Invasive Alien Species in Northern Ireland

Crataegus laevigata, Midland Hawthorn

Crataegus laevigata
© Graham Day
Crataegus laevigata
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Crataegus laevigata (Poiret) DC., Thuill., auct.
 
Introduction
The hawthorn population of Northern Ireland mainly consists of the common hawthorn (Crataegus monogyna Jacq.), with occasional individuals which are hybrids between the common and midland hawthorns (C. x media Bechst.). The midland hawthorn itself is extremely uncommon and probably always occurs as a planted shrub or tree.
 
Description
The midland hawthorn is similar to the common hawthorn but has less indented leaves, which are a darker glossy green. It is generally less thorny and frequently has drooping branches. Flowers, which have two styles instead of the normal one of common hawthorn, are produced about two weeks earlier. The midland hawthorn is rarely encountered as the pure species in Northern Ireland, but its hybrids with the common hawthorn are not infrequent. They show various combinations of characters intermediate between the parent species, and some examples, which are probably F1 hybrids, are very close to midland hawthorn proper. However, all hybrids appear to have only a single style. The supposed F1 hybrids flower about a week before adjacent common hawthorns.
 
Country of origin
The English Midlands. Probably from commercial nurseries supplying hedging plants.
 
Location in Ireland
The pure species is very rare and is an insignificant element in the flora of Northern Ireland. The hybrid hawthorns are locally frequent in hedgerows. Outside hedges, hybrids are also very rare, but they have invaded the sand dunes at the Umbra, County Londonderry, where a large number of trees and bushes are evident.
 
Life cycle
Hawthorns are sexually-reproducing woody plants, producing attractive hermaphrodite flowers in about May, which are succeeded by small red fruits called haws which are food for birds. The hard one-seeded stones inside the haws pass through a bird's gut unharmed. Cross-fertilisation, by insects, is necessary for fruit formation.
There are no barriers to hybridization between the species except the small differences in flowering time, and, in England, a difference in habitat preferences. Hybrids are fully fertile.
 
Wildlife and habitat impacts
There is no known negative impact of the midland hawthorn or the hybrid hawthorns on animal wildlife.
Concern has been expressed that they might introgress with common hawthorns and alter the genetic integrity of the native populations of this species. However, it appears that many of the hedges in Ulster were planted with introduced hawthorns, so the common hawthorn of hedges theoretically poses an identical threat. It is unlikely that any clear and easy distinction can now be made between native plants of common hawthorn and those derived from England. Furthermore, continued introduction of hedging hawthorns from outside Ireland, including from Holland, continues to the present time.
 
Key vectors
The midland hawthorn and the hybrids were all introduced into Northern Ireland from England with hawthorn quicks purchased for planting hedge boundaries. Secondary spread from hedges into other habitats such as scrub appears rare, but would be by bird-sown seed. This has occurred at the Umbra dunes, County Londonderry, with hawthorns derived presumably from local planted hedgerows.
 
Management measures
The main interest lies in identifying populations of common hawthorn which are probably of native Irish origin for use as a source of native seed. Such populations need to be well away from hedges to rule out the likelihood of interbreeding with hedgerow hawthorns of either species or the hybrid.
It will have to be accepted, however, that perhaps most lowland hawthorn trees and shrubs are contaminated by genes from introduced common hawthorn and hybrid plants, or are directly descended from them. There is now no realistic possibility of reversing this situation.
 
Further information
Text written by:
Paul Hackney, Keeper of Botany, Ulster Museum