An Illustrated Guide to Arizona Weeds

The University of Arizona Press

General Information


Economic losses from weeds are of continuing concern to Arizona farmers and ranchers. Weeds decrease farm income by robbing the soil of precious moisture that would otherwise be available for crop production, by utilizing soil nutrients needed by cultivated plants, by lowering the quality of farm products because of weed impurities, and by increasing the cost of labor, equipment, and irrigation. Many weeds also harbor some of the worst crop insect pests, and are alternate hosts to organisms causing crop diseases. Some weeds are parasites on useful plants.

On rangelands, weeds may seriously decrease grazing capacity for livestock by competing with the good forage plants for moisture and nutrients. Other undesirable range plants may be poisonous, causing reduced weight gains, lowered animal production, or even death. Some range weeds are troublesome to domestic livestock because of their thorns, barbs, stiff hairs, or sharply pointed seeds. Such may cause mechanical injury to the eyes, mouth parts and intestines, and to the skin and the hide. They may lower the value of wool and mohair, or cause wounds that invite attack from screwworms.

Weeds in the yard dampen the home owner's interest in improving and maintaining gardens that beautify the home, and make the community attractive and livable. Weeds also raise the homeowner's costs for water and fertilizer.


A weed is any plant that grows where it isn't wanted, or is unwanted because of certain undesirable characteristics. Wild roses growing in a pasture are weeds. Bermudagrass is excellent as a summer lawn, but a pernicious weed in flower beds, crop lands, or ditchbanks. Even in lawns, bermudagrass may be undesirable and thus a weed to people afflicted with hay fever. A weed may be almost a personal matter.

Plants that come into crops, irrigated lands, lawns, and gardens; that become established on disturbed soil along roadsides, ditchbanks, neglected fields, and waste places; that invade and increase on rangelands, replacing the cover of perennial grasses or sometimes causing livestock poisoning, are weeds. Such plants include fungi, ferns, grasses, broadleaved herbs, shrubs, and even trees.

CROP, GARDEN AND LAWN WEEDS - Many of the most serious of these weeds in Arizona have been introduced into the United States from the Old World, as wild oat (Avena fatua), nutsedge (Cyperus rotundus, C. esculentus), johnsongrass (Sorghum halepense), junglerice (Echinochloa colonum), hoary cress (Cardaria draba), knotweed (Polygonum argyrocoleon, P. aviculare), field bindweed (Convolvulus arvensis), curly dock (Rumex crispus), and Russian knapweed (Centaurea repens).

Some native plants have also become established as serious pests, as Palmer amaranth (A maranthus palmer), silverleaf nightshade (Solanum elaeagnifolium), Texas blueweed (Helianthus ciliaris), alkali side (Sida hederacea), Wright groundcherry (Physalis wrightii), and red sprangletop (Leptochloa filif ormis).

WEEDS OF ROADSIDES, DITCHBANKS, AND WASTE PLACES - May be very aggressive and troublesome. Two of the worst offenders, Russian thistle (Salsola kali var. tenuifolia), and puncturevine (Tribulus terrestris), are both Old World introductions.

The common sunflower (Helianthus annual), crown beard (Verbesina encelioides var. exauriculata), alkali heliotrope (Heliotropum curassavicum), slimleaf bursage (Franseria confertiflora), spiny aster (Aster spinosus), and horseweed (Erigeron canadensis), are either native weeds, or were introduced from other parts of the United States.

RANGE WEEDS - Are mostly native plants of low forage value. Opportunity for their increase comes about through overgrazing, drought, and other disturbing influences particularly detrimental to the good forage plants. The junipers (Juniperus spp.), mesquite (Prosopis juliflora), burroweed (Haplopappus tenuisectus), broom snakeweed (Gutierrezia sarothrae), and cholla (Opuntia spp.), are examples of such native invaders.

