Poison Hemlock

It’s that time of year in Northwest Ohio that two toxic plants are coming into flower. Poison Hemlock and Wild Parsnip are both in full flower but are toxic in different ways.

This week's recording of the OSU Turfgrass Team Times. This edition was recorded on Friday, June 6, 2025.

Poison Hemlock (Conium maculatum) and Wild Parsnip (Pastinaca sativa) have reached developmental stages that place these weeds outside of effective management options in southwest Ohio. Wild parsnip is producing seeds while poison hemlock is in full bloom meaning seeds can still develop on plants that are cut or sprayed with an herbicide.

Poison hemlock overwintering rosettes are already coming back to life. Now is an easy time to look for established poison hemlock plants because they are going to be greener than most any other plants that are not growing.

As I drive down the freeway, I am seeing Poison Hemlock everywhere. What’s a gardener to do?

Poison hemlock is currently in the growth stages in southern Ohio that are susceptible to early-season management options. Seeds have germinated and last season's rosettes are beginning to "bolt."  Eliminating these plants now can significantly reduce infestations.

Poison hemlock (Conium maculatum) is in full flower in much of Ohio with some plants showing signs of senescence in the southern part of the state meaning flowers are mature and seeds will soon follow. This means it’s too late to reduce the seed bank by targeting flowering plants with herbicides or mowing.

Poison hemlock and wild parsnip are currently in a growth stage that makes them susceptible to early-season management options. Targeting these dangerous plants now will prevent flowering and seed production.

Poison hemlock (Conium maculatum) is one of the deadliest plants in North America. Wild parsnip (Pastinaca sativa) sap can produce severe, painful skin blistering. Both are commonly found growing together in Ohio and both are beginning to bloom meaning the clock is quickly winding down for controlling these non-native nasties.

Poison hemlock and wild parsnip are currently in a growth stage that makes them susceptible to several management options. Targeting this stage for control will prevent flowering and seed production.