Study

The effect of timing of cutting on giant hogweed Heracleum mantegazzianum growth and seed production in the Mulkear River catchment, Co. Limerick, Ireland

Summary

Giant hogweed Heracleum mantegazzianum (native to eastern temperate Asia), is an invasive perennial plant that has significantly increased its geographical range in Ireland in recent years, being especially abundant along stream and river banks. Dense stands of this tall plant can develop, suppressing and excluding native herbaceous plant species, which play an important role in river bank stabilisation. The effect of cutting on giant hogweed growth and seed production is described here, the findings of which helped form the basis of a giant hogweed control programme on the Mulkear River catchment in western Ireland.

Study area: Seven sites (Dargle, Mulkear, Newport, Portmarnock, Shanganagh, Shannon and Tolka) in Mulkear River catchment, Co.Limerick western Ireland where giant hogweed Heracleum mantegazzianum has established large dominant stands were studied in 1993.

Giant hogweed phenology: The phenology of the plants (lifecycles in response to seasonal changes) was examined. At each site, 10 plants were marked after emergence in spring (using different coloured threads to distinguish between seedling, immature and mature plants). At one site, Portmarnock, where spoil had been deposited over tarmac to a depth of 15 cm, the effect of this limited substrate depth on growth and seed production was compared with plants from an adjacent site with no soil depth restriction. At each site, a total seed count was made for 10 mature plants, selected to include a range of heights, so that the relationship between plant height and seed number could be examined. Seedling density was measured at Portmarnock and Mulkear in five 1 m² permanent quadrats on seven occasions between March and August 1993. Similar quadrats were established in late September at 2 m intervals from the edge of the parent plant stand to determine the extent of seed dispersal beyond where hogweed was already established.

Cutting: The effects of cutting and timing of cut, on growth and seed production, were examined. To determine the effect of cut timing on seed production, plants in circa 100 m² plots at Portmarnock and Mulkear were cut to ground level in late March (early cut) and mid-May (late cut) 1993. These were compared with uncut plants (controls). Total seed counts were subsequently made from 10 mature plants, to include the range of plant heights in each of the plots. The length and width of 50 seeds from the centre of the terminal umbel in each treatment at Portmarnock were measured to examine the possibility that smaller (possibly less viable) seeds might result after a late cut.

Effect of substrate depth: The effect of limited substrate depth on plant height and seed for plants at Portmarnock was very apparent. These plants were an average of 1.49 m tall compared to those on an adjacent site with normal soil which averaged 2.78 m (around the average height found for the other six sites). Likewise, seed numbers were significantly less on the shallow substrate (average of 13,884/plant) compared with those in normal soil (average of 42,068/plant). Plants growing on shallow soil also had fewer umbels. The shallow soil inhibited growth of the normal tap-root, these plants having developed a highly-branched, laterally-spreading root system which provided good anchorage.

Seed production: Average seed number per plant for the seven sites ranged between 13,884 at Portmarnock and 68,748 on the Dargle. The highest number of seeds
recorded from a single plant was 107,984 (a 3.3 m tall plant on the River Tolka).

Seed dispersal: Seed dispersal is passive and normally mediated by water or wind. The seeds are large and flat and, even in conditions of strong wind, are unlikely to travel more than a few metres. Quadrat seed counts in the vicinity of dense hogweed stands at Portmarnock and Mulkear showed that over 95% of seeds fell within 10 m of the colony, with a few isolated seeds observed up to 50 m distant.

Effect of timing of cut: No giant hogweed plants were killed as a result of cutting, whether cut in March or May. There were significant differences in plant height and numbers of seeds produced between treatments, uncut plants were tallest and produced most seed, late cut (May) plants were shortest and produced least seed (see Table 1, attached). Seeds from early-cut plants were significantly longer than those from uncut plants, although there was no difference in seed width, whilst seeds produced by late-cut plants were both shorter and narrower (possibly indicating that these smaller seeds were less viable).


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