Understanding weed life cycles

A weed is defined as a plant out of place, or a plant growing where it is not desired. Weeds compete with turfgrasses for light, water, soil nutrients and space. In a turf, weeds disrupt uniformity and are often considered to be aesthetically unpleasing. With a few exceptions, weeds will not provide year-round vegetative cover. A year-round turf cover is especially beneficial to prevent erosion and runoff, and in areas that might receive moderate to heavy use during the non-growing season. Weeds in athletic fields can adversely affect playability and athlete safety.

Weeds of turf are most often categorized by their life cycle. The most important thing one can learn about a specific weed is its life cycle. Knowledge of the life cycle is valuable when attempting to manage a specific weed because not all strategies are appropriate for all life cycles. Also, the timing of when a specific control strategy is implemented will vary based on a weed’s life cycle. A weed can be a summer annual, winter annual, biennial or perennial (Tables 9 and 10).

Annual weeds

Annual weeds reproduce by seed and complete their life cycle in one year. The annual life cycle is further divided into summer annual weeds and winter annual weeds. Summer annual weeds germinate in spring or early summer, grow vegetatively, flower and produce seed and die in late summer or fall. Examples of summer annual weeds include crabgrass, yellow foxtail, goosegrass, barnyardgrass, ragweed, purslane, spotted spurge, pigweed, carpetweed and prostrate knotweed. Winter annual weeds germinate in late summer and fall. They grow vegetatively and go dormant with the arrival of cold weather. In the spring they continue to grow vegetatively early and then switch to a reproductive phrase during which they flower and produce seed. After flowering, they die with the onset of warm weather. Winter annual weeds can be just as problematic and troublesome as summer annual weeds. Examples of winter annual weeds include shepherd’s-purse, common chickweed, sticky chickweed, annual bluegrass, henbit, pineappleweed, spring whitlowgrass and corn speedwell.

Biennial weeds

Biennial weeds reproduce by seed and complete their life cycle in two years. Biennial weeds germinate in spring to early summer and grow vegetatively forming a leaf rosette at the end of the first growing season. After over-wintering as a rosette, the biennial weed resumes growth, flowers, produces seed and completes the life cycle in the second growing season. Examples of biennial weeds are common mullein, burdock, bull thistle, wild carrot and garlic mustard.

Perennial weeds

Perennial weeds live for three or more years and reproduce by seed and vegetative propagules including roots, rhizomes, stolons, tubers, and bulbs. Dandelion, broadleaf plantain, narrowleaf plantain, broadleaf dock, false dandelion, spotted cat’s ear and fall dandelion are examples of simple perennials and reproduce by seed. Spreading perennials reproduce by seed and vegetative propagules and can form sizable clonal patches in turf. Examples of spreading perennials are quackgrass, red sorrel, creeping woodsorrel, Canada thistle and ground ivy. Other perennials such as wild garlic and star-of-bethlehem arise from bulbs. Yellow nutsedge arises from small tubers called nutlets.

Table 9. Life cycles of common broadleaf turf weeds

Table 10. Life cycles of common grass and grass-like turf weeds