05-Aug Conquerors of Concrete: The War Between Useful & Invasive Weeds

The most common and familiar is the Broad-leaved dock Rumex obtusifolius known to many for its use in treating nettle stings. The leaves are rubbed in the affected areas neutralising the formic acid which causes the sting. The very large leaves with blunt ends are too coarse to be enjoyed by cattle, which is another reason they are seen everywhere, as well as them having a six month flowering period.

The Curled dock R. crispus is only a little less common. Like many docks it thrives on disturbed ground. ‘Curled’ refers to the wavy edges of its leaves which makes it easier than most to recognise. Probably the third most common dock is the red-veined dock R. sanguineus. This one is identified by its red or purple stem and leaf veins. It tends to be found in areas of rough grass. When cut it issues a blood-coloured juice which in the good tradition of the Doctrine of Signatures meant it was likely to have been used to staunch wounds. Another close relation is the Wood dock R. sanguineus var viridis. This is likely to be the only dock that you find in local woods, although Sharp dock R. conglomeratus may occur if the wood is a damp one. A number of other docks prefer living beside or near water. Good places to look for these are Surrey docks, Hampton Court, Hamlands and Thamesmead.

The Great Water dock R. hydrolapathum tends to grow in the west of London. It is a large plant with long leaves that used to be used to clean teeth. The Patience or Fiddle dock R. patientia is a large alien dock. It prefers riverbanks and at one time was even grown as a vegetable. Its popularity may have deteriorated with it also being a competent laxative.

The Great Water dock R. hydrolapathum tends to grow in the west of London. It is a large plant with long leaves that used to be used to clean teeth. The Patience or Fiddle dock R. patientia is a large alien dock. It prefers riverbanks and at one time was even grown as a vegetable. Its popularity may have deteriorated with it also being a competent laxative.

The old dish of boiled beef and sour docks refers to the sour tasting Sorrel R. acetosa. Another strikingly tall alien dock seen along the banks of the Thames is the Greek dock R. cristatus. If a dock is seen with golden yellow leaves it may well be the rare Golden dock R. maritimus and if one has very waisted leaves it is likely to be the Fiddle dock R. pulcher. The latter shows an odd preference for churchyards.

This is also a time when Polygonum spp. particularly Redshanks Polygonum persicaria, Water pepper P. hydropiper and various other knotgrasses tend to be noticed more. One attractive member of the group Bistort P. bistorta, was seen earlier in the spring but most of the others come into flower now. Redshanks or Persicaria as it is often called, is very common and easily recognised by the large black spots on its leaves.

These were originally attributed, like so many other plants, to spots of Christ’s blood that had fallen from the cross. Water pepper tends to get noticed for its unusual habit of growing in dried up puddles along paths in woodland. It is famous for its fiery pepper taste when a little is nibbled and placed on the tip of the tongue. The very similar looking P. mite has not got this peppery character and so goes under the unfortunate name of Tasteless Water Pepper. If a more prostrate Polygonum sp. is found it may well be the rare Small Water Pepper P. minus. A pink-flowered species seen floating in water is likely to be Amphibious Bistort Persicaria amphibia. This exists in two forms, one of which is perfectly happy to live on land and the other which is much happier in water. An even paler pink flower may indicate Pale Persicaria P. lapathifolia which is almost as common as Redshanks in our area.

There are many other non-native Polygonum spp which tend on the whole to be more vibrant reds and pinks. With gardeners selecting the colours it is no surprise they have found their way into our gardens. However, being inherently invasive they can quickly take over.

The word ‘Polygonum’ means ‘many knees’ and the group we call knotgrasses illustrate this by producing their flowers along their stems which have many swollen joints. Knotgrasses have small flowers that vary from reddish-pink to purple or white. The two most likely to be seen are the Prostrate knotgrass P. aviculare which has no petals but green, white, purple and pink sepals and the Equal-leaved knotweed P. arenastrum which tends to be green, white and pink. Any found at the edges of fields could be the Cornfield knotgrass P. rurivagum whose flowers are more crimson in colour. The seeds of the whole group are highly nutritious and sought after by birds. They were also another important food of Neolithic man. There are one or two closely related twining species that tend to cause problems.

Black bindweed Fallopia convolvulus is a particularly troublesome weed in gardens and the Russian vine F. baldschuanica can quickly grow to an astonishing forty-five metres in length. This has meant it has become a common sight for rail commuters along miles of railway embankment.

Lastly there is the most hated plant in London, Japanese knotweed F. japonica. Athough it only arrived from China in 1900 it has already managed to march across the whole of London. It has a more beautiful and less troublesome hybrid with the Russian vine which can be found on Railway Fields nature reserve in Haringey.

Finally, a more loved but less often seen relation is buckwheat Fagopyrum esculentum. The seeds of this plant are made into the nutty-tasting flour used in Breton pancakes and Russian blinis. For this reason it was once grown as a crop but now only tends to occur where birdseed has been discarded.