Plants that you don't want to touch

The following article was written 4 seasons ago, but in light of the recent concern of species that are poisonous to the touch like Giant Hogweed and Wild Parsnip, we thought it would be worth revisiting. The risk of experiencing a severe skin reaction to the following plants depends on how much contact you make with the plant. If you casually brush up against the foliage, it is unlikely that you will experience anything, but if you are extremely aggressive in breaking up vegetation and handling their leaves and stalks without any protective clothing, then the results can be quite uncomfortable.

Poison Ivy (Rhus radicans)

Likely the most common and familiar of these three plants, poison ivy is widespread throughout Ontario in forests, fencerows, meadows, roadsides and natural areas.

Plant form: Grows in patches on the ground, or climbs fences, trees or buildings. The tall climbing form is mainly confined to areas along Lakes Erie, Ontario and the lower Ottawa valley

Life cycle: Perennial, spread by seeds (dispersed by birds) and by underground woody rhizomes. Stems also root in contact with soil.

Appearance: A deciduous woody vine with 3 leaflets (hence the saying "Leaves of three - let it be"). The longer petiole on the centre leaf is an identifying characteristic. Leaves are shiny green all summer, turning bright red in the fall. Clusters of white berries are produced by late summer and may be retained on the stems all winter.

Caution: All plant parts are poisonous, including roots, stems and leaves. The oil resin may be released onto clothes, pets, or prunings. Sensitive individuals need to use extreme care year-round. Inhaling smoke from burning plants can cause severe reactions in breathing passages.

May be confused with: Fragrant sumac - which has 3 leaflets, but no extended middle petiole; Virginia creeper - which has 5 leaflets instead of 3.

Figure 1 - The three leaves of Poison Ivy, note the centre leaf has the longer petiole (stalk)

Figure 2 - Berries of Poison Ivy

Figure 3 - The 5 leaflets of Virginia creeper, which often gets confused with poison ivy

Giant Hogweed (Heracleum mantegazzianum)

This member of the Carrot family has gained notoriety from recent reports in the media. It's a distinctive looking plant in the landscape now as a mature plant along roadsides, streambanks, and waste areas.

Life cycle: Perennial, reproducing only by seed. Giant Hogweed is a monocarpic plant, meaning it only produces seed once in its lifetime.

Appearance: Within an established stand there will be great variability in plant age and size with younger plants being less than 30 cm in height while older plants can be as high as 5 m tall, with a hollow reddish- purple stem up to 10 cm in diameter. Flowers from June to August, generally plants are taller than 2 m when flowering, with a compound umbel up to 1.2 m across, with 30 to 50 branches, each with an umbel with 30-20 flowers.

Caution: Giant hogweed contains furocoumarins (psoralens), which make human skin hypersensitive to sunlight, causing cellular damage at the surface. They absorb long-wave ultraviolet light and become photodynamic.

Figure 4 - The large compound leaf of Giant Hogweed

Figure 5 - The red/purple blotchy and hairy stem of Giant Hogweed compared to the relatively green and hairy stem of Cow-parsnip.

Figure 6 - A very tall and mature stand of Giant Hogweed.

May be confused with:

Angelica, but Angelica is void of any hairs on the stem and their compound leaves contain numerous leaflets (See Figure 7)

Figure 7 - The leaf of the non-poisonous Angelica

Figure 8 - The purple stalk of Angelica, that is hairless.

Wild carrot, but Wild carrot is much smaller and the compound leaves have a "lacy" appearance that looks identical to cultivated carrot.

Figure 9 - The lacy compound leave of wild carrot

The native Cow-parsnip (Heracleum maximum), a close relative of Giant Hogweed and also contains the furocoumarins that cause skin dermatitis. Cow parsnip is much smaller in height, lacks the purple blotchiness on the stems and has compound leaves that contain distinct lateral leaflets (Figure 10).

Figure 10 - The leaf of cow parsnip.

Wild parsnip (Pastinaca sativa)

This wild plant is very similar to cultivated parsnip. Wild parsnip occurs throughout Ontario on roadsides, meadows, fencerows and abandoned fields.

Plant form: Medium plants usually 0.5 to 1.5m in height. The whole plant has a distinctive parsnip odour.

Life cycle: Biennial, reproducing only by seed.

Appearance: A highly branched plant, with hollow green stems. The pinnately compound leaves, similar to parsnip, are green and will have 2 to 5 pairs of opposite sharply toothed leaflets. The yellow flowers are held high above the leaves, clustered in compound umbels up to 20 cm across.

Caution: Similar to giant hogweed, wild parsnip also contains furocoumarins which can cause severe skin dermatitis, when activated by sunlight.

Figure 11 - Leaf of wild parsnip

Figure 12 - Wild parsnip's yellow flower