Weeds are an indeterminate group of plants which influence directly or indirectly every section and individual in the community, while to agriculture and cognate rural industries some 600 species constitute a major factor limiting production and causing significant economic loss. Whether or not a plant is a weed depends, in the main, on its place of growth relative to man's intentions – no plant is an absolute weed. For example, Pinus radiata in a plantation is a useful plant, but established in hill-country pasture is a weed.

Why are plants regarded as weeds? Weeds occupy ground space to the exclusion of useful plants, competing for light, soil nutrients, and moisture, or may grow where no vegetation is required. Some plants, too, have specific characteristics which under most circumstances warrant their classification as weeds. Reasons such as the following explain why some New Zealand plants are classed as weeds.

1. Compete with and smother crops in cultivated land, e.g., black nightshade (Solanum nigrum), fathen (Chenopodium album), spurrey or yarr (Spergula arvensis).

2. Invade pastures and reduce grazing, e.g., gorse (Ulex europaeus), rushes (Juncus spp.), thistles (various species).

3. Block drainage channels and impede navigation in lakes, e.g., Canadian pondweed (Elodea canadensis), watercress (Nasturtium spp.), water hyacinth (Eichhornia crassipes).

4. Poisonous to livestock and humans, e.g., Cape tulip (Homeria collina), hemlock, (Conium maculatum), ragwort (Senecio jacobaea).

5. Cause undesirable flavours in agricultural produce, e.g., King Island melilot (Melilotus indica), land cress or twin cress (Coronopus didymus), wild garlic (Allium vineale).

6. Reduce value of wool, e.g., bur clover (Medicago polymorpha), burdock (Arctium spp.), piripiri – corrupted to bidibidi, (Acaena spp.).

7. Cause mechanical injury to livestock and humans, e.g., barley grass (Hordeum murinum), African boxthorn (Lycium ferocissimum), needle grass (Stipa variabilis).

8. Act as hosts to insect pests and fungous, bacterial and virus diseases, e.g., weeds of the cabbage family (Cruciferae) carry clubroot; hawthorn (Crataegus monogyna) carries the bacterial disease, fire blight; and many weeds are host to aphids significant in the spread of virus diseases.

9. Scramble over and smother shrubs and trees in gardens, hedges, native bush reserves, e.g., German ivy (Senecio mikanioides), ivy (Hedera helix), wild clematis (Clematis vitalba).

10. Invade and replace lawn and green swards, e.g., dandelion (Taraxacum officinale). Mercury Bay weed (Dichondra repens), Onehunga weed (Soliva spp.).