WATER PEPPER AND WILLOW WEED (Persicaria hydropiper and persicaria persicaria)
These two damp-loving weeds are often confused, so we’ll sort them out for you.
Water pepper grows in lowland areas in both Islands in NZ, where it is generally found on lake margins, drains, and poorly-drained areas of pasture and grows up to 75cm high. It has noticeable red stems and bright green, lance shaped leaves about 12cm long and with wavy margins. The pink or rose bud-shaped flowers are carried at the end of each stalk in spike-like racemes which curve downwards. In New Zealand it blooms from about November to June. It is thought to have originated in Eurasia, but is now found in many countries.
In pasture it can be found near seldom-used gateways where damp ground is occasionally churned by animal feet, and also where the ground dips and remains moist for longer periods. Roots are shallow and the plants are relatively easy to pull out.
Its name derives from its partiality to damp soil and the sharp peppery taste of leaves and stems. At the base of each leave is a membranous sheath surrounding the stem.
Livestock tend to avoid it, and it is regarded as poisonous in countries such as the US, Australia and Europe. However the leaves are reputedly used by Japanese cooks to add a peppery taste without masking other delicate flavours. The tiny dark brown or black seeds can be favoured by waterfowl and other grain eating birds.
Willow weed is lower growing (about 40cm) and often sprawling. The stems may be reddish, but not as brightly so as water pepper. Leaves are dark green and larger (up to 15cm long and 4cm wide) with a central dark blotch on the upper surface which, in parts of the US sees it called Spotted Lady’s Thumb. Young leaves may be tinged red or brown, and the sheath at the base of each leaf has a distinct fringe of hairs.. The flowers are pink, cylindrical racemes, with a very long flowering of Jan-Dec.
Willow weed can be very invasive, particularly in cultivated ground and among crops as it will tolerate drier ground than water pepper. It will reproduce itself from even a small piece of stem, and produces a great number of tiny three-sided seeds which can remain dormant for long periods but still remain viable. So removal must be done carefully, and it may take several seasons to eradicate it. However, growing plants can be killed by frost.
Although usually avoided by livestock, there have been recorded deaths of sheep and pigs from it in other countries, and there have been suspected cases of livestock poisoning in NZ.
In the US both plants are often called ‘smartweed’ because both can cause skin irritations or allergic reactions.