Watch out for those annoying weedy pests

July 15, 2013
Poison hemlock can be identified by its small white flowers.

A host of unwanted plants, including poison hemlock, hogweed and Canada thistle have been pestering residents of Southwestern Pennsylvania this summer.

“It’s a little scary to live in the world of weeds,” said Dr. Bill Curran, weed specialist at Penn State Extension. “When I drive around, I notice things most people don’t notice,” he said. “You almost have to become a little numb or it would bother you.”

Poison hemlock is a plant that can be identified by its small white flowers. It is an invasive biennial species that has made itself very comfortable in North America since it was first introduced for its pleasant blossoms.

“It’s been around for a long, long time,” Curran said. “But in the last five to seven years, I’ve noticed it has become much more common.”

Curran said poison hemlock is most often viewed along roadways and in meadows. During its second year of its biennial cycle, a bolt 3 to 7 feet tall protrudes from the weedy plant base. White flowers blossom and then turn to seed before the plant dies.

Although it is poisonous when ingested, the weed poses little threat to humans and livestock this late into the summer. Curran said grazing animals avoid the plant in the field, and it is not very tolerant to mowing, so hemlock does not thrive in lawns or alfalfa fields. However, farmers who suspect they may have seen the flowery stalks in the past month or so should inspect their hay supplies.

Poison hemlock was brought to the continent by colonists, but quickly spread to the wild. It is quite common in Europe. The deadly seeds and roots were in the potion used to execute Socrates.

Despite its deadly potential, hemlock has some better-known relatives.

“It’s a member of the carrot or parsley family,” Curran said. “Some of our most common vegetables are in that family.”

The family apiaceae also includes edibles such as anise, cilantro and celery along with other poisonous weeds such as wild parsnip and giant hogweed.

“Giant hogweed is a noxious weed,” Curran said. “There’s an active program to try and eradicate it. But it is also an escaped ornamental someone brought over because they liked pretty flowers.”

Giant hogweed is a ruddy plant that can cause itching when touched. It blossoms into an umbrella-shaped cluster similar to that of poison hemlock.

Curran said hogweed is among a number of unwanted plants that might be becoming more common in Pennsylvania.

Among them, Canada thistle may prove especially problematic if it continues to spread.

“It’s not poisonous, but it’s a very aggressive plant,” Curran said. “It’s a perennial and lives for more than two years and spreads quickly through both seed and rhizomes. So, it’s very tricky.”

Rhizomes are underground structures that sprout from a plant’s root system to produce clone plants elsewhere in a field. Curran said this time of year, Canada thistle is close to going to seed. The rugged, spiny-leaved plant also produces a chemical that irritates the skin. It sprouts a burst of purple or pink flowers that later reveals feathery spores that distribute through the wind.

“Now that it’s flowering, get out there and mow it back,” Curran said. “You don’t want it to go to seed; you want to get it under management. Now is the time to get it knocked back.”

For more information on dangerous plants, contact the Penn State Extension Washington office at 724-228-6881.



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