Weeds compete with crop plants, which reduces productivity. Pests and diseases damage crop plants, which reduces productivity.

Properties of annual weeds are

• rapid growth
• short life cycle
• high seed output
• long-term seed viability

Properties of perennial weeds with competitive adaptations

• storage organs
• vegetative reproduction

Most of the pests of crop plants are invertebrate animals such as insects, nematode worms and molluscs.

Plant diseases can be caused by fungi, bacteria or viruses, which are often carried by invertebrates.

Weeds, other pests and diseases can be controlled by cultural methods including ploughing, weeding and crop rotation.

Pesticides include

• herbicides to kill weeds
• fungicides to control fungal diseases
• insecticides to kill insect pests
• molluscicides to kill mollusc pests
• nematicides to kill nematode pests.

Selective herbicides have a greater effect on certain plant species (broad leaved weeds).