Hogweed
Other names
Cow parsnip
Latin names
Heracleum sphondylium L.
Weed Type
Biennial Weeds
Occurrence
Hogweed is a biennial, or monocarpic to polycarpic perennial native in grassy places, along hedges, on rough ground, roadsides and banks. It may show a preference for chalk soils. Hogweed is chiefly a problem on pasture but may encroach onto arable land from the hedgerow or headland. However, although common in the hedge bottom it is rarely found further than 2.5 m into the arable field. In a survey of weeds in conventional cereals in central southern England in 1982, hogweed was found in 2, 3 and 1% of winter wheat, winter barley and spring barley fields respectively. It can be a weed of perennial crops such as fruit.
Hogweed is a variable species and 9 geographical variants have been recognised. Two of the nine subspecies occur in Britain. Subspecies sphondylium is widespread but ssp. sibiricum occurs only in parts of East Anglia and may have been introduced. Hybrids occur between the common hogweed and the giant hogweed (H. mantegazzianum).
The willow carrot aphid Cavariella aegopodii can overwinter on hogweed.
Biology
Hogweed flowers from June to September. The flowers are self-compatible and usually insect-pollinated. There are several hundred seeds in each flower umbel. The average seed number per plant in ruderal habitats is 5,030. Seed is shed slowly from August until winter.
Ripe seeds contain a rudimentary embryo that requires 2-3 months at low temperatures to after-ripen. Seed has given 3% germination after 14 days at 5°C and 69% after 96 days. Seeds do not after-ripen fully at higher temperatures and there was no germination of seed kept at room temperature. Seeds buried in soil develop a light requirement for germination.
Seed mixed into the surface soil and stirred periodically emerged from January to June with a peak in March-April. No seedlings appeared outside this period. The majority of seedlings emerged in the first year with only the odd seedling appearing over the following 4 years. Seed sown into short turf in October emerged from March to June with a peak in late-March. Around 50% of the emerged seedlings survived into the summer. In closed communities, seedlings may emerge but not develop further until an opening in the vegetation occurs.
The stems and foliage die down in winter leaving a stout taproot that overwinters.
Persistence and Spread
Based on the seed characters, hogweed seed should persist for less than 5 years and does not form a persistent seedbank.
The seeds are winged and flattened, and may be scattered a short distance by the wind.
Management
The taproots may be dug out and should be collected up after ploughing. There was a noticeable increase in hogweed seedlings in cereals following a change to direct drilling and minimum cultivations.
In grassland, hogweed is favoured by liming and increased levels of potassium.
The leaf canopy developed by hogweed is vulnerable to cutting and grazing. In pasture the weed should be cut regularly to prevent seeding. On roadside verges, increasing the cutting frequency reduces the frequency of hogweed. Cattle and other herbivores readily eat hogweed leaves. Rabbits may dig up the emerging shoots in June but generally avoid the growing plant.
Updated October 2007.