Mark Schonbeck, Virginia Association of Biological Farming
Every kind of weed has an identity—its species. Following are some tools and techniques for identifying the weed species in your fields.
Identifying all the weeds on a farm is not easy, and it is usually not necessary. However, correctly identifying major weeds can be an important first step toward effective control. Two weed species can look very similar at certain growth stages, yet differ greatly in life cycle, modes of reproduction, effects on crops, and responses to control tactics.
A plant’s species name can be spelled out in Latin (its scientific name, e.g., Amaranthus retroflexus) or in plain English (its common name, e.g., redroot pigweed). Scientific names are more precise, as each species has just one valid scientific name at any one time. They are also less descriptive and a little harder to pronounce, especially for those of us who did not take Latin in school.
Common names are more user-friendly but less precise. A given weed might have two or more common names. For example, common lambsquarters is also called fat-hen or white goosefoot. Some common names have been attached to two different weeds; for example, lambsquarters has also been called pigweed in some regions, and the term witchgrass has been applied both to the perennial quack grass and to an annual weed closely related to fall panicum. In order to minimize such confusion, weed scientists have adopted official common names, such as common lambsquarters for the species Chenopodium album, and redroot pigweed for Amaranthus retroflexus.
Botanists position each plant species in the enormous family tree of the plant kingdom, which illustrates current best estimates of each species’ genetic and evolutionary relationships with other plant species. A genus (plural genera) is a group of closely related species that share many characteristics of appearance, growth habit, and genetic makeup. Latin names are usually in two words, denoting the genus (e.g., Amaranthus) and the species (e.g., retroflexus).
Some plant species have distinct variants called subspecies, for which the Latin name has three words. Subspecies may evolve naturally as a species adapts to different environments (as many weeds do), or may be developed through plant breeding (e.g., cabbage, broccoli, kohlrabi, cauliflower, collards, and kale are all subspecies of Brassica oleracea). Subspecies can cross-breed, a fact that is important to understand. For example, Japanese millet (Echinocloa crus-galli ssp. frumentacea), a useful grain and cover crop, is a subspecies of barnyard grass (Echinocloa crus-galli), the world’s third worst weed!
The next larger grouping above genus is family. Some of the better-known plant families in agriculture include the grass family (including cereal grains, corn, sorghum, millets, and pasture grasses); the legume family (including peas, beans, clovers, and alfalfa); the brassica family (including cabbage, broccoli, cauliflower, radishes, and mustards); and the nightshade family (including tomato, potato, pepper, and eggplant).
Knowing a weed’s plant family is important, as many economically important crops have weedy relatives in the same family, which may harbor insect pests or pathogens of the crop, or in some cases cross-breed with the crop itself. Occasionally, the weedy relative can play a beneficial role, acting as a trap crop to divert pests from the cash crop, or supporting important natural enemies of the pests as well as the pests themselves.
Identify weeds with the help of a good field guide, manual, or taxonomic key to the agricultural weeds in your region. Collect a representative specimen or several specimens (recommended), and examine them closely, including foliage, stem, flowers, roots, and other belowground parts. Familiarize yourself with some of the jargon used in your field guide or key (most references have a glossary of terms).
Plants are identified by visible characteristics that remain roughly constant among all individuals within a species. These can include:
Characteristics that are more variable within a species, yet can help identify the weed include:
Other distinctive characteristics that help identify some weeds include:
For more on how to recognize identifying characteristics of weeds, see A Basic Illustrated Glossary of Plant Identification Jargon below.
The more defining characteristics you can observe, the faster and easier it is to get a positive ID on the weed. Useful tools for identifying weeds include a ruler or folding rule to measure plants and plant parts; a hand lens or magnifying glass for examining small plant parts or features; a trowel, spade, or digging fork for exhuming intact root systems and other underground structures; and a weed identification guide or manual (Fig. 1).
Figure 1. Some basic weed identification tools. Figure credit: Mark Schonbeck, Virginia Association for Biological Farming.
Noting when and how a weed emerges and grows can aid in identification. Summer annuals are mostly frost-tender, usually emerge between the spring frost-free date and late summer, and die at the first fall frost. Winter annuals emerge any time between the end of summer and early the following spring, flower and set seed in spring or early summer, and usually dry up with the onset of hot weather. Thus a weed that is still thriving after a fall frost is almost certainly not a summer annual such as pigweed, purslane, or galinsoga, and a weed that is succulent and vegetative in July is probably not a winter annual like henbit or yellow rocket.
How a weed first emerges from the soil can give clues to whether it is coming up from a seed or from a rhizome or other perennial underground structure (Fig. 2). Dig up the emerging weed to see if it is a true seedling (easy to dig up, few fine roots, remains of seed or seed coat may or may not still be visible), or sprouting from a perennial root or other structure (harder to dig up, attached to a larger root, rhizome, tuber, or fragment thereof).
