In White Admiral 65, Rob Parker presented a brief but fascinating account of the value of ground ivy as an important source of food, in the form of nectar, for one of our rare butterflies, the Dingy Skipper. This was in fact a follow on from Caroline Wheeler's account, in White Admiral 64, of a herbalist's uses for the plant. The plant itself has many interesting features so here is the response of a botanist.

Plate 1: Detail of Ground Ivy showing flowers & ivy-shaped leaves

        Ground Ivy, Glechoma hederaea, is a member of the Lamiaceae, the deadnettle family. It is a common plant in Suffolk, growing in woods, grassland and waste places, especially in disturbed areas on damper and heavier calcareous soils. Ground Ivy has had a very good spring this year, producing large, low-lying masses of blue-violet flowers. The plant produces ivy-shaped leaves, though they are not waxy like true ivy (Photo 1, below). The flowers are most prolific between April and May; the petals are fused together to form a narrow tube that gradually expands into a wide throat and then opens out into four lobes (Photo 2: right). The lower, large petal lobe is bifid and bears several purple spots, which serve to guide appropriate insects to the nectar, which is produced at the base of the flower.

In Ground Ivy evolution has worked to produce flowers that are elegantly designed for pollination by bumble-bees, honey-bees and similar insects. These insects have to force their heads into the flower as far as the beginning of the throat to be able to reach the nectar. In so doing the top of the insect’s head makes contact with both stigma and anther so that if the insect visits another flower of ground ivy, cross pollination is likely to occur. However, bee flies, as well as Rob’s butterflies have longer tongues so they can take nectar without any contact with stigmas or anthers, so there is no benefit to the plant. Insects with short tongues are excluded from the sugary nectar by the length of the petal tube. Small beetles are deterred from crawling down the tube to reach the nectar by a thick mass of white hairs on the lower petal lip at the entrance of the throat, in the throat itself and in the tube (Photo 3, below). Each pollinated flower can produce four seeds, although the normal number is usually lower than this, depending on the availability of appropriate insects.

From the above description of ground ivy flowers and their pollination it will come as a surprise that hardly any of the ground ivy you see on your walks will have actually come from germinated seeds. In the last month or two of winter, an overwintering plant begins to grow again and will soon develop a horizontal stem (stolon). As the stolon grows it periodically sends up new plants, and later in the season, when growth is optimal, two new plants are produced every week for each stolon. Stolons can reach a length of two metres and produce 17 plants. New plants themselves can also start stolons so the outcome is a carpet of ground ivy. Multiplication is therefore by vegetative propagation, the plant carpet being a clone of the genetically identical plants. Ground Ivy plants are not long lived like the carpets of bluebells or ramsons we see in Suffolk woods. Most live for much less than a year and the colony can be regarded as shifting, moving to disturbed areas to exploit temporary local conditions and moving on by horizontal growth elsewhere. As winter draws near the rate of plant death increases, but a few young plants survive and over-winter in the green state.

So why does Ground Ivy use all that energy and resource to produce nectar-rich flowers when it reproduces so easily and effectively by vegetative propagation? I can think of three reasons. Firstly the seed can stay dormant in soil for long periods, so that if the area where a colony is growing becomes overgrown it can rest dormant for years until conditions become suitable again. Secondly, the seed is probably the best method of dispersal, enabling the plant to form new colonies in new areas. Ground Ivy has no specialised method of dispersal, but they are often taken and transported by ants. Most seeds will be eaten but some will escape and germinate. Finally, seeds can contain new combinations of genes because they are the product of sexual union. Clonal colonies, being genetically fixed, can hardly adapt to environmental change (which we hear about so much these days) or from any increased competition from other plants growing nearby. Insects that pollinate Ground Ivy will occasionally bring in pollen from other clonal colonies. The resulting “hybrid” seeds will have wide genetic diversity and a small number of them may well have greater vigour and adaptability to changing conditions. If these become dispersed and then germinate into new plants, they will produce their own clonal colonies, perhaps replacing existing, less adapted clones and competing better with other species.

And so the cycle of new improved individuals being formed by crosspollination, followed by clonal multiplication very, very slowly goes on and on. Asexual propagation and sexual reproduction have separate roles to play and by working together they ensure the success of the species, and this is very evident in Suffolk during the spring.