Hoary cress
Other names
chalk weed, devil
Latin names
Cardaria draba (L.) Desv. (Lepidium draba L.)
Weed Type
Perennial Broad-leaved Weeds
Occurrence
Hoary cress is an introduced, erect rhizomatous perennial found on waste ground, roadsides, railways, arable land, pasture and on sandy soil by the sea. It is often limited to hedgerows and field margins but can invade agricultural land. Hoary cress occurs on soils ranging from light sands to medium-heavy clays. It prefers neutral to alkaline soils. Dense colonies can develop that exclude other plants.
The sub-species draba was first recorded in 1829. It was introduced into Swansea and several other ports in 1802. It was also introduced into Britain as seeds in the hay filled mattresses of soldiers returning from the Walcheren expedition in 1809. The hay was ploughed-in by a farmer on the Isle of Thanet. It was still rare and local in 1860 but gradually spread further afield. By 1887 it was known in several parts of southern Britain, the eastern counties and Wales. Hoary cress was considered a serious weed in Hertfordshire by at least 1925. It is now found throughout the UK but is most abundant in the south and east. In a survey of arable weeds in 1971-1973 it was common in only 2% of tetrads surveyed. It may have been more frequent in grassy areas than on arable land.
It is a very variable plant due to the introduction of several different clones. Plants exhibit some variation in leaf shape.
Hoary cress is reported to be grazed by sheep. When eaten by cows it can taint dairy products. Analysis has shown that it contains moderate levels of calcium and no oxalic acid.
Biology
Flowering takes place from April to November. The flowers are automatically self-pollinated. The seeds mature from May to July. The inverted heart-shaped pods may contain 2 seeds but usually only 1 develops. Seed number per flowering stem may be 2,300. The seed numbers per plant range from 1,000 to 5,000.
Hoary cress seed germinates over a wide temperature range from 0.5 to 40°C. At 0.5°C germination takes 5 months, at 35°C it takes 23 days. Seeds germinate at or near the soil surface.
Seedlings develop a taproot over 50 cm long within 9 weeks of germination. Lateral roots grow out and new shoot buds are formed 3 weeks later. These grow upwards as leafy shoots or develop into rhizomes. Lateral roots grow out horizontally mainly in the top layers of soil before turning down to form secondary vertical roots. These give rise to more laterals that again turn down to form verticals and so on. At nodes along the rhizomes are buds that can develop into branch rhizomes. New shoots form at the point where laterals turn down to form verticals. After 6 months, roots may have penetrated to 1 m and spread 2 m horizontally. Eventually the vertical roots may grow down 3 to 5 m, well below cultivation depth. The horizontal roots extend to over 4 m. The upright shoots give rise to a rosette of leaves where the rhizomes emerge from the soil. Seedlings that emerge in April may remain in a rosette state in the first year and flower from June on in the following year.
The above ground shoots appear in May and persist until the hard frosts. Carbohydrate reserves in the roots are lowest in April and highest in July.
Persistence and Spread
Hoary cress seed remains viable in soil for at least 3 years. In dry-storage, seed viability was 84% after 1 year and 31% after 2 years. No seeds were viable after 3 years dry storage. Seeds in running water gave 12% germination after 1 month.
Seeds are dispersed by water and transported in soil. The seed coat become mucilaginous when moistened. Hoary cress seed has occurred as an impurity in alfalfa and red clover seed.
Initially dispersal is probably by seed but, once established, plants spread locally by producing new shoots from the underground rhizomes. Buds can arise from any part of the root system and dispersal can be by root buds as well as seed. Hoary cress can regenerate from short segments of root. Vegetative spread is encouraged by mechanical cultivation. If the crown of the plant is damaged new shoots develop from the upper part of the roots. Shoots only form on the deeper roots when the upper parts are killed or damaged.
Management
Avoid spreading the seed in crop seed, farmyard manure and straw, and on harvesting implements. Root pieces can also be spread on machinery. New infestations should be eradicated without delay by hand digging. Hand pulling only removes the tops and regeneration from the roots soon occurs. Regular and frequent cutting to prevent seeding and exhaust food reserves will reduce vigour. Spring cultivations keep the weed in check but do not eradicate it. Hoary cress was eradicated in 2 years by hoeing at 7.5-10 cm at intervals of 2-4 weeks during the growing period.
Hoary cress is difficult to control by normal cultivations. Fallowing alone does not eradicate the weed but deep ploughing to 38 cm followed by a fallow can be effective. Two bare fallows or a bare fallow followed by a cropped fallow were thought to be effective. Thorough cultivation in hot dry weather will reduce the weed.
Hoary cress is favoured in winter cereals where it is able to get away early with little crop competition. The weed can be kept in check by rotations that include a large proportion of spring crops. Control with smother crops of mustard, vetch, maize or clovers and grasses has had limited success. Sowing down to a long ley has given promising results. Hoary cress survives mulching with straw or paper.
Grazing by sheep is reported to have a good effect but cattle are thought to avoid it.
Updated October 2007.