds, while the webworms are killed off.

A mature Parsnip Webworm emerging from its webby tent to feed on flowers and tender green seed capsules. Bishops Mills, 23 July 2021

Stands of Parsnip with lots of webworms are ragged and brown from the caterpillars’ activity, while those along the 416 and other roads are uniform and golden, and it seems to me that this difference is due to the control activities knocking back webworm populations. I’ve measured this in two different stands: a 20m long, 1m wide stand between a field of wheat and County Road 18 had one webworm web per metre on 20 July, while one 2m tall plant in our “brushy oldfield” in Bishops Mills, had 20 webs, destroying about 80% of the flower area, and other plants in the site were 19 with webs, many with 50% of the flower area webbed, and four, mostly under overhanging tree limbs, without any webs on 19 July. Both of these surveys were early in the flowering season, so webbing would be expected to increase as the caterpillars grow.

If it’s necessary to remove Parsnips from an area, the plants can be pulled up in the evening, or on an overcast day, while wearing clothes. I’ve managed Parsnip in neglected gardens which were yellow with their bloom, by working after sunset, wearing gloves, and piling the pulled plants so the seed heads are off the ground. If the seed can’t fall down onto bare soil it won’t (in my experience) germinate.

One Parsnip plant isn’t a threat, nor is contact with unbroken leaves, because the dermatitis isn’t contact dermatitis, but is due to the sap from broken stems and leaves. It’s the combination of sap+sun that’s the problem. Extensive stands can be repeatedly mowed, when the Sun isn’t out (and washing up afterwards): mowing only once will just result in the sprouting of smaller flowerheads.

The tastiest way to diminish any threat from the sap of the tall second-year plants is to dig and eat the roots of the first-year plants after frost. These are just as edible as their domesticated cousins, though the daughter’s infant name for them -“Rattails” – suggests their average size. The leaves are also edible as a potherb, and in internet discussions it’s only those who haven’t tried them who are spooked by potential toxicity.

If, however, no one is going to crush the plants, it would be more interesting to leave a stand of Parsnips alone, leave the stems standing through the winters, see how much of a webworm population builds up, and to what extent it diminishes the Parsnips in subsequent years. I’ll be interested to hear of anyone’s experiences with Parsnip or webworms at bckcdb@istar.ca.

 

1 COMMENT

  1. Along the lines of “it would be more interesting to leave a stand of Parsnips alone, leave the stems standing through the winters, see how much of a webworm population builds up, and to what extent it diminishes the Parsnips in subsequent years,” in 2022 the stand in “our brushy oldfield in Bishops Mills” has many fewer Parsnips than last year, and these are fairly heavily web-wormed, and a small stand near one of our porches had 2m tall plants, very heavily web-wormed, which Clay Shearer has put up on iNaturalist – https://inaturalist.ca/observations/126466010

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