Giant Hogweed Biology
Giant Hogweed Growth & Development (Information pertaining to control)
Giant hogweed plants are long lived perennials (persisting through the year or through many years). Often, the tall majestic plants seen in brochures and in the media are more than five years old and can be as old as 25 years. It can take a giant hogweed plant three to four years before it builds the energy to produce a flower. After a flower is produced it will most likely die, however, giant hogweed is perennial in nature and if the main root has developed side shoots they could grow new plants the following year.
Giant hogweed plants are most commonly recognized when they are mature and flowering. Since giant hogweed plants may not have flowers for a couple years of development one must also look for the numerous younger plants with large basal leaf rosettes (some with stems and leaves) when identifying plants to control. In the first few years of growth, the leaves and stem of a non-flowering plant will die back over the winter. After the mature plant flowers and sets seed, the whole plant including the root will die. If the flowering plants are damaged or cut above the root before the flowers open, the plant may survive a couple more years. As a rough guide, giant hogweed plants start growing in early spring, flower in June-July and set seed in August. Giant hogweed plants have a long branching taproot up to 24 inches (60cm) long and 6 inches(15cm) in diameter at the crown. Their large taproots store substantial below-ground resources which can give some defense against physical and chemical control practices.
Seed Viability & the Importance of Seed Removal
Giant hogweed plants reproduce by seed - an average plant produces anywhere between 20,000 and 100,000 seeds. Since most seeds fall within a few meters of the parent plant, seedlings develop under very crowded conditions and thus seedling mortality is high. The majority of seeds (95%) are found within the top two inches of the soil layer and 30 feet from a colony. The seeds are dispersed short distances by wind but can dangerously travel longer distances by water (can float up to three days). Seeds may stay alive in the seed bank for more than five years. Since the dispersal of giant hogweed is almost entirely by seeds it is VERY important to prevent the plant from flowering and setting seed.
Habitat & Competition
With their tremendous growth, large leaf area and prolific seed production, giant hogweed plants are able to out compete and replace native vegetation. Giant hogweed plants shade out surrounding vegetation with their giant leaves and tight growth pattern. Bare soil is created below the plants, which leads to soil erosion in winter. This in conjunction with the loss of plants with finely branched roots able to hold the soil is of special concern along slopes and stream banks. Giant hogweed typically spreads first along rivers or stream corridors as the seeds are able to disperse longer distances. Once the plants become more prevalent they are able to spread throughout a variety of habitats.
Giant hogweed grows in wet areas along streams and rivers, on waste ground, near houses, in vacant lots, and along railways and roads. It prefers moist soil and can quickly dominate ravines and stream banks.