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Poison Hemlock Spreading In Pa.

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Poison hemlock spreading in Pennsylvania

Updated: 9:48 AM EDT Jun 9, 2021

State agriculture officials are warning Pennsylvanians about an invasive and dangerous plant that's blooming right now.

The poison hemlock plant is poisonous to both people and animals.

Penn State Extension says the plant has a smooth, hollow stem and can be identified by purple spots on its stems.

It tends to grow near meadows and streams and smells like parsley when it is crushed.

Pennsylvania's botany and weed specialist says it spreads easily, and there are ways you can get rid of it safely.

"I do advise people to wear long sleeves, long pants, gloves if working with it-- Preferably don't mow it if you can avoid it because people can breathe in those particles," said Trilby Libhart, Pennsylvania botany and weed specialist.

The state also says another plant, the giant hogweed, has almost been eradicated in Pennsylvania, but experts say a lot of people mistake cow parsnip for it.                                                                                                                                                                                                                                            SEE VIDEO REPORT     ;    https://www.wtae.com/article/pennsylvania-poison-hemlock-spreading/36672792

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Poison Hemlock: Facts About this Early Season Weed

Poison hemlock resumes its growth early in the spring. Learn more about this weed that is widespread across the state.
Poison Hemlock: Facts About this Early Season Weed - News 


Poison hemlock as it begins vegetative spring growth. Source: D. Lingenfelter, Penn State Weed Science images

Plant family: Poison hemlock (Conium maculatum) is in the carrot or Umbelliferae family and the leaves and foliage resemble carrot and parsley.

Look-alikes: Wild carrot (also called Queen Anne’s lace), wild parsnip, spotted water hemlock, and purple-stemmed angelica can sometimes be confused with poison hemlock because they are in the same family and have some similar features. Cow parsnip and giant hogweed are also in the same family; however, they are much larger in stature and should not be easily mistaken with poison hemlock.

Lifecycle: Poison hemlock germinates from seed and is a biennial plant with a basal rosette of leaves during its first year. Once it overwinters, in late April/early May, it bolts into an erect branched plant producing prominent white flowers in an umbel generally in June and July. It sets and drops seeds soon after. This weed only reproduces by seed and its taproot dies as the main plant desiccates.

Vegetative stage of poison hemlock. Note purple blotches on the main stem. Source: D. Lingenfelter, Penn State Weed Science images.

Characteristics: In addition to its fern-like, glossy green, lacy leaves, the plant has a hairless, hollow main steam with purple blotches/spots which branches. Its large flower structures are composed of many smaller white flowers. Plants typically grow 2 to 6 feet tall and have a parsnip-like odor when the leaves are crushed.

Poison hemlock at flowering. Source: D. Lingenfelter, Penn State Weed Science images

Poisonous traits: The sap can cause dermatitis (skin irritation) on some people. So be sure to wear protective clothing and gloves when handling the plant or when weed-whacking. It is poisonous when ingested by humans and livestock. The plant contains several closely related pyridine alkaloids with the main one being coniine, a colorless, volatile, and strongly alkaline oil. All parts of the plant are poisonous, and some studies have shown toxicosis at 0.25% fresh wt. (of the animal’s weight) for horses and 0.5% for cattle. That would be 2.5 to 5 lb. of material per 1000 lb. animal. Mature seeds are the most poisonous. Significant poisoning can result in muscle paralysis and suffocation.

Habitat: It grows along roadsides and in no-till fields, fencerows, pastures, fallow areas, and waterways.

Control: Individual plants can be easily dug out with a shovel. Larger infestations can be cut with a mower or string trimmer or may require the use of a herbicide. Several herbicides are effective for control. Herbicide applications are most effective when they are sprayed in the fall when poison hemlock is in the rosette stage or before it bolts in the spring. Effective herbicides include 2,4-D + dicamba, Crossbow (2,4-D+triclopyr), or glyphosate as a spot treatment.

Other interesting facts: Poison hemlock is native to Europe, northern Africa, and western Asia and was introduced to North America as an ornamental garden plant. In the annals of history, it was noted that the Greek philosopher Socrates chose to drink poison hemlock tea for his execution.

Additional Poison hemlock information:

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I have whined steadily about this stuff which is extremely toxic to animals and mildly caustic to humans. At an early stage, it can be sprayed pretty easily, but as it spreads over miles of roadway, then it becomes a water contaminant problem to be dealt with.  Farmers can be losing animals to it or even pets if they seem to like chewing on it.

It's one of those "ignore it as long as possible" situations.



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Many of the plants we grow in our gardens are poisonous. You can't eradicate them all. Don't eat the weeds or the garden plants if you don't know what you are doing. A word to the wise. Queen Anne's lace blooms much later and has that little black/purple floret in the center. Enjoy it. It is very pretty. 

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