What is a weed?
A weed may be defined as any plant or vegetation that interferes with the objectives of farming or forestry, such as growing crops, grazing animals or cultivating forest plantations.
A weed may also be defined as any plant growing where it is not wanted. For example, a plant may be valuable or useful in a garden, or on a farm or plantation – but if the same plant is growing where it reduces the value of agricultural produce or spoils aesthetic or environmental values, then it is considered a weed. However, some plants are weeds regardless of where they grow.
The term ‘pest plant’ (or ‘plant pest’) is commonly used by New Zealand’s central and regional government agencies.
Internationally, the term ‘invasive alien plant’ is often used. This is because weeds have usually been introduced, accidentally or deliberately, from other countries. This is true of New Zealand, where only a handful of native species are considered weeds.
Very few plants were regarded as weeds by European botanists when they arrived in New Zealand – the ancestors of Māori must have brought these with them from Polynesia. They included punawaru (Sigesbeckia orientalis), kohiriki or cobblers’ pegs (Bidens pilosa), small-leaved nightshade (Solanum americanum), horned oxalis (Oxalis corniculata) and bladder hibiscus (Hibiscus trionum).
Māori methods of horticulture involved burning the land to clear it, and growing crops for no more than two years before allowing the area to revert to bush. This meant that invaders such as bracken (Pteridium aquilinum) did not have time to become a problem. The land was then left for many years before being cleared and cultivated again.
Māori vocabulary scarcely acknowledged the existence of a ‘weed’. There were too few fast-growing annual plants for them to need such a word.
On Captain James Cook’s second voyage to New Zealand, crews from the Resolution and Adventure planted vegetable gardens in Queen Charlotte Sound, in 1773. Some of the plants, notably cabbage, turnip and radish, spread or were deliberately spread. By the 1840s, European vegetables and the weeds that came with them were found in many parts of the country.
Charles Darwin, visiting New Zealand aboard the Beagle in 1835, was dismayed to see some unwelcome colonisers. His journal entry for 24 December notes:
‘In many places I noticed several sorts of weeds, which, like the rats, I was forced to own as countrymen. A leek has overrun whole districts, and will prove very troublesome, but it was imported as a favour by a French vessel. The common dock is also widely disseminated, and will, I fear, for ever remain a proof of the rascality of an Englishman, who sold the seeds for those of the tobacco plant.’
Some weeds grew and spread more vigorously than they had in England. Many were introduced accidentally as impurities in agricultural and garden seed, or as seeds in packing material, bedding, and imported hay and straw.
Settlers also brought plants for their use – culinary and medicinal herbs, hedging plants, and trees for timber and firewood. Several of these were useful initially, and widely spread by farmers and growers. Too late, some were found to cause more harm than good when they escaped from gardens and plantations and became weeds in other environments. These include wilding pines (Pinus spp.), used in forestry and now a pest in tussock grassland, and sweet briar (Rubus rubiginosa), once a rootstock for grafted roses.
Gorse, hedging and thistles
Gorse is one of New Zealand’s most notorious weeds. It was deliberately introduced, often sold as seed or seedlings in the 1800s, and planted for stock fodder or as hedgerows. At that time timber was the only available fencing material and gorse was a cheap alternative. But gorse quickly spread to cover hillsides and pasture.
Other plants grown for hedging included broom (Cytisus scoparius), elaeagnus (Elaeagnus x reflexa), hakea (Hakea spp.), boxthorn (Lycium ferocissimum) and barberry (Berberis glaucocarpa), all of which are now regarded as weeds. Hedgerows of gorse, barberry or boxthorn still persist today in some parts of the country.
Some thistles too may have been deliberately introduced, distributed and planted by Scottish settlers as heraldic or Scotch thistles. Cirsium vulgare has kept the name Scotch thistle, and is now a common pasture weed everywhere.
Although sometimes regarded as a weed, mānuka is the source of mānuka honey, which is quickly becoming popular. It has an antibacterial property found only in honey produced from Leptospermum plants. Some farmers are now being encouraged by beekeepers to preserve areas of mānuka scrub.
Mānuka (Leptospermum scoparium) has been the scourge of generations of hill-country farmers, spreading quickly across new pasture. Tutu (Coriaria spp.) poisoned stock (and unlucky circus elephants on at least three occasions) in earlier days, and honey made from it is poisonous. Bracken or rauaruhe (Pteridium esculentum) was a problem in newly cleared land.
Native weeds that pose local problems include mātātā or ring fern (Paesia scaberula), matagouri (Discaria toumatou), and tauhinu (Ozothamnus leptophyllus).