Poison Hemlock Identification and Management Resources

Poison hemlock (Conium maculatum) is a biennial plant, meaning it takes two years for the plant to complete its lifecycle.  During the first year, the plant grows as a short, less than 18 inches in height, rosette with its leaves arranged in a cluster at the base of the plant, similar to a dandelion's growth habit.  In the second year, in March and April, the plant will still appear as a low-growing rosette but as spring progresses the plant will bolt/flower and send a flowering stalk from the base of the plant until it is about four to six feet in height.  The flowers are umbrella-shaped and white in color.  At this point, it is most often confused with wild carrot (Queen Anne's Lace) or wild parsnip.   Wild carrot is usually 18 to 24 inches in height when flowering compared to poison hemlock's towering height of four to six feet at the same stage.  Poison hemlock will be less than 24 inches in height when it is not flowering.  The other key difference is that wild carrot has numerous, fuzzy white hairs along the stems (Figure 1).  Wild parsnip will reach the same height as poison hemlock during flowering but will have yellow flowers and no purple coloration on the stems.

Figure 1: Close-up of a wild carrot plant showing numerous white hairs along the stems.  Also, note the absence of red or purple spots on the stem. Photo: Liz Bosak, Penn State Extension

The key characters to look for are red to purple spots along the stem and a complete absence of hairs.  It is critical to look very closely because poison hemlock plants vary widely in the number and intensity of purple spots on the plant.  Coloration, when it is in the rosette stage, can be very faint, just light reddish/purplish spots that you will only see if you look closely (Figure 2). 

Figure 2: Rosette, 1^st year, stage of poison hemlock.  Note that the red/purplish spots are difficult to see at this distance.  Photo: Liz Bosak, Penn State Extension

Remember to wear gloves when handling poison hemlock!  Once the plant begins to flower, the spots typically become very purple and can cover most of the flowering stem/stalk (Figure 3).  To learn how to identify poison hemlock and its common lookalike wild carrot, watch this Penn State Extension video.

If you have any more questions about identifying poison hemlock, please contact your local county Extension office. 

Figure 3: Flowering stage, 2^nd year, of poison hemlock.  Observe the pronounced purple mottling/spotting along the flowering stalks.  Photo: Liz Bosak, Penn State Extension

Management of this weed is particularly important for livestock and hay producers.  Small populations can be removed by hand digging but remember to avoid contact with your skin by wearing gloves.  For more information about managing poison hemlock, please see:

• Livestock Producers Should Recognize and Manage Poison Hemlock by Dwight Lingenfelter.