Brassica plants have a characteristic cotyledon shape with a distinct notch at the apex. The flower has four petals arranged in a cross with six stamens. This plant family includes several winter annual weeds such as shepherd's purse, as well as vegetable crops such as cabbage and broccoli. The mustard family consists of herbaceous plants of cool to temperate climates.
While the weedy members of this family are well known and sometimes troublesome to vegetable growers, only one—shepherd's purse—is listed among the world's 76 major weeds Holm et al. Brassica weeds can be a reservoir of pests and diseases of related vegetable crops, yet flowering brassicas weedy or cultivated often attract the natural enemies of the brassica pest complex.
The pink family includes common chickweed, mouse-ear chickweed, corn spurry, and corn cockle, as well as ornamental flowers such as carnation, pinks, sweet William, baby's breath, and Maltese cross. Common chickweed is particularly troublesome in garlic, early greens, other cool season vegetables, and cool-season forage crops, but it usually does not seriously impact warm-season vegetable or row crops. The pink family is characterized by simple, opposite leaves arising from swollen nodes, regular flowers, usually with five petals, but sometime with the petals deeply notched or lobed to give the appearance of 10 petals per bloom, as seen in the chickweeds Fig.
Figure 7. Weeds in the pink family have oppositely-arranged leaves, and small five-petaled flowers with the petals deeply lobed, giving the appearance of 4 a. Common chickweed b is a widespread winter annual or occasionally perennial weed in this family.
Common lambsquarters, whose young foliage is edible and as nutritious as spinach, chard, and beet also members of the goosefoot family , hardly needs introduction to vegetable and grain farmers Fig. Considered the world's tenth worst weed Holm et al. It is highly prolific, and one large plant can shed , viable seeds. Figure 8. Young a and mature b lambsquarters plants. Kochia is another summer annual weed in the goosefoot family.
It has become a problem in annual crops in the northern Great Plains region. Dayflowers are often mistaken for dicots because their leaves are wider than those of most monocots. They have three-petaled blue to purple flowers in some species, one petal is smaller and paler than the other two , with six stamens.
Asiatic dayflower and spreading dayflower are sprawling summer annual herbs that have become naturalized in moist soils near streams and bottomland woods throughout the eastern United States. They are occasional weeds of gardens and fields. A more recent and more invasive introduction is the Benghal dayflower, which spreads aggressively by rhizomes, and can form flowers and seeds either above or below ground. It has become a severe problem in cotton and other crops in parts of Florida and the Gulf Coast.
Figure 9. Asiatic dayflower a and Benghal dayflower b. The morning glory family includes the incredibly deep-rooted 3 to 18 feet wandering perennial field bindweed, recognized as the world's 12th most damaging weed Holm et al. The parasitic dodders are also members of this plant family. Figure Hedge bindweed climbs this fence and covers the ground between clumps of grass weeds a.
Uprooted bindweed shows the vining habit and the underground network of brittle, white fleshy rhizomes b. The one-inch fragment separated from the rest of the plant can regenerate a new plant from as deep as six inches.
Ivyleaf morning glory c and tall morning glory d are two species of weedy annual morning glory that commonly occur in vegetable and row crops in warm-temperate climates.
Family characteristics include showy, funnel- or tube-shaped flowers, simple alternate leaves, and a climbing, vine-like habit of growth. Many of these weeds climb any upright support, including crop plants, whose growth and leaf opening can become seriously hindered as a result. Weeds in the Convolvulaceae thrive in summer heat, as does the sweet potato, one important food crop in this family.
Sweet potato vines do not climb, but instead form a thick canopy, which effectively suppresses many weeds including yellow nutsedge.
While this family includes no major crops in the United States, and only a few weed species, one of the latter is purple nutsedge, once cited as the world's worst weed Holm et al.
This small, heat-loving plant causes serious economic losses in most tropical and warm-temperate crops around the world. A wandering perennial, purple nutsedge is highly allelopathic against many crops, and can form an underground biomass density of rhizomes, bulbs, and tubers that rivals or exceeds the total biomass of a vigorous 9-ft crop of sorghum. Yellow nutsedge is also quite aggressive and has invaded colder regions up to the Canadian border.
