Long, narrow, pointed, willow-like leaves with wavy margins and a strong peppery taste when eaten (try it!)
Tiny pink or cream-coloured flowers in slender spikes at the tops of the shoots
Most commonly found in wet places, along drains and ditches and in poorly drained pastures
At the base of each leaf a membranous sheath (called the ochrea) surrounds the stem
Stems are green when young, turning red with age, and the roots slender.
Similar weeds
Two related species – American willow weed (Persicaria punctata) and the native swamp willow weed (Persicaria decipiens) – are also found in wet or damp places
American willow weed is larger. The yellow-green leaves have straight margins and flowers are white. Common from Auckland to Waikato in the North Island and sporadic in Nelson and Marlborough in the South Island
Swamp willow weed is more prostrate; leaves are pale to dark green or often red. Leaves usually with a dark blotch (thumb-print). The flower heads are usually erect and sometimes branched. The flowers are pink-tinged to pink. This species is common throughout New Zealand in swamps, ditches, and along lake and stream sides.
Biology
Origin
Persicaria hydropiper is native to Eurasia and North Africa
Accidentally introduced to New Zealand and first recorded here in 1906.
Life cycle and habitat
Fresh seeds tend to be dormant and a period of cold damp storage is necessary before germination can occur
Seeds have been shown to remain viable in the soil for up to 20 years
Seedlings emerge in spring and the plants flower and die over summer
Because of their bitter taste plants are not palatable to livestock and are rarely grazed. As a result they stand out above other pasture species
Plants commonly occur alongside drains and in the wetter parts of pastures, and also in other damp places such as river or creek banks.
Benefits
Persicaria hydropiper is used as a spice, food flavouring, and garnish during food preparation in Japan and other countries
P. hydropiper has a range of traditional medicinal uses in Europe, where it has been used as a diuretic, treatment for gastric issues and stimulant for blood flow.
Impacts
Occurrence
Very common in damp pastures and near waterways throughout lower areas of both North and South Islands.
Impact on pasture
Only rarely grazed and therefore replaces more useful pasture species in damper areas within pastures
Its unpalatability may prevent stock from grazing close to the plants, especially when plant populations are dense.
Impact on livestock
Not often considered to have any adverse effects on livestock, but has been suspected of poisoning animals in New Zealand
Regarded as poisonous to livestock in Australia, the USA and Europe
Dermatitis can occur in both livestock and humans from contact with this plant.
Control
Grazing management
The best advice is to avoiding pugging in affected areas over winter and early spring, though this is often very difficult to achieve.
Other methods
The competition of dense infestations can be reduced by mowing
Pasture renovation should be accompanied by improvements to drainage.
Chemical control
Controlling water pepper selectively in pastures can be difficult to achieve. The main key to successful control is to spray the plants in early spring while they are still young
Very young seedlings are more susceptible to 2,4-DB than MCPB. As the seedlings become older, 2,4-D will be needed to control them, and this is more effective than MCPA. However, as plants become more mature, 2,4-D is less effective
Another option is to use thifensulfuron (eg Harmony), which is slightly more effective than 2,4-D
Most farmers don’t try to control water pepper until it is too large, by which time probably the best option is an aminopyralid/triclopyr mix (such as Tordon PastureBoss). However, this will kill clovers, so it is better to treat it much earlier with 2,4-D
In paddocks with water pepper plants usually establish in the same areas each year, so if farmers feel that spraying the weeds is justified, then they should consider spraying in early to mid-spring while the plants are small.
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
Menu
