Leaves are deep green, lobed and sparsely hairy. Each leaf has two or three pairs of rounded leaflets with a larger terminal lobe
Upright flower stems carry few leaves and many tiny white flowers in small clusters
Flowers are 4-5 mm across with four petals, each twice as long as the four sepals, and four stamens with yellow anthers
Flowers are followed by small, slender, upright pods 15-25 mm long. Two valves split explosively and coil upwards, spreading the seeds up to a metre from the parent plant.
Similar species
Wavy bitter cress (Cardamine flexuosa) is similar but often slightly larger with a wavy erect stem and six stamens in the flower. It is scattered throughout New Zealand and found in wetter environments than bitter cress
The native New Zealand bitter cress (Cardamine debilis) is similar to bitter cress but has larger flowers, each with six stamens. It is found throughout New Zealand, in a range of forest or tussock habitats
Cuckoo cress (Cardamine pratensis) is a rhizomatous perennial herb up to 60 cm tall. It has larger pink flowers and is found in wet places and along river banks in Northland, Auckland, Waikato, Manawatu and Horowhenua in the North Island and in Westland and Southland in the South Island.
Biology
Origin
Originally from Europe and western Asia, bitter cress has now been introduced to North America, eastern Asia, Australia and New Zealand
First recorded in New Zealand in 1901.
Life cycle
Flowers August to December. Plants can set seed within a few weeks of germinating, so several generations can occur in a year
One plant can produce up to 5000 seeds that can germinate immediately. However, if the seeds are in environments that do not favour germination they can survive for several years
Seeds are small and sticky when wet so can stick to footwear, enabling them be transported and dispersed
In a moist environment bitter cress can probably germinate, grow and set seed at any time of the year.
Benefits
Leaves have a hot, cress-like flavour and have been used as a garnish or flavouring in salads.
Occurrence
Now found throughout the North Island and in eastern areas of the South Island, as well as on Stewart and Chatham Islands
Commonly found in damp gardens, along driveways, in cultivated or disturbed ground and can also be found growing in plant pots.
Impacts
Bitter cress is probably not very competitive as a weed although if many individual plants germinate together they could limit the growth of later germinating species
Seeds generally do not germinate in the presence of competitive or shading vegetation such as pasture
Plants can act as hosts for common garden pests such as aphids
Wavy bitter cress is more abundant in Poverty Bay and has been noted as a weed of arable crops in that region.
Impacts on pasture
Bitter cress is rarely a pasture weed although it could interfere with establishment of a new pasture if present in high numbers
Young stems with their soft, finely-divided leaves break easily but later become tougher and can sprawl for long distances.
Control
Cultural control
Hand weeding is very effective but must be repeated regularly, preferably before the plants set seed. Individual plants are easy to dislodge and uproot. Uprooted fruiting plants can continue shedding seed.
Bitter cress is usually found in open places and the seeds probably don’t germinate under the cover of vegetation.
Bitter cress also germinates from very close to the surface (less than 10 mm deep) and will not grow if the soil surface is dry. It germinates best in compacted, damp soils.
Chemical control
Bitter cress (Cardamine hirsuta) is readily controlled by a wide range of herbicides including 2,4-D, glyphosate and many of those normally used in crop production.
Further information
Popay I, Champion P, James T 2010. An illustrated guide to common weeds of New Zealand. New Zealand Plant Protection Society, Christchurch, New Zealand. 416 p.
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