The University of Arizona

About Weeds

Weeds compete for water, nutrients and sunlight and reduce the vigor and growth of ornamental landscape plants. A weed is any plant that interferes with the management objective for an area of land at a particular time. Weeds are usually plants that are very prolific, invasive, competitive, harmful, destructive, or difficult to control. Weeds detract from the appearance of landscapes and can be hosts for disease causing organisms and insects. They may also provide shelter and food for insects and rodents that feed on ornamental plants and invade buildings. The important aspects of this weed definition are that a plant is a weed only in the eyes of the person considering it a weed and that a plant is only a weed at a specific time and place. Thus, landscape managers should be cautious about using weed control measures such as soil sterilents that preclude alternate land uses in future years.

Weed Life Cycles

Weeds can be grouped by life cycle in ways that are useful for discussing weed identification and control.

Herbaceous plants in temperate climates produce nonwoody aerial stems that grow, flower, produce seed and die each year, and are either annuals, biennials or perennials.

Annual plants live for only one growing season. They are referred to as winter annuals if they germinate, grow, flower and produce seed during the fall and winter or as summer annuals if they germinate, grow, flower and produce seed during the spring and summer. Common summer annuals in desert landscapes include spurges, pigweeds, and puncturevine. Common winter annuals include London rocket, thistles, little mallow, redstem filaree, annual bluegrass, mediterraneangrass, and wild barley.

Biennial plants require parts of two seasons to grow, flower and produce seed. In Arizona, biennials typically germinate in the fall, grow a basal rosette of leaves during the winter, and then flower and produce seed in the spring and early summer. Some annual weeds in Arizona may grow as biennials during parts of two seasons when favorable moisture and temperature conditions exist (e.g., camphorweed, little mallow, and Russian thistle). Biennials are not common in Arizona, are not that different from winter annuals in our climate, and can be managed using the strategies discussed for annual weeds. The survival or reoccurrence of annual and biennial weeds in a landscape depends on the production of seed by weeds in the landscape and on the movement of seeds in to the landscape by animals, wind, and water.

Herbaceous perennials live indefinitely and produce new aerial stems each year from stolons and underground roots and stems (rhizomes and tubers) that survive for several years. Most perennials also reproduce by seed so new seedlings germinate every year. Herbaceous perennials are some of the most difficult to control landscape weeds because the underground vegetative structures are difficult to kill. Common perennial weeds in landscapes include bermudagrass, purple nutsedge and woodsorrel.

Woody perennials such as trees and shrubs have woody aerial stems that persist from year to year in contrast to herbaceous plants. Woody plants have large underground root systems that make them difficult to remove or control in landscapes. Common unwanted woody perennials in desert landscapes include desert broom and salt cedar as well as mesquites and palo verdes growing in the wrong place.

Weed Types

Weeds that commonly occur in desert landscapes can also be grouped by type depending on the number of seed leaves or cotyledons and type of leaf venation.

Dicots or dicotyledons (also called broadleaf plants or broadleaves) have seedlings with two seed leaves or cotyledons. Dicots or broadleaf plants usually have leaves with pronounced netted or branched venation. Dicot flower parts usually occur in groups of four or five.

Monocots have seedlings with one seed leaf or cotyledon. Grasses and sedges are the two groups of monocots that commonly occur in Arizona desert landscapes. Grasses and sedges have leaves that are long and narrow with a parallel venation pattern and have flower parts that typically occur in groups of three.

Grasses have leaves arranged in two ranks or rows along the stem, the edges of leaf sheaths are not united, the stems are cylindrical or oval in cross section and the stems are usually hollow except at the joints or nodes in the stem.

Sedges are grass-like plants that have leaves arranged in three ranks or rows along the stem, have leaf sheath edges that are united, have stems that are triangular in cross-section and the stems are solid.

Being able to distinguish between dicot and monocot weeds is important because some herbicides will control dicot but not monocot weeds and vice versa. In addition, herbicides that control grasses may not control sedges. Purple and yellow nutsedge are two common sedges in Arizona landscapes that can be very difficult to control because they are perennials with extensive underground root and rhizome systems. Some common dicot and monocot weeds that occur in Arizona landscapes are listed in Table 1 and Table 2, respectively.

Descriptions and illustrations of many Arizona weeds can be found in An Illustrated Guide to Arizona Weeds by Kittie F. Parker, from the University of Arizona Press.

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