hairy cress, popping cress
Cardamine hirsuta L.
A native, annual to biennial plant found in open and cultivated ground, and on rocks and walls throughout the UK. Hairy bittercress is recorded up to 3,800 ft. It is a common weed of gardens, greenhouses, paths, railways and waste ground. It is a particular problem in container-raised plants from nurseries and garden centres.
Hairy bittercress is variable in size and leaf shape. Waved bittercress, C. flexuosa, closely resembles hairy bittercress and is also variable in habit. It is usually annual or biennial but occasionally perennial. A related introduced weed, New Zealand bittercress, C. corymbosa, has become troublesome in polytunnels. It is similar in appearance to hairy bittercress and has the same explosive seedpods but is generally smaller.
Hairy bittercress is found in flower all through the year but mainly from March to August. It is automatically self-pollinated. Seed is shed in May and June and sometimes into the autumn. There are around 20 seeds per seedpod. The average seed number per plant is 600 but a large plant may yield several thousand seeds. Plants can be found in fruit for 8 months of the year.
There is little germination of the fresh seeds. The seed after-ripens at high temperatures. The higher the temperature the greater the temperature range at which subsequent germination will take place. Germination is increased by a period of dry storage.
Hairy bittercress seed germinates from April to December. There are peak flushes of seedling emergence from July to August and November to December but this varies in different years. Autumn is the main period of seedling emergence. Hairy bittercress can complete its lifecycle in 5-6 weeks. The cycle is longer in rich soils and shorter in poor ones. Seedlings can survive the severest frost.
Hairy bittercress forms a relatively persistent soil seedbank.
When the seedpods are ripe the seeds are dispersed explosively for up to 1 m if the plants are shaken by the wind or by weeding operations. The seeds become sticky when wet and can be spread on tools and clothing.
Seedlings should be killed by early cultivations and regular hoeing to prevent hairy bittercress plants flowering and setting seed. Stem fragments are capable of re-rooting following cultivation in moist conditions. Hairy bittercress seedlings should be removed from container plants before the weed can set and shed seeds. The standing areas must also be kept weed-free. Improved drainage may discourage this moisture-loving weed.
Updated October 2007.