Holm et al. Cultivated strains of yellow nutsedge are grown in India for their edible tubers chuffa. Sedges are characterized by grass-like leaves deployed in a three-ranked arrangement, and solid stems distinctly triangular in cross section Fig. In contrast, true grasses have rounded stems, which are hollow between the nodes, and leaves in a two-ranked arrangement. Sedges are characterized by leaves arranged in three ranks around a solid, triangular stem a , whereas true grasses have two-ranked leaf arrangement and hollow, cylindrical stems.
This vigorous yellow nutsedge shoot b arises from a tuber in the top few inches of soil. Cypress spurge Fig. Other common weeds in this family include leafy spurge a major rangeland weed in the western United States , Virginia copperleaf, wooly croton, and spotted spurge. One of the characteristics of this plant family is the acrid, milky juice exuded from cut tissues, which can be a severe eye irritant.
Flowers are small and inconspicuous, often imperfect stamens and pistils borne on separate flowers , and often lacking petals. In many species, several flowers are grouped together in a cuplike structure, which itself resembles a flower. Cypress spurge, Euphorbia cyparissias L. The legume family plays a special role in organic and sustainable agriculture, as these plants fix atmospheric nitrogen N through their symbiotic relationship with Rhizobium bacteria.
Legumes provide protein-rich beans, peas, pulses, and livestock forages such as alfalfa, clovers, and lespedeza. This plant family includes a few summer annual weeds, such as hemp sesbania, and showy crotolaria, as well as kudzu, a highly invasive exotic perennial vine, which can overgrow and kill mature forest trees Fig.
In addition to their N-fixing root nodules, legumes typically have compound leaves trifoliate, pinnate, or palmate , and bear seeds in pods that dry and split open along both sutures. Many legumes have characteristic pea blossoms with petals differentiated into banner, wings, and keel; and 10 stamens with the filaments of 9 or all 10 fused into a strap or tube Fig.
Many legumes have a characteristic pea flower a , though some species such as mimosa have more regular flowers. Weeds in this family range from white clover b causing occasional problems in low-growing vegetables, to kudzu c smothering mature trees.
The Lamiaceae includes several winter annual and perennial weeds, including purple deadnettle Fig. The mint family is characterized by aromatic foliage rich in essential oils, opposite leaves borne on stems that are square in cross section, and usually a distinctly bilabiate flower with four stamens. The flowers are rich in nectar and often attract and support natural enemies of insect pests. Common weeds in the mint family include the winter annual purple deadnettle a and the perennial ground ivy b.
Weedy members of this family that are familiar to vegetable and grain growers include henbit, deadnettles, and ground ivy. Mint itself can become a troublesome weed, as its perennial rhizomes readily spread from the mint patch into neighboring crops.
The mallow family includes the summer annual weeds velvetleaf, prickly sida, and spurred anoda, as well as the perennial common mallow. Velvetleaf is a major weed of corn and other row crops, especially in fields that are plowed annually.
The mallow family also includes several important crops: cotton, okra, kenaf, hibiscus, and ornamental mallows. The mallow family is characterized by showy flowers with five sepals, five petals, and numerous stamens whose filaments are fused into a tube surrounding the style of the pistil.
Emerging seedlings of some species bear one roundish and one distinctly heart-shaped cotyledon Fig. The flower has numerous stamens fused into a tube surrounding the pistil.
Yellow woodsorrel Fig. While it is most often a weed of turf, landscapes, and greenhouse plants, it can occur in vegetable fields, especially under reduced tillage. It has trifoliate compound leaves, and small, regular flowers with five yellow petals. Yellow woodsorrel, Oxalis stricta L. Pokeweed is a well-known and spectacular herbaceous perennial.
It comes up from heavy fleshy taproots every spring and can grow as tall as 10 feet Fig. The foliage and berries are poisonous to humans and livestock, though newly emerging foliage can be eaten if properly prepared.
American pokeweed, Phytolacca americana L. The plantain family includes several weedy species, such as broadleaf plantain and buckhorn plantain, which are most troublesome in turfgrass, though they can occur in cultivated fields and gardens as well. The weedy plantains are low-growing perennials characterized by parallel leaf veins unusual in dicots. The sessile without petiole leaves grow in a basal rosette, from which arises a leafless stalk bearing a dense spike of tiny, inconspicuously colored flowers Fig.
