Davey Tree Flipbooks

16 GIANT HOGWEED (Heracleum mantegazzianum) PLANT DESCRIPTION Giant hogweed is a large broadleaf biennial weed typically found in late spring to midsummer as an 8–10 foot tall upright, single-stemmed, branching plant with flat- topped, white flower heads. Giant hogweed can also be seen as a seedling in the spring. Larger than most other herbaceous plants, it can occasionally attain a height of 20 feet. LEAF: Giant hogweed has very large, broad, jagged leaves that are generally wider than they are long. Leaves are hairless and grow in groups of three. STEM: The main stem is very large, hollow, and is covered in hairs. Stems also have vertical ribs and purple spots and develop branches as they prepare to flower. FLOWER: The white flower heads at the ends of branches are composed of many small flowers arranged in an upright, flat- topped or umbrella formation. SEED: Seeds develop as flat, round, green disks covered in hairs that eventually turn brown. REPRODUCTION: Seeds are the primary means of dispersal. Most seeds drop around the parent plant, but can also be spread by wind, water, and animals. A mature giant hogweed plant is capable of producing thousands of seeds that remain viable for up to five years. WHY IT IS NOXIOUS: Use caution around giant hogweed; contact with sap can cause extreme skin blistering, especially when combined with sunlight exposure. Giant hogweed can displace desirable plants and create sight distance and safety concerns if allowed to grow near the road. SEASONAL IDENTIFICATION WHEN TO FIND: Giant hogweed is most noticeable in summer after flowering and seed set. This weed is larger than most other plants. It has very large leaves, stems, and white flat-topped, umbrella-shaped flower heads. SPRING IDENTIFICATION = EASY TO MODERATE Seedlings and young plants can be found growing low to the ground among other plants. The leaf is the best identification feature at this time. Stems of second year plants begin to develop in late spring. SUMMER IDENTIFICATION = EASY Stems develop hairs as the plant grows tall. Key features are the large stems, leaves, and flat-topped white flower heads. Flat, round, hairy green seeds develop in late summer and are often found on plants that are still in flower. FALL IDENTIFICATION = EASY Dead giant hogweed can be seen standing with stout, hollow stems and seed heads that are still intact. This weed skeleton is taller than most other weeds. WINTER IDENTIFICATION = MODERATE Giant hogweed's main stem can persist into winter. If found, look around for seedlings and rosettes that may overwinter. SIMILAR-LOOKING SPECIES WILD CARROT (Daucus carota) flower heads are also white, flat topped, and umbrella shaped, but they are much smaller and much more densely arranged. The stem is solid and covered in hairs. POISON HEMLOCK (Conium macuulatum) flower heads are white but are round or globe shaped and much smaller. WILD PARSNIP (Pastanica sativa) flowers have a similar form but are yellow. After it dies, wild parsnip most closely resembles giant hogweed; however, the stems are smooth and have larger raised vertical ribs. LOW HIGH ohio distribution roadside distribution growth seeds ability to dominate difficulty of control WHERE TO FIND: Giant hogweed is not prevalent in Ohio but has been located in isolated stands in eastern Ohio and neighboring states. Giant hogweed prefers moist soil around waterways and is tolerant of shade but can also grow in full sun in dry soils around ditches. CONTROL METHODS: MECHANICAL: Hand pull using gloves. Use caution! Giant hogweed is toxic to humans and can burn the skin causing blisters. Mow repeatedly to reduce height or reduce seed production. CHEMICAL: Make directed foliar applications in spring to summer with non-selective herbicide. CULTURAL: Intensive grazing by sheep or goats can weaken and reduce giant hogweed colonies. Plant tall fescue in fall to provide competition. RECOMMENDATION SUMMARY: Giant hogweed is giant and toxic! Giant hogweed is rarely found in mowed areas. Always use caution and avoid contact with plant sap. For best control results, make directed foliar applications with non-selective herbicide up until flowering. Later season applications will still provide some control even after seeds begin to develop. Plant sap is toxic! Use caution when hand pulling to control or cutting to reduce seed or plant height. Giant hogweed is so large it displaces other vegetation leaving bare patches. When controlled, the treated area should be planted with tall fescue to avoid leaving bare patches and provide competition. Failure to control giant hogweed allows mature plants to produce thousands of seeds that remain viable for up to five years, resulting in localized spread of this large noxious weed. biennial x 2 RIVM LEVEL OF CONCERN

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