Water hemlock, Cicuta maculata L., is an invasive, poisonous weed that reaches up to 10 feet tall and grows back year after year. The plant is unsightly when it occurs in lawns and home landscapes, and it is poisonous to pets and humans. Water hemlock is even more of a problem for farmers since the weed can kill livestock if consumed. Killing water hemlock involves removing the underground root system.
Put on gloves, long sleeves and trousers before treating or handling water hemlock to prevent contact with the skin.
Water the location where the water hemlock is growing to a 3- or 4-inch depth using a garden hose. This makes removing the weeds easier. If the ground is already damp, as is often the case where the weed grows, do not add additional water.
Locate the largest clumps of water hemlock and dig them out of the ground with a shovel or a hand spade. Keep the blades 12 inches away from large weeds and 6 inches away from smaller ones so that you do not cut through the root system.
Fill a garden sprayer with one of the following herbicides: phenoxy, glyphosate, 2,4-D and picloram. Dilute the chemical with water as instructed and then spray each remaining water hemlock weed with the mixture until each is saturated. Follow all safety precautions recommended by the manufacturer.
Examine the ground closely for any trimmings or plants that were dropped and place them into the trash bag as well. Seal up the bag and throw it into a dustbin for disposal.
Inspect the site once per week for the remainder of the growing season to identify any small water hemlock seedlings that sprout up. If seedlings are detected, use a hand spade to dig up the roots and dispose of them in a trash bag.
You can kill water hemlock any time of the year, but the University of Nevada, Reno, Cooperative Extension notes it is easiest to control in the spring.
Contact a doctor immediately if your skin comes in contact with the hemlock or any of its parts such as its sap. Do not let children or animals near hemlock.