POISONOUS RANGE WEEDS - Some plants normally produce some specific substance which, when eaten in sufficient quantity, causes illness or death in livestock; other plants that ordinarily provide good forage may under certain periods of physiological change in the growth pattern produce poisonous substances.

Johnsongrass (Sorghum halepense), patota (Monolepis nuttalliana), Palmer amaranth (Amaranthus palmer), and Russian thistle (Salsola kali var. tenuifolia) may, under favorable growth conditions, accumulate high concentrations of nitrate and become poisonous even in small quantities. Johnsongrass may also produce hydrocyanic acid under a variety of environmental conditions, as when growth is stunted, or growth is interrupted and then resumed, as a result of freezing or drought. Other range plants producing hydrocyanic acid are arrowgrass (Triglochin spp.), whitethorn, (Acacia constricta), and mountainmahogany (Cercocarpus spp.).

The soil environment becomes important when it contains high concentrations of the element selenium. Certain plants, such as species of loco (Astragalus spp.), aster (Aster, spp. Machaeranthera spp.), and saltbush (Atriplex spp.), absorb the selenium and may accumulate toxic amounts, causing selenium poisoning in livestock. Such plants have the ability to change insoluble selenium to a soluble form so that other plants growing nearby may also cause poisoning by absorbing the selenium.

In most poisonous plants, however, the presence of the toxic principle is not influenced by physiological or environmental variations. There are numerous poisonous compounds which may be present normally in certain plants. The effect of these substances is usually relative to the animal's body weight and the amount of the compound eaten, which often is cumulative. Some of these important stock-poisoning plants in Arizona are loco (Astragalus spp.), larkspur (Delphinium spp.), western whorled milkweed (Asclepias subverticillata), snake-weed (Gutierrezia spp.), cocklebur (Xanthium spp.), burroweed (Haplopappus tenuisectus), jimmyweed (H. plurifolius), pinque (Hymenoxys richardsonii var. floribunda), and threadleaf groundsel (Senecio longilobus).

Some plants are more poisonous when young, as larkspur (Delphinium spp.) or cocklebur (Xanthium spp.), which is poisonous in the seedling stage. Others, like the lupines (Lupinus spp.), are more dangerous in seed. All parts of the plant either green or dry, as in hay, are poisonous in western whorled milkweed (Asclepias subverticillata), loco (Astragalus spp.), western sneezeweed (Helenium hoopesii), or western bracken (Pteridium aquilinum var. pubescens). Some weeds are about equally poisonous to all kinds of livestock, while others affect only certain kinds of animals. Thus pingue (Hymenoxys richardsonii), western whorled milkweed (Asclepias subverticillata), or lupine (Lupinus spp.) are harmful chiefly to sheep, and larkspur (Delphinium spp.), snakeweed (Gutierrezia spp.), or whitethorn (Acacia constricta) mainly to cattle.

THE "GOOD" WEEDS - Not all weeds are "bad," but when plants grow in weedy places they suffer from the stigma of being classed as weeds. Many of these are native plants that come in great abundance to the moist, disturbed areas along roadsides and waste places, or that grow in sterile caliche soil, their native habitat, in adjacent mesas, or vacant lots. These weeds are usually not aggressive, nor poisonous to livestock, nor do they invade croplands and thus compete in cultivated areas. The highways throughout Arizona from spring through late summer are lined with colorful weeds that often help to beautify our state. Many unimportant weeds are described here because they are abundant and people often want to know their names. Some of these are: globemallow (Sphaeralcea spp.), trailing four-o-clock (Allionia incarnata), woolly tidestromia (Tidestromia lanuginosa), Gordon bladderpod (Lesquerella gordoni), western clammyweed (Polanisia trachysperma), Rocky Mountain beeplant (Cleome serrulata), hairy bowlesia (Bowlesia incana), skeleton weed (Eriogonum deflexum), desert senna (Cassia covesii), and slimpod senna (C. Ieptocarpa).


