Story: Weeds of agriculture
Page 2 – Weeds in pasture, crops and forestry
Weeds in pasture
Weeds can become established and persist in grazed pasture. Some, such as scrubweeds, thistles and dock reduce the pasture available to stock.
Others may directly harm livestock, by:
- poisoning – ragwort (Senecio jacobaea), St. John’s wort (Hypericum perforatum)
- damaging skin and eyes – thistle, barley grass seed (Critesion spp.)
- causing infections such as scabby mouth in sheep – Californian thistle (Cirsium arvense)
- affecting the quality of wool and pelts – thistle, barley grass seed, Bathurst burr (Xanthium spinosum)
- tainting dairy produce – twincress (Coronopus didymus).
Among the most prevalent thistles are Scotch thistle, nodding thistle (Carduus nutans) and Californian thistle, some of which have been common in pastures since before 1900. Others (for instance, nodding thistle) have become a problem only in the last 50–60 years.
Mouse-ear hawkweed (Hieracium pilosella) is allelopathic – it produces toxins that inhibit the growth of other plants. Mouse-ear hawkweed spread rapidly in the 1950s, and is a serious problem in the high-country farmland of the South Island.
Two species of Nassella are also a problem in pasture – nassella tussock (Nassella trichotoma) and Chilean needle grass (Nassella neesiana). Other Nassella species are also potential problems.
The most widespread is Nassella tussock, a perennial tussock native to South America which can form dense mats on pastures. It is unpalatable to sheep and cattle. A mature plant can produce 120,000 seeds a year, which can remain in the soil for a decade or more. Seeds are borne by wind and water, and carried on animals, humans and machinery. Plants are found in the drier parts of New Zealand. However, prolonged efforts by Nassella Tussock Boards, and more recently, regional councils, have limited its spread.
In pasture, any plants other than sown grass species and clovers tend to be regarded as weeds because they may be less productive. However, organic farmers regard a ‘mixed herb ley’ of a mixture of species as healthier for stock. They often consider so-called weeds such as dock, plantains and dandelions to be useful components of pasture.
Weeds in field crops
Many weeds that grow among crops and in gardens were introduced either accidentally or deliberately by early British settlers. They include groundsel (Senecio vulgaris), chickweeds (Stellaria media and Cerastium spp.), fathen (Chenopodium album), nightshade (Solanum spp.), amaranthus (Amaranthus spp.) and willow weed (Persicaria persicaria) – plants that germinate readily, and grow, flower and fruit quickly to produce copious seeds.
Sometimes weeds contaminate harvested crops. For example, black nightshade berries may be collected along with peas. Weeds can also host destructive pests such as aphids, caterpillars, fungal rots and viruses.
In gardens, weeds are mainly an unsightly nuisance, but they also compete for water, nutrients and space. The more troublesome and difficult to eradicate are couch grass (Elymus repens), which has underground rhizomes or lateral roots, bindweed or Calystegia spp., also with rhizomes, and Oxalis, which multiplies rapidly with many small bulbs that are difficult to eradicate.
Presumably as a result of climate change, weed species from warmer regions are gradually spreading southwards. They include the summer grasses (Digitaria, Setaria and Panicum spp.), paspalum (Paspalum dilatatum) and kikuyu grass (Pennisetum clandestinum).
Is gorse worse?
It is impossible to say which is New Zealand’s worst pastoral weed. But in terms of lost production, land covered and money spent on control, it’s easy to argue that gorse is the worst. Scrub weeds like gorse, broom and others waste useful pasture space. But gorse can also be an excellent nursery for native seedlings.
In forestry, weeds cause the most problems in the first two years of tree growth. They can reduce the growth rate of young trees and sometimes distort their growth patterns. In some forests pampas grass (Cortaderia selloana) has become established and is now widely regarded as a weed. Herbicides are used extensively at the start of new forestry projects, and mowing and other mechanical methods of weed control are also used on less rugged terrain.