Our Mixed-up Hawthorns

Source: Byatt, Jean. “Our Mixed-up Hawthorns.” Sussex Botanical Recording Society Newsletter, no. 66 (May 2008). http://sussexflora.org.uk/wp-content/uploads/2016/03/Newsletter_May_2008.pdf.

 

Two species of native hawthorn grow in SE England; both are diploid species 2n=34. In the absence of spatial barriers inter-specific hybridisation frequently occurs.

Crataegus monogyna is widespread in Europe and is very tolerant of different climatic and soil conditions. C. laevigata is much more habitat specific. It grows best in a temperate climate in the understorey of woodlands on clay soils. Hence, as Sussex is such a wooded county, and was more so in the past, it is perhaps more frequent here than elsewhere in England, with the possible exception of parts of adjoining counties.

Except in places where there is a very extensive area of unbroken, undisturbed woodland with a suitable soil, it is unlikely that a pure population of C. laevigata survives. Hence where you find C. laevigata you should also expect to find hybrids if you search for them. In practice it is C. laevigata you may need to search for, not vice versa. C. laevigata is not necessarily found in woodland as it may also grow in older hedgerows where it could have been planted in the past.

Hawthorns can be very long-lived, so once present and undisturbed they may survive for hundreds of years. There was a plant of C. monogyna growing in Richmond Park that I estimated could have been there since Stuart times. It was quite hollow and the surviving branches were propped up. It was on its last legs then and may well now be dead. Hence it is not surprising that where conditions remain suitable you may find C. laevigata in quite a small area of woodland in the remnants of what was once a large forest.

The best time to look for C. laevigata is in spring, as it first comes into flower about a fortnight before C. monogyna, sometime in May. Its flowers are larger, although less frequent than those of C. monogyna, and should have two to three styles. I have been surprised that in the area around Crowborough the plants of C. laevigata which I have found have only two styles rather than occasionally three. In case of uncertainty you can of course count the nutlets later in the year.

Hawthorn leaves are important in identification. They may very considerably in size. The surface of the leaves of C. laevigata is, as its name suggests, much shinier than that of C. monogyna, and their shape is distinctive, as shown below:

The leaves of C. laevigata are better adapted to photosynthesis in shade than those of C. monogyna. In contrast to C. laevigata, C. monogyna can grow quite tall in its preferred soil, chalk, to such a degree that it shades out all competitors, including its own lower branches, forming an almost monospecific woodland. Most other plants can then only survive near the margins, the most successful being Mercurialis perennis (Dog’s Mercury), which has specific photosynthetic pigments enabling it to be fairly shade-tolerant. More commonly of course we encounter C. monogyna growing in more open habitats on a variety of soils, and forming unwelcome scrub on chalk grassland.

I append below a table to assist in distinguishing the two species, since until you are confident about the parents you cannot expect to be able to identify their hybrids. Hybrids can embody a large range of variation, but generally they will exhibit either a mixture of the characters of the parents, intermediate states, or a combination of the two. These varying states occur to a greater or lesser degree as the plants may also back-cross and form hybrid swarms.

Table showing main features of Hawthorns

	C. monogyna	C. laevigata
Habitat	Most	Mainly woodland on clay soils
Flowering time		Starts 2 weeks earlier than C. monogyna
Flower diameter	Up to c.14mm	May be over 14mm
Style number	1	2-(3)
Mature leaves	Laciniate – often 5-7 lobes	Shallowly 3-5 lobed to more or less entire
Lowest lateral leaf sinus	In lower 1/3 of leaf blade often 2/3 cut to midrib	In upper 1/2 of leaf blade cut less than 2/3 to midrib
Lowest lateral lobe	Lower margin more or less entire	Lower margin serrulate more or less to base

A note of caution

I have found plants growing near the south coast in the Pevensey and Exceat areas which have some fruits with two styles but which do not correspond with the Crataegus hybrids found more generally inland. For example, they often lack the finely serrulate lobe margins noted above. It is possible that in warmer times in the geological past another species of hawthorn was present in this area. There is a similar but more complex situation in the Netherlands, making identification of hybrid plants very uncertain. I think that a possible candidate for this third species is another diploid hawthorn, C. azarolus, which has 1-2(-3) styles and was widely cultivated in Europe in the past, although it only survives around the eastern Mediterranean and in SW Asia today.

These puzzling plants may be present on other parts of the Sussex coast and it is worth looking out for them.