Grouped according to their growth habits, there are three principal classes of weeds: annual weeds, biennial weeds, and perennial weeds.

ANNUAL WEEDS - Live one growing season. They flower, produce seed, and then die down entirely. Most annuals, and also biennials, are dependent upon seed alone for reproduction and are usually prolific seeders. There are two general types of annuals: summer annuals and winter annuals.

SUMMER ANNUALS - Are true annuals. They germinate in the spring, mature and produce seeds during the summer, and die in the fall or winter when frost occurs. Some examples are junglegrass (Echinochloa colonum), red sprangletop (Leptochloa filiformis), Russian thistle (Salsola kali var. tenuifolia), Palmer amaranth (Amaranthus pallmeri), and puncturevine (Tribulus terrestris). A few summer annuals also produce new plants from prostrate stems rooting at the nodes or joints as large crabgrass (Digitaria sanguinalis) and southwestern cupgrass (Eriochloa gracilis).

WINTER ANNUALS - Germinate in late summer or fall and live through the winter as a small rosette of leaves. In the spring they mature rapidly and produce flowers and seeds. Many of these weeds also grow as summer annuals. In central and southern Arizona, such annuals as London rocket (Sisymbrium trio), shepherdspurse (Capsella bursa-pastoris), nettleleaf goosefoot (Chenopodium murale), silversheath knotweed (Polygonum argyrocoleon), spiny sowthistle (Sonchus asper), wild oat (Avena fatua), and many others grow luxuriantly during the short mild winters.

BIENNIAL WEEDS - Live two growing seasons. They germinate in the spring and spend the summer and winter in a rosette form, storing up food in thick roots, usually a taproot. Growth is resumed the next spring, producing a flowering plant which sets seeds, matures, and dies. There are few biennial weeds in Arizona. Common mullein (Verbascum thapsus) and golden corydalis (Corydalis aurea) are true biennials. Some weeds, commonly annual in colder areas, may become biennial in habit in Arizona, as little mallow (Malva parvipora), camphorweed (Heterotheca suboxillaris), white sweetclover (Melilotus alba), and annual yellow sweetclover (M. indica).

PERENNIAL WEEDS - Live more than two growing seasons. The aboveground parts usually die at the end of the growing season. At the next growing season, new growth arises from the underground structures, which live from one season to the next. In addition to maintaining these old plants, new plants are always produced from seed, and often also from vegetative structures. Based on their methods of producing new plants, most perennial weeds may be classed as simple perennials or creeping perennials. A few possessing unusual structures are classed as specialized perennials.

SIMPLE PERENNIALS - Depend entirely on seed for the production of new plants, except where occasionally pieces of the root crown may be broken off and new plants started. Simple, non-grass perennials usually have a thick taproot topped by a root crown from which new growth arises each growing season. The taproot may be elongated, as in curly dock (Rumex crispus), tuberous side (Sida hederacea), and dandelion (Taraxacum vulgare); or it may be short and covered by a mass of fibrous side roots as in broadleaf plantain (Plantago major) and buckhorn plantain (P. Ianceolata) Simple perennial grass weeds have a thick tuft of fibrous roots, as dallisgrass (Paspalum dilatatum), squirreltail (Sitanion hystrix), and foxtail barley (Hordeum jubatum).

CREEPING PERENNIALS - Produce new plants from seed also, but their principal means of reproduction is vegetatively from creeping or horizontal stems and creeping or horizontal roots.

CREEPING OR HORIZONTAL STEMS - Are branches that arise from the lower stem nodes, spreading out horizontally from the plant, and may be underground or aboveground.

