A weed can be defined as a plant that grows where it is not wanted.
Weeds do much damage to crops. When we consider loss¬es from weeds for all of the crops, including pastures, plus damage by insects and diseases harboured by weeds, plus the lower quality of milk and its products and grain and wool because of weeds, the figures run into unbelievable millions. And this is not all of the loss. The extra labour required to keep weeds under control is probably the greatest loss they cause. A large proportion of the labour of producing crops is spent in destroying weeds.
We cannot list here all of the causes, of the great losses from weeds, but we shall list a few of them: 1. Weeds rob other plants of water in the soil. 2. They rob other plants of food materials. 3. Weeds often shade and smother small cultivated plants because they grow faster. 4. They harbour plant diseases. 5. They harbour insects and injurious worms. 6. They cause lower quality of grains and wool. This is due to weed seeds in grain, and to burs in wool. 7. Some kinds of weeds poison livestock and people. 8. Weeds cause bad flavours in milk, butter and cheese. 9. They cause' hay fever and other diseases which some people get from breathing the pollen from weeds.
Weeds spread because they are more hardy than cultivated plants, because they have so many seeds, and because the seeds have so many ways of being scattered. Weeds grow faster than most cultivated plants and are more difficult to kill. Weed seeds live a long time. The seeds of some weeds buried in the soil may live fifteen to thirty-five years. There is a popular expression: one year of seed gives seven years of weeds.