What Makes a Plant a Weed? 

A weed is a plant growing in the 'wrong' place; where it is not wanted or where it causes harm. A weed in a garden may make the garden look untidy and it may shade out or strangle garden plants. A weed in the New Zealand environment may exclude native species, change the environment to make it less suitable to native animals or impact on the farming economy.

Garden Weeds

In the garden, we think of weeds as those plants that grow between and amongst the plants we are growing on purpose. The weeds make the garden look untidy and left unchecked can fill and shade or strangle the plants we have taken money and time to purchase and plant. Weeds may also harbour and be reservoirs of pests and diseases that affect our cultivated plants. They might also reduce airflow between plants encouraging disease spread.

Weeds will grow wherever they are given a chance; in flower beds, gaps in pathways, vegetable patches, lawns, pots and waste areas. Weeds are often weedy because of their ability to take advantage quickly of opportunities, germinate and grow, or to spread by vegetative reproduction into adjoining space.

There are several ways to make your garden less suitable to weed growth:

• Weed seeds germinate when they are exposed to light, warmth and moisture. If you cultivate an area you are likely to expose weed seeds to light and the warmth from the sunshine and provide bare soil where they are not shaded by other plants. If, after cultivation, you cover the exposed soil with a covering of deep mulch, chip bark or weed-suppressing mat it is possible to prevent light getting to the seeds and prevent germination or to deny young seedlings of light preventing their growth.
• Plant mature plants that will shade out and outgrow weeds.
• Encourage strong healthy growth of cultivated plants by the targeted use of good organic fertiliser.
• When cultivating, remove all weed root and stem fragments. Many weeds have roots and stems that will grow from fragments left behind in the soil. Products such as Kiwicare Weed Weapon Preventer can prevent germination of weed seeds and bulbs in undisturbed soil.
• In lawns let the grass grow longer so that it shades out weeds. Cutting lawns tightly gives low growing weeds the light they need.

It should be understood that many of the foreign plants we cultivate in our gardens may naturalise and become weeds in the environment. In the Auckland region alone approximately four species naturalize each year. Gardeners should take care to do what they can to prevent garden plants from spreading to the environment in general. A list of the most invasive of garden plants can be found on the Weedbusters website.

Agricultural Weeds

Weeds can be damaging to crops, reducing productivity, contaminating harvests, harbouring pests and diseases and increasing production costs.

The agricultural and horticultural industries spend considerable time and money improving productivity by reducing weeds amongst their crops. It is estimated that in New Zealand the cost to primary production is hundreds of millions per annum. Small scale production in home vegetable plots is similarly affected by weeds.

Some weeds that grow in the pasture are harmful to grazing animals, e.g. ragwort is poisonous, and thistle damages skin and eyes. Unwanted plants in fields and new forest plantations affect growth by competing for nutrients and space.

Environmental Weeds

The New Zealand environment is uniquely vulnerable to invasive species and this is true of plant species as it is for invertebrates and mammals.

According to Weedbusters ‘There are over 24,700 introduced plants growing in gardens and nurseries in New Zealand; 10% of these will naturalise (establish in the wild), and 10% of these will become serious pests.’ i.e. 247 serious pest weeds in our environment. 

The Department of the Environment has more than 300 species on its list of plants that damage the environment.

It could be argued that all plants that are non-native should be classed as weeds. This would be impractical as many of our crops would then be weeds. But there are introduced plants that have found New Zealand conditions suitable for weedy spread and growth and which exclude native plants, changing the environment to the detriment of native invertebrates, reptiles and birds. They can also change the appearance of the New Zealand landscape. Take the example of wilding pines. These self-seeding weeds from the timber plantations change the appearance of mountains, hills and valleys of many parts of New Zealand and support a very different variety of life than the existing native ecosystems.

Environmental weeds left un-controlled would change the face of New Zealand within a few decades.

While in each of these areas the reason for plants being classed as weeds is different there is a great overlap of weed species. Many garden weeds are also weeds to agriculture and the environment. Many environmental weeds originated in gardens. And weeds that are a pest to agriculture will often have reservoirs in gardens and wild areas. So controlling weeds where ever they are will be of benefit to our gardens our primary industry and our environment.

Kiwicare produces a wide range of herbicides for control of a wide range of weeds. All are used in home gardens and many are used in agriculture and environmental protection. The range includes organic options, general burn down herbicides, selective herbicides and targeted application products like Invade Gel.

Search for your weed in the Kiwicare garden problem solver.

David Brittain
Kiwicare