Creeping underground stems are called rhizomes or rootstocks. These are true stems, consisting of a succession of nodes or joints, with leaves (usually scalelike) on the nodes. and a bud in the axil of each leaf. New plants with leafy shoots and adventitious roots, also the secondary branches, always arise at the nodes and not just at any point as they do in horizontal roots. Creeping stems may be found near the surface or penetrate deeper by gradually sloping downward, never vertically straight down as in roots. Weeds of this type are difficult to eradicate for the rhizomes may store considerable food or extend over large areas. Also, the bud at the node is capable of sending up a new plant even when the rhizome is cut into pieces. Johnsongrass (Sorghum halepense) and red sorrel (Rumex acelosella) are examples.

Creeping aboveground stems, called runners or stolons, trail along on top of the ground. They are long, slender, and leafy, forming roots at nearly every node; or nearly leafless and taking root only near the tip. These weeds are usually not as hard to eradicate as those with rhizomes. Rocky Mountain iris (Iris missouriensis), cinquefoil (Potentilla anserina), and white clover (Trifolium repens) are examples. A few weeds possess both creeping underground stems (rhizomes) and aboveground stems (stolons), as bermudagrass (Cynodon dactylon).

CREEPING OR HORIZONTAL ROOTS - Are lateral branch roots. These are true roots, arising from a vertical root, usually an elongated taproot, and lack nodes and leaves. Adventitious buds may form at any point along these roots and produce new plants by sending up leafy shoots. Creeping roots can penetrate to a much greater depth than creeping stems. Most of the taproot may extend below cultivating depths, and produce new deeper, horizontal branches when the upper ones are destroyed. Also, the horizontal roots can grow straight downward at any point, thus establishing very deep, extensive root systems, which are virtually impossible to eradicate.

Some of the worst prohibited noxious weeds reproduce by creeping roots, as Russian knapweed (Centaurea repens), hoary cress (Cardaria draba), field bindweed (Convolvulus arvensis), Canada thistle (Cirsium arvense), silverleaf nightshade (Solanum elaeagnifolium), and blueweed (Helianthus ciliaris).

SPECIALIZED PERENNIALS - A few perennial weeds reproduce by means of unusual structures such as tubers, bulbs, and bulblets.

TUBEROUS PERENNIALS - In addition to reproducing by seed and vegetatively by means of creeping stems or creeping roots, a few weeds also produce new plants from small tuberous enlargements formed at the ends of the underground stems or roots. These are storage organs, capable of sending up new plants at any time. They often remain in the ground when the rest of the plant has been grubbed out.

The tubers formed at the ends of underground stems are actually modified rhizomes. Before maturity they are covered by scaly leaves at the nodes, indicating that they are stem structures, as in yellow nutsedge (Cyperus esculentus) and purple nutsedge (C. rotundus). Tubers formed by creeping roots are clearly root structures, as in hogpotato (Hoffmanseggia densiflora). These lack the nodes and scalelike leaves of the stem tubers, even at a very early stage.

BULBOUS PERENNIALS-Usually belong to the lily family, as wild onion. These weeds are common in Arizona, but none are discussed in the text. They seldom reproduce by seed, but rather by the multiplication of bulbs and bulblets (tiny bulbs) under the ground. Aerial bulblets may also be formed, often in the leaf axils.


Noxious-weed seeds are so declared by law. They may be divided into two classes: prohibited noxious-weed seed-produced by weeds which are so highly detrimental and difficult to control that the sale of commercial planting seed which contain any of these seeds is prohibited; and restricted noxious-weed seed from very objectionable weeds, but more easily controlled, thus a limited amount of such seed is permitted. Each of the states has its own seed laws in which the "prohibited noxious-weed seed" and the "restricted noxious-weed seed" are listed by both their common and scientific names. In addition, the maximum amount of restricted noxious-weed seed permitted is stated. The laws also require that seed labels state the quantity per ounce or per pound of any restricted noxious-weed seed present.

PROHIBITED NOXIOUS-WEED SEED IN ARIZONA - The Arizona Seed Law designates the following as "prohibited noxious-weed seed" in Arizona: (The plants with asterisks are not known to date in Arizona, but are in nearby states. By prohibiting their seed in commercial seed, the chance of their entry is greatly reduced.)

