Symptoms of poisoning can occur within 30 minutes to 2 hours, depending on the animal, and include nervousness, trembling, muscular weakness, knuckling at the fetlock joints, loss of coordination, dilation of pupils, a weak and slow heartbeat, coma, and eventually death from respiratory paralysis. Poison hemlock can cause fetal deformation if a pregnant animal eats the plant. Toxins can pass into the milk of an animal, affecting the offspring, and in the case of dairy cattle can affect the safety of milk that humans consume. To reduce livestock exposure to poison hemlock's toxins, prevent grazing when the plant is the only available green forage, such as in early spring. Habitat and Associated Plants Poison hemlock grows at low elevations bordering pastures and cropland and gradually invades perennial crops.
Poison hemlock tends to be more competitive in wet soil conditions but it can survive in dry sites.
It is common for the weed to first spread downstream and then move to dryer upland sites. Seed Germination, Productivity, Longevity, and Dispersal Poison hemlock reproduces solely by seeds. Seeds often drop next to the parent plant and regenerate, forming dense stands of the invasive plant. The seeds are not well developed for long range dispersal so the plant relies on seed spread primarily by water, birds and rodents. Since the plant stalk persists through winter, seeds have the potential to disperse from September to late February.