CINCINNATI, Ohio -- If Socrates were alive, the first thing he would tell you is "Stay away from poison hemlock!" The toxic weed, reputedly used to execute the famous Greek philosopher in 329 B.C., has become common in Ohio during the past decade, posing a health risk to suburban dwellers, hikers, farmers and anyone who might come into close contact with it. "Poison hemlock has become one of the most prevalent weeds in Ohio, especially along highways in the Columbus area and between Dayton and Cincinnati," said Joe Boggs, Ohio State University Extension South District horticulture specialist and Hamilton County agent. "It was a bit of a landscape oddity a few years ago, but now you don't have to look hard to find it." A biennial member of the carrot family, poison hemlock, Conium maculatum-- which has nothing to do with the evergreen hemlock tree -- can cause respiratory failure and even death when ingested by humans or animals. In addition to roadsides, poison hemlock thrives along borders of fields, railroad tracks, irrigation ditches, stream banks, meadows and waste areas. Flowering takes place between June and August during the second year of growth. "Very wet springs in Ohio during the past two years have favored the spread of poison hemlock," Boggs said. "During the first year, the weed is easier to control. But in the second year, seeds appear and spread really easily, surviving chemical applications." Sometimes planted as an ornamental, poison hemlock may grow to a monstrous size, towering up to 10 feet. It produces small white flowers that are typical of its family. The herb has a smooth, purple-spotted stem; dark, glossy-green, fern-like triangular leaves; and tiny, ridged fruit. Underground, it has a fleshy white taproot. Both the leaves and roots have a disagreeable parsnip-like odor. What makes this European native so dangerous, Boggs said, is the presence of toxic alkaloids, including coniine and gamma-coniceine. Although these substances are more concentrated in young leaves, stems and seeds, all parts of the plant are poisonous and should not be touched. "Two years ago, a couple of school kids from the Cincinnati area ended up in the emergency room after playing with poison hemlock branches," Boggs recalled. "This is quite a concern because several poison hemlock-related deaths have been reported around the country." In most cases, poisoning has occurred from people mistaking poison hemlock's roots for parsnips, its leaves for parsley or its seeds for anise. The difference between wild parsnip and poison hemlock is that parsnip has deep-yellow flowers. A master of disguise, poison hemlock also resembles spotted water hemlock, which is equally poisonous, and the non-toxic wild carrot (also known as Queen Anne's lace). But unlike wild carrot, poison hemlock does not have hairs on its stems or leaves, nor does it grow branching, feathery bracts beneath its flower clusters. Boggs pointed out that the surge of housing developments in former farmland near urban centers is exposing many people who know little about plants to poison hemlock. "Poison hemlock is not considered a big deal within the farming community because they know how to recognize it," Boggs said. "The problem is with city people who are moving into suburban areas and don't know the difference between poisonous and safe plants. They might end up having poison hemlock right in their back yard." Toxic plant exposure is the fourth most common cause of nationwide poisoning, accounting for more than 100,000 annual reports to poison control centers -- more than 80 percent of them in children. There are more than 700 plant species in the United States that can cause poisoning if a part of the plant is swallowed. OSU Extension is the outreach arm of Ohio State's College of Food, Agricultural, and Environmental Sciences.