Bindweed, field (Convolvulus arvensis L.)

Blueweed, Texas or blueweed (Helianthus ciliaris DC.)

Camelthorn (Alhagi camelorum Fisch.)

Cress, hoary or whitetop (Lepidium draba L., Cardaria draba Desv., or *L. repens Boiss. )

* Horsenettle (Solanum carolinense L. )

Horsenettle, white or silverleaf nightshade (Solanum elaeagnifolium Cav.)

Knapweed, Russian (Centaurea repens L., or C. picris Pall.)

Morningglory (Ipomoea spp.)

Nutsedge, purple (Cyperus rolundus L.)

Nutsedge, yellow (Cyperus esculentus L.)

Perennial sorghum (Sorghum spp.) as johnsongrass (S. halepense L.), sudangrass (S. sudanese [Piper] Stapf.), and *sorghum almum (S. almum Parodi)

*Sowthistle, perennial (Sonchus arvensis L.)

*Spurge, leafy (Euphorbia esula L.)

Thistle, Canada (Cirsium arvense (L.) Scop.)

*Whitetop, hairy or hoary cress (Cardaria pubescens (C. A. Mey.) Roll. var. elongata Roll., or Hymenophysa pubescens C. A. Mey.)

RESTRICTED NOXIOUS-WEED SEED IN ARIZONA - Seeds of the following weeds are declared as "restricted noxious-weed seed" in Arizona: (The numerals after each plant name indicate the maximum number of these particular seeds allowed per pound in commercial planting seed. A total of 500 of all such seed per pound is allowed.)

Dock, curly (Rumex crispus L.) . . . . . . . . . . . . . 300

Dodder (Cuscuta spp.) . . . . . . . . . . . . . . . . 45

Mallow, alkali (Sida hederacea [Dougl.] Torr.) . . . . . . . . 300

Mustard, wild or wild turnip (Brassica spp.) . . . . . . . . 300

Oat, wild (Avena fatua L.) . . . . . . . . . . . . . . 100

Puncturevine (Tribulus terrestris L.) . . . . . . . . . . . 100

Sandbur (Cenchrus pauciflorus Benth.) . . . . . . . . . 100

Thistle, Russian (Salsola kali L. var. tenuifolia Tausch.) . . 300


All weeds have both a common name and a scientific name. A single common name for a given weed has been recommended by the Weed Society of America in their report of Common and Botanical Names of Weeds. This recommended name is printed in capital letters in this book. Additional common names applied to a given weed are also listed, not as a choice of common names, but to facilitate the weed's identification.

The use of common names has resulted in such confusion that it is necessary to use a plant's scientific name. No laws cover the usage of common names; the same common name is often given to a variety of weeds, particularly closely related species. Also, most weeds have numerous common names. To end this confusion, the name recommended by the Weed Society should be universally adopted, even though it might not always be as popular as another. Now that each particular weed species has been given a precise common name, it should be used.

A plant scientific name is governed by strict international botanical law and is an exact name for one particular species. Sometimes a name must be changed due to increased knowledge. The original name then becomes a synonym of that species, and though outmoded, it still refers to that species and cannot be applied to another. The author's name is an essential part of the scientific name, and is abbreviated at the end of the Latin name.

For the sake of uniformity, the same scientific name as given in Arizona Flora is used unless another name is now the accepted one. (The older name is then placed in parentheses.)


The purpose of this book is to assist the farmer, rancher, homeowner, or agricultural student to recognize common Arizona weeds. Most weeds may be identified by comparison with the drawings, and checking the plant description. To identify a weed not described in the book, use Kearney & Peebles, Flora of Arizona, or send the weed to the Herbarium, University of Arizona, Tucson, for free identification. The specimen should be pressed and dried before sending, and should include all parts-roots, stems, leaves, flowers, and fruits.

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