Plants can be grouped by their life cycles:

Annual plants have a one-year life cycle. They grow from seed, produce seed for the next generation, and mature in one year or less.

Summer annuals are plants that arise from seeds in the spring. The seeds were in the ground over the winter. They grow, produce seed, mature and die before winter. Examples include annual morningglory, barnyardgrass, crabgrass, foxtails (green, yellow and giant), kochia, lambsquarter, marijuana, partridge pea, pigweed, ragweed (common and giant), and Russian thistle.

Winter annuals grow from seeds that germinate in the fall, over- winter as young plants, set seed, mature, and die in the spring or summer. Examples include annual bluegrass, bedstraw, Carolina geranium, chickweed, downy brome, groundsel, henbit, little barley, ripgut brome, wild mustard, wild oats, wild radish, wild rye, and yellow rocket.

In milder climates the differentiation between winter and summer annuals can become less distinct. Because the root systems of annual plants do not persist from year to year, defoliating these plants usually controls them. Treating annual plants after seed set is usually not recommended. Knowing the growth habits of annuals is important in planning how and when to control them.

Biennials complete their life cycle within two years (Figure 2). In the first year they grow from seed, develop a root system, and a compact cluster of leaves (rosette) on the soil surface. In the second year, they mature, flower, produce seed, and die. There are no grasses with a biennial life cycle. Biennial broadleaves include bull thistle, burdock, common mullein, evening primrose, musk thistle, sweetclover, teasel, wild carrot, and wild parsnip.

Control should be directed at the first-year plants. After the seedhead has been produced, which is often the most visible part of the plant, the plant begins to senesce and die. Controlling annual and biennial plants with herbicides after flowering and seed production is of little or no benefit. Mechanical cutting to reduce the height of the flower stalk may be more practical.

Perennial plants live more than two years. Some live for many years. Most will go through the vegetative, seed production, and mature stages each year for several years. Others may grow for several years before they produce seeds. Some perennial plants, such as dandelions, die back each winter in northern climates. Others, such as shrubs and trees, may shed their leaves, but do not die back to the ground. In milder climates, some perennials can remain green year round. Most perennials grow from seed, but many also produce vegetative reproductive structures. Perennials are quite variable, but can be grouped into three broad categories based on root characteristics: simple, bulbous or tuberous, and creeping.

Simple perennials will spread by seed and root segments. These plants have persistent root systems but they do not usually spread by root segments unless broken into parts by mechanical methods. They include bluestem (big and little), broomsedge, plantain (buckhorn and broadleaf), chicory, curly dock, dandelion, goldenrod, spiderwort, vaseygrass, white heath aster, and most trees and shrubs.

Some perennials reproduce vegetatively from underground bulbs or tubers, as well as by seed (Figure 3). Bulbs are swollen underground leaf bases. Some examples include wild garlic and wild onions. Tubers are swollen tips of rhizomes. They contain buds that are capable of resprouting. Examples of plants with tubers include Jerusalem artichoke, and yellow and purple nutsedge. Bulbs and tubers can be spread by soil disturbance, and can resprout when the parent plant has been controlled.

Creeping perennials spread vegetatively from stolons (horizontal stems running on the soil surface usually rooting at the joints) (Figure 4), by rhizomes (underground horizontal stems modified for food storage and asexual reproduction) (Figure 5), or by seed. Creeping perennials usually occur as a patch that continues to enlarge each year. Roots of creeping perennials can be located off the right-of-way while the spreading stems or vines continue to reinvade the treated area. This makes the herbicide treatment appear to be ineffective. Repeated treatments with a translocated herbicide may be necessary. Examples include bermudagrass, blackberries, Canada thistle, cogongrass, common milkweed, Dalmatian toadflax, hemp dogbane, horsenettle, horsetail, Japanese knotweed, Johnsongrass, leafy spurge, multiflora rose, Phragmites (common reed), prairie cordgrass, purple loosestrife, quackgrass, red sorrel, scouringrush, St. Johnswort, yellow toadflax, field bindweed, hedge bindweed, Japanese honeysuckle, kudzu, poison ivy, trumpetcreeper, Virginia creeper, and wild grape.

Perennial plants are also best controlled while seedlings. Mature perennials are difficult to control because their persistent roots and stems enable the plants to resprout. Defoliating perennial plants provides only temporary growth suppression by killing the above ground plant. Herbicides that move through the plant (translocate) are most effective if applied after rapid vegetative growth has stopped and the plant has begun to store food reserves in its roots. Having the herbicide move to the roots and underground stems is important when controlling perennial plants.