Comfrey is a perennial plant with thick, fleshy roots. The plant self-propagates through seed and root division, and the large root system can run deep in the soil. New plants emerge from the smallest piece of root when severed from the parent. This attribute makes killing comfrey a challenging task. Constant attention must be paid to hold the plant in check while eradicating it from the growing area. Comfrey begins its seasonal growth cycle in early April, and by late May the plant is well on its way to flower production.
Mix the glyphosate according to label instructions at the chemical's heaviest concentration. The thick fleshy roots of comfrey store all the food required for early spring growth; the goal is to force as much chemical to the food storage area of the plant.
Spray the emerging crown of leaves to each individual comfrey plant in early April. Concentrate the spray to the centre cluster of leaves. The leaves will show signs of wilting from the chemical application.
Keep the plants mowed to a low height of 3 inches or less in order to weaken the chemically treated plant and prohibit bloom formation.
Apply a second glyphosate treatment three weeks after the first application. Keep the plant mowed to a low height between herbicide treatments. Continue subsequent treatments throughout the growing season. Successful killing of comfrey will show itself in the following spring when no crown of leaves emerges from the soil.
Do not dig up comfrey roots. One missed placed portion of root may spread the plant to unwanted areas. Keep people and animals away from all areas treated with chemicals.