Ralph Waldo Emerson described a weed as "a plant whose virtues have not yet been discovered." But most gardeners consider a weed an unwelcome plant growing their garden. Yet an invasive plant may be an undesirable intruder to gardens in one climate, a meek and tame beauty in another climate.
Plants are usually identified and classified in field guides. A field guide can be a helpful tool in identifying a weed. Field guides are usually organised by climate and/or geographic location. Look for a field guide suited to your region.
Identify a weed by inspecting the few obvious physical characteristics: life cycle and growth habit, stems, leaf pattern, root system, flower and seeds. Finally, armed with this information, consult a field guide book or website for a photo and description of the plant.
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Check the plant to see if it is herbaceous or woody. Herbaceous plants have soft green stems and die back to the soil annually (annual and perennial plants). Woody plants have hard fibrous roots and do not die back annually (shrubs and trees).
Check the leaf pattern of the plant. An "alternate" leaf pattern features leaf lobes that are staggered along the stem on opposite sides of the stem. When the leaf lobes are attached to the stem at the same location but on opposite sides of the stem, they are said to be in an "opposite" leaf pattern.
Inspect the leaf of the plant. A single leaf with one leaf blade on a petiole stem is called a "simple" leaf; an example is a maple tree leaf. Numerous blades along an attached stalk or petiole are "compound" leaves; an example is a locust tree leaf. Note the appearance of the leaf: broad and flat; flattened leaves; needle-like leaves; scalelike leaves. Check the characteristics of the leaf: smooth-edged, serrated-edged, lobed, toothed. Crush the leaf and take note of any odour.
Inspect the stem or stems of the plant, whether the plant is monocot or dicot. A monocot plant grows a leafy blade with parallel vein structure; an example of a monocot plant is corn. A dicot plant grows with two or more leaves sprouting from a stem system; an example of a dicot plant is a bean plant.
Inspect the root system of the plant. The two types of root systems are taproot and fibrous root. Taproot plants have a single thick root with a few small branches; a carrot is a plant with a taproot system. Fibrous roots are hairy and branch out into thick clumps; turf grass is a plant with fibrous roots.
Look for any bulbs growing under the ground; identify the bulb shape. The various bulb shapes are: true bulbs, corms, tubers, and rhizomes. The true bulb is a singular, pear-shaped bulb with a protective skin; an example is a tulip bulb. A corm bulb is similar to a true bulb and has a protective skin, but looks more swollen and has a flattened basal plate; an example is a gladiolus bulb. A tuber bulb has no basal plate and no protective skin, and often produces several buds; an example is a potato plant. A rhizome bulb is unique in that it grows horizontally under the soil surface, half-stem system and half-root system. Rhizomes can be very invasive; an example is an iris.
Inspect the flower of the plant, if possible. Note the colour, the size, and the pattern of the petals. Look to see if the flower petals are patterned in rotating groups of three, such as with an Asian lily, or have a ring of floral parts in multiples of four or five, such as with a tulip.
Check the seeds or fruits of the plant, if possible. Note the size, colour, and transportation mechanism of the seed. Examples of transportation mechanisms for seeds are: barbs or burrs, such as with the cockle burr or burdock plant; wings, such as with the maple tree; parachute, such as with the dandelion. Check the plant for berries, and note the characteristics: hairy, shiny, fleshy, juicy.
Consult a field guide book or website, and compare your identification results with the descriptions of plants in the guide. Use the process of elimination until you identify the plant.
Consult your local cooperative extension department for identification and advice. Look in the phone book for your local branch, or visit the USDA Invasive and Noxious Weed website.
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