How many plants have these weedy characteristics? Relatively few, in fact. There are approximately 250,000 species of plants in the world, but only about 200 species are considered to be major weed problems (Holm, et al., 1977). In addition to this small number of species there are relatively few plant families that contain major weeds. Of the 300 plant families, 75 families comprise 75% of all flowering plants, and of these only 12 families comprise 68% of the world’s worst weeds (Holm,et al., 1977). Within these 12 families, just 3 families comprise 43% of the world’s worst weeds, with 37% being in the Poaceae (grass family) and Asteraceae (composite family). Most of the major families of weeds also contain members that are major crops, such as grains in the Poaceae, beans/peas in the Leguminosae, and vegetables in the Solanaceae and Brassicaceae, to name a few. Other families have few crop representatives but many weeds, such as the Asteraceae.
The definition of a weed and the plant characteristics that contribute to its weediness are good to know. However, other factors such as habitat, growth form or seed type, and life cycle are important in identifying the most appropriate management practices for weeds in humans’ various plant-related activities and useful in determining specifically what weed is present in any given situation.
Classification on Habitat:
Habitat refers to whether the weed grows in a terrestrial or an aquatic environment. Weeds can be a problem in both habitats and can include epiphytic and parasitic types. Growth form or seed type can be used to classify plants in 3 categories. Gymnosperms, such as pines, have seeds not enclosed in an ovary. Examples include larch, fir, spruce, hemlock, Douglas fir, cedar, and redwood. Most gymnosperms are not considered to be weeds. Monocots, or flowering plants with one seed or cotyledon, generally have narrow leaves with parallel veins, but some monocots have large leaves with palmate-type veins, such as water hyacinth. Examples include lilies, irises, sedges, grasses, palms, orchids, cattails, sugar cane, and banana. Many of our most serious weed problems are monocots. An important distinction is that all grasses are monocots, but not all monocots are grasses. Dicots, or flowering plants with two seed leaves or cotyledons, include maple, oak, pigweed, common lambsquarters, and sunflower. Many of our most serious weed problems are dicots.
Classification on Life Cycle:
Life cycle refers to a plant’s life span, season of growth, and method of reproduction and determines the methods needed for management or eradication. Plants have been divided into three life cycle categories: annual, biennial,and perennial.
Annual plants complete their life cycle (from seed to seed) in 1 year or less. Normally, they are considered easy to control. This is true for any one crop of weeds. However, because of an abundance of dormant seed and fast growth, annuals are very persistent and they actually cost more to control than perennial weeds. Most common weeds are annuals, and there are two types—winter and summer. Winter annuals germinate in autumn or winter and overwinter as a rosette, resume growth in early spring, and produce fruit and seed and die by midsummer. The seeds often lie dormant in the soil during the summer months. In this group, high soil temperature (125°F or above) has a tendency to cause seed dormancy—to inhibit seed germination. Examples include chickweed, downy brome, hairy cress, cheat, sheperds purse, field pennycress, corn cockle, cornflower, and henbit. These weeds are most troublesome in winter-growing crops such as winter wheat, winter oats, and winter barley.
Summer annuals germinate in the spring, grow through the summer, and mature, form seed, and die by autumn. The seeds lie dormant in the soil until the next spring. Summer annuals include cockleburs, morningglories, pigweeds, common lambsquarters, common ragweed, crabgrasses, foxtails, and goosegrass. These weeds are troublesome in summer crops like corn, sorghum, soybeans, cotton, peanuts, tobacco, and many vegetables. A biennial plant lives more than 1 but less than 2 years. During the first phase of growth, the seedling usually develops vegetatively into a rosette. Following a cold period, vegetative growth resumes followed by floral initiation, fruit set, and death. There is confusion between the biennials and winter annuals because winter annuals normally live during 2 calendar years and during 2 seasons. Biennials generally grow later into the second season and tend to be larger plants. Examples include wild carrot, common mullein, bull thistle, wild lettuce, and common burdock. Several biennials are weed problems in minimum- or no-tillage systems and perennial crops.
A perennial plant lives for more than 2 years and is characterized by renewed growth year after year from the same root system. Most perennials reproduce by seed, and many are able to spread vegetatively. They are classified as simple, creeping, or woody.
Simple herbaceous perennials reproduce by seed and have no natural means of spreading vegetatively unless injured or cut; the cut pieces may produce new plants. For example, a dandelion or dock root cut in half longitudinally may produce two plants. The roots are usually fleshy and may grow very large. Examples include common dandelion, dock, buckhorn plantain, broadleaf plantain, and pokeweed.
Creeping herbaceous perennials:
Creeping herbaceous perennials reproduce by seed and by vegetative means, including creeping aboveground stems (stolons), creeping underground stems (rhizomes), or a spreading root system that contain buds. Examples include red sorrel, perennial sowthistle, quackgrass, bermudagrass, johnsongrass, and field bindweed. Some weeds maintain themselves and propagate by means of tubers, which are modified rhizomes adapted for food storage. Examples include purple and yellow nutsedge and Jerusalem artichoke.
In all cases, creeping perennials have tremendous vegetative reproductive capacity and are the most difficult weed problems to manage regardless of the tools used. Cultivators and plows often drag pieces about a field. Herbicides applied and mixed into the soil may reduce the chances of establishment of such pieces. Continuous and repeated cultivation, or mowing for 1 or 2 years, and use of persistent herbicides is often necessary for control. An eradication program requires the killing of seedlings as well as the dormant seeds in the soil.
Woody perennials are plants whose stems have secondary thickening and an annual growth increment. These plants can be weed problems in pastures and many perennial-cropping systems. Examples include poison ivy, wild brambles, and multiflora rose.