Dodder

This afternoon I went out to Barnack Hills and Holes to do some recording with a group of local botanists. It was fun having people with a range of experience, and there was a lot of discussion and informal learning going on. I think I might have agreed to lead a grass identification day next summer, as this was the plant family that most people were struggling with.

Barnack is a wonderful reserve and has many rarities, but I was particularly pleased to see huge quantities of dodder Cuscusta epithymum. This annual parasitic herb can be identified by its reddish twining stems and soft pink flowers - there are no leaves as it gains all its nutrition from its host, which at Barnack appears to mostly be bird's-foot-trefoil.

Dodder seeds sprout at or near the surface of the soil. Although dodder germination can occur without a host, it has to reach a green plant quickly and is adapted to grow towards the nearby plants by following chemosensory clues. If a plant is not reached within 5 to 10 days of germination, the dodder seedling will die. Before a host plant is reached, the dodder, as other plants, relies on food reserves in the embryo.

After a dodder attaches itself to a plant, it wraps itself around it. If the host contains food beneficial to dodder, the dodder produces haustoria that insert themselves into the vascular system of the host. The original root of the dodder in the soil then dies. The dodder can grow and attach itself to multiple plants.

Dodder is a species which is particularly characteristic of lowland heath and chalk downland, predominantly south of the Thames-Severn line. There are scattered sites as far north as Yorkshire and the Isle of Man, but there has been a significant decline in its distribution as a result of ploughing up of sites and scrub invasion; this decline has been particularly marked in the north of its range.

The butterfly sitting on the dodder is another speciality of the site, a male chalkhill blue Lysandra coridon. This was out in profusion yesterday thanks to the bright sunshine, and the ground almost appeared appear to shimmer with the activity of hundreds, if not thousands, of males searching for a mate just a few inches above the ground. The distribution of this species follows the distribution of the caterpillar's food plant, horseshoe vetch, which in turn follows the distribution of chalk and limestone grassland. This species is therefore restricted to England, south east of a line running from West Gloucestershire in the west and Cambridgeshire in the east.

I suspect that there are very few sites where you could get both these species in one image!