Broadleaf or common plantain, Plantago major L. This plant family also includes plants that were formerly classified in the Scrophulariaceae or Figwort family, such as yellow toadflax Fig. The grasses comprise a large family of monocots—clearly the most agriculturally important plant family on Earth. In addition to many of the world's staple food and fodder crops corn, rice, wheat, millets, sorghums, other annual cereal grains; and many perennial and annual forages , the grass family contributes 29 of the 76 species listed as the most important agricultural weeds worldwide, including 10 of the top 18 Holm, et al.
Bermuda grass, barnyard grass, jungle rice, goosegrass Fig. The weedy species include both annuals e. Two major weeds in the grass family include goosegrass a and johnsongrass b. The most definitive characteristics of grasses are that the single cotyledon remains underground inside the seed after germination, and that the seedling shoot emerges as a rolled or folded set of leaf blades protected by a sheath coleoptile that opens at the soil surface to release the expanding leaf blades.
The growing point remains underground for some weeks after emergence unlike broadleaf plants Fig. As a result, young annual grass weeds are more difficult to control by cultivation or flame weeding than annual broadleaf weeds of the same size. Mulching is also less effective on grasses than other weed seedlings, as the coleoptiles can penetrate organic mulches more readily than the delicate shoots of broadleaf seedlings.
Grass seedlings emerge as a pointed coleoptile, which protects the new leaves as the seedling pushes through the soil and surface residues. The growing point remains below ground for several weeks and can regenerate the plant if the top is severed or frozen. Most broadleaf seedlings bring both growing point and cotyledons above the soil surface upon germination, leaving them more vulnerable to cultivation or unfavorable weather conditions such as frost.
Other grass characteristics include narrow, linear leaves often times as long as wide ; small, inconspicuous flowers borne on a spike or panicle at the top of the stem, small dry fruits each containing a single seed or grain, and a strong, fibrous root system. Perennial grasses may or may not have stolons or rhizomes, but never taproots. Quack grass, Bermuda grass, and johnsongrass are all wandering creeping perennials that propagate through strong, aggressive rhizomes or stolons.
The buckwheat family includes the knotweeds, smartweeds, curly dock, broadleaf dock, and red sorrel, as well as buckwheat, a valuable grain and cover crop. Wild buckwheat and mile-a-minute are rampant annual vines that climb, shade, and bind other plants much like bindweeds and morning glories. The docks and wild buckwheat are considered among the 76 globally important weeds cited by Holm et al. One distinguishing characteristic of the buckwheat family is the ocrea, a prominent membranous or leafy sheath encircling the stem and base of the petiole at each node Fig.
Let us look at them. Plant embryos in seeds have structures called cotyledons. A cotyledon is the central portion of a seed embryo to which the epicotyl immature shoot and radicle immature root are attached.
The number of cotyledons differs in these two groups of plants and that forms the basis for the main classification of monocots and dicots. A seed of a monocot plant has one cotyledon and that of a dicot plants has two cotyledons. Roots can develop either from a main radicle that is one large taproot with many small secondary lateral roots growing out of it, or can be a fibrous mass of roots that arise from the nodes in the stem, called adventitious roots.
Monocots have adventitious roots, whereas dicots have a radicle from which a root develops. Leaves have more than one characteristic that help differentiate a monocot from a dicot. If the leaf has a stalk, then the plant is a dicot.
But, in the case of a monocot plant, the leaf is sessile, which means it is attached directly by its base without a stalk. The next characteristic that helps in the identification is the venation. If the leaves have parallel venations that are long and thin, then the plant is monocot. If the leaves have a branched venation, then the plant is a dicot.
Monocot flowers tend to have a number of petals or other floral parts that is divisible by three, usually three or six. Dicot flowers on the other hand, are likely to have parts in multiples of four or five four, five, ten, etc.
This character is not always reliable, and is not easy to use in identification of some flowers with reduced or numerous parts. Monocot and dicot plants have different pollen structures. In a monocot, the pollen grain produced by the flower has a single furrow or pore through the outer layer. In a dicot plant, the pollen grain has three furrows or pores. Monocot plants normally have a weak stem, whereas dicots have a strong stem. Vascular tissues are seen as long strands and are called vascular bundles.
In the dicot plant, the vascular bundles are arranged in a ring form, whereas in a monocot, these bundles appear scattered through the stem, with more of the bundles located towards the periphery outer edge of the stem than at the centre.
Note: There are some exceptions to this classification however, as some species of plants belonging to monocots can have characters belonging to dicots, since the two groups have a shared ancestry.
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