Many are invasive. They all compete for space, nutrients, moisture and light. Weeds are capable of growing in almost any soil or site. Actually, some cultivated plants can be considered as weeds in certain circumstances. This results in the appearance of many unwanted seedlings the following spring.
Annual weeds complete their life cycle within one year.
They set seed that germinates freely. Some will produce several generations in a single season. It is important then to remove annual weeds before they flower and set seed. Perennial weeds survive from one year to the next. Many perennial weeds are able to become prolific because of poor cultivation techniques that leave small pieces of the rhizome in the soil after digging or rototilling.