Unwanted wild plants, particularly those springing up in a cultivated area, are known as weeds. Generally, weeds are hardy flowering plants which reproduce easily. Many weeds can be wind-pollinated and these can have small, insignificant flowers. Weeds can be native or non-native.
When weeds grown so quickly that they take over the environment of other species, or make it difficult or impossible to farm an area, they are known as noxious pests. Sometimes, even cultivated plants can become so invasive that they are considered weeds. This can happen when plants are moved to an area they are not native to, particularly areas with warm, hospitable climates and fertile soil.
There is no hard-and-fast rule for what makes a plant a weed. Even useful plants can become naturalised in an area and become weeds. Describing a plant as a weed is therefore a matter of human perception. Blackberries, from the family rosacea, have sweet, edible fruit which humans prize. However, they are considered noxious weeds in some parts of the world. Calla Lilies (species of Zantedeschia) are expensive in high-end florists in some cities, but they are considered pests in some parts of Australia. Wisteria is a prized decorative plant, but can become invasive.