Figure 2. The small weeds in this photo are true seedlings, having germinated over the past two weeks. The larger weeds with somewhat arrow-shaped leaves are shoots of hedge bindweed that have emerged from rhizome or rhizome fragments within the top foot or so of soil. Figure credit: Mark Schonbeck, Virginia Association for Biological Farming.
Flowering specimens are easiest to identify to species, because they display the greatest number of defining traits. In fact, plant families are delineated to a large degree by flower structure. However, growers often want to identify major weeds in earlier growth stages, even as emerging seedlings, in order to determine best management strategies while the weeds are small and relatively easy to control. This can be challenging to say the least. If a particular species has you stumped, let a few individuals develop into mature, flowering plants, then identify them.
Some weed manuals include good photographs and descriptions of seedling characteristics that allow identification of the weed’s plant family or genus, if not species. Seedling characteristics include:
Once you have a weed specimen in hand, use one of these methods to identify its species:
The direct comparison method is especially useful when you want to verify the ID of a weed with which you are familiar, or for which you have narrowed it down to a short list of possibilities. A strong likeness of the specimen to a photo or diagram, and a close match with the manual’s written description suggests a correct ID. A clear discrepancy in one or more defining characteristics indicates an incorrect ID—try again. This method can be quick and efficient when the list of possible species is reasonably short—if the manual is well organized, is written for your region, and includes the weed in question! Time-consuming pitfalls include random guessing, and trying to verify ID of a weed that the manual does not include.
The dichotomous key is the time-honored method by which botanists and agricultural scientists have identified weeds, native vegetation, or cultivated plants for the past century or more. The dichotomous key can provide a definitive ID when skillfully used. Starting at the beginning of the key, read each pair of characteristic descriptions or categories, and choose the one that best matches the specimen at hand. Each choice gives a reference number directing you to the next pair of characteristics to examine. It works much like a treasure hunt.
Dichotomous keys often use a lot of botanical jargon, so make sure the key has a good glossary of terms before buying the book. One disadvantage is that some of the dichotomies may refer to characteristics not shown by the specimen (e.g., flower color when the specimen is vegetative), or may be difficult to see. In this case, you will need to explore both sides of the dichotomy. Errors early in the process can send you on a lengthy “wild goose chase” until the error is discovered.
Several land grant universities and the Weed Science Society of America have developed interactive keys, based on computer databases that catalogue all of a region's or the continent's main agricultural weeds. The interactive key allows you to begin with the most readily visible or measurable traits of the specimen at hand. There is no set order in which to answer questions about the weed; instead, you can click on leaf shape, life cycle, flower color, root structure, or other characteristics, choosing the best match from a list of several alternatives. The key maintains a list of weeds that match the description being developed, narrowing the list down with each choice until the weed is identified or a short list of possible species remains. The database includes verbal descriptions and photos of each weed to assist with positive ID.
Any of these methods is only as good as the weed manual, key, or database used, the quality of the specimens available, and the observation skills of the user. If the weed in question is not included in the key or guide being used, you can waste a lot of time searching for it in vain! It is important to choose a manual or database that is written for your region and includes all of the region's major agricultural weeds. If a particular weed cannot be found in the reference you are using, it could mean any of the following:
If you cannot identify an important or abundant weed species present on the farm:
Farmers who bring fresh specimens of new or unfamiliar weeds in to Extension or university departments for positive ID can help with early detection of a new exotic invader or changes in weed geographic ranges related to climate changes or other factors.
Figure 3. Structures on a broadleaf weed or crop. Figure credit: Mark Schonbeck, Virginia Association for Biological Farming.
Figure 4. Structures on a grass weed or crop. Figure credit: Mark Schonbeck, Virginia Association for Biological Farming.
Figure 5. Broadleaf seedlings. Figure credit: Mark Schonbeck, Virginia Association for Biological Farming.
Figure 6. Roots and other underground structures. Figure credit: Mark Schonbeck, Virginia Association for Biological Farming.
Figure 7. Leaf shapes. Figure credit: Mark Schonbeck, Virginia Association for Biological Farming.
Figure 8. (a) Leaf margins. (b) Leaf venation. Figure credit: Mark Schonbeck, Virginia Association for Biological Farming.
Figure 9. Leaf structures. Figure credit: Mark Schonbeck, Virginia Association for Biological Farming.
Figure 10. Arrangement of leaves on stem. Figure credit: Mark Schonbeck, Virginia Association for Biological Farming.
Figure 11. Flower structures. Figure credit: Mark Schonbeck, Virginia Association for Biological Farming.
Figure 12. Types of flowers. Figure credit: Mark Schonbeck, Virginia Association for Biological Farming.
Figure 13. Inflorescences (arrangement of flowers in clusters). Figure credit: Mark Schonbeck, Virginia Association for Biological Farming.
This article is part of a series on Twelve Steps Toward Ecological Weed Management in Organic Vegetables. For more on weed monitoring and identification, see:
This is an eOrganic article and was reviewed for compliance with National Organic Program regulations by members of the eOrganic community. Always check with your organic certification agency before adopting new practices or using new materials. For more information, refer to eOrganic's articles on organic certification.