The mode of action of some herbicides is to destroy weeds by damaging leaf cells and causing them to dry up. Others alter the uptake of nutrients or interfere with the plant’s ability to grow normally or to conduct photosynthesis.The mode of action often dictates when and how a herbicide is used. Herbicides must (1) adequately contact plants, (2) be absorbed into the plants, (3) move to the site of action in the plant without being deactivated, and (4) reach toxic levels at the site of action.
Those that inhibit germination or seedling growth are used as preemergent herbicides; they are applied to the soil to control weed seedlings before they break through the soil surface. They rely on rainfall or are incorporated into the soil to place the herbicide in close contact with the germinating weed seed. Some products (e.g., trifluralin) do not move within the plant so injury symptoms are confined to site of uptake. Others (e.g., atrazine) are systemic and enter through the roots and move upward. In general, symptoms will be most obvious where the product tends to accumulate.
Other types are used as postemergent herbicides and are applied to the foliage of emerged weeds. Some postemergents have contact activity, meaning they kill the plant by destroying leaf and stem tissues. Other postemergents are translocated (moved within the tissues of the plant) from leaves and other green parts to growing points.
Chemical and physical relationships between the leaf surface and herbicide often determine the rate and amount of uptake. Uptake also can be affected by plant size and age, water stress, air temperature, humidity, and herbicide additives. Differences in the amount of herbicide uptake within the plant often explain the year-to-year variation in herbicide effectiveness.
Like soil-applied herbicides, postemergence herbicides differ in their ability to move within a plant. Nonmobile (contact) postemergence herbicides must thoroughly cover a plant for good control. Mobile herbicides move within the plant to the site of action.
Plants that can rapidly degrade or deactivate a herbicide can escape the toxic effect. The ability of some plants to rapidly degrade a herbicide is the basis whereby plants are differentially susceptible to some herbicides. However, plants under stress (hot or cold temperatures, high humidity, or physical injury) may be affected by herbicides to which they normally are tolerant. Misapplication, especially excessive rates, can overwhelm the ability of the plant to degrade or deactivate the chemical and result in plant injury.
This annual grass reproduces by seed and by rooting at the lower joints of the stem (Figure 28a). The stems are erect or arise from a creeping base and usually form mats. Small, inconspicuous flowers are produced in two rows along one side of the 3 to 10 finger-like branches at the top of the stem (Figure 28b).
In Kentucky, crabgrass seeds start to germinate in late April or early May. It thrives best during warm summer months when bluegrass is producing the least amount of growth. Crabgrass grows to maturity and develops seed from July until September, then dies in autumn.
Figure 28 Crabgrass, Digitaria spp. (A) creeping base and rooting at lower joints of stem. (B) arrangement of flower and seeds on finger-like head.
This smooth, flat-stemmed, coarse annual grass reproduces entirely by seeds. Stems are branched, arise from tufts, and often form mats. Flowers and seeds are produced in two rows along one side of the 2 to 10 finger-like branches at the top of the stem (Figure 29b). Goose-grass may look like crabgrass; however, goose-grass differs in that it has a flattened stem and does not take root at the lower joints of the stem (Figure 29a).
Goose-grass germinates in April and May, produces seed from June to September, and dies in autumn.
Figure 29. Goose-grass, Eleusine indica. (A) stems arising from tufts. (B) finger-like flower and seed head.
Nimblewill is a shallow-rooted perennial grass that spreads by seeds and aboveground stems. The stems are slender, branched, and spread along or near the surface of the ground. Tiny, inconspicuous flowers and seeds are arranged loosely on nodding or ascending branches along the upper part of the stem (Figure 30). The leaf blades are usually less than 1/4 inch wide and not more than 2 inches long. Nimblewill is grayish green compared with the darker green bluegrass.
New growth of nimblewill starts from the crowns or aboveground stems in late March or April. It continues to grow throughout the summer and early autumn. The roots remain alive throughout the year, but the tops die in autumn, leaving dense brown mats in the lawn during the winter.
Figure 30. - Nimblewill, Muhlenbergia schreberi. Entire plant, showing slender creeping stems, ascending seed heads.
This plant is a winter annual that reproduces by seed. Stems are flattened, grow in tufts, and sometimes take root at the lower joints (Figure 31a). The leaves are very soft, and the seed heads are arranged in the shape of a pyramid (Figure 31b).
Annual bluegrass thrives best during cool weather. Germination occurs from early fall to early spring. The plant grows to maturity, develops seeds, and then dies in late spring or early summer, leaving unsightly brown areas in turf. Annual bluegrass is often lighter green than Kentucky bluegrass. It usually grows from 4 to 8 inches high when left unmowed.
Figure 31. - Annual bluegrass, Poa annua. (A) plant showing tuft of stems. (B) pyramid-shaped seed head.
Figure 32. - Foxtail, Setaria spp. Entire plant showing erect stem with seed head at the top.
Chickweed is a juicy-tissued, shallow-rooted winter annual that reproduces by seed and trailing stems that take root at the lower joints (Figure 33a). The leaves are arranged in pairs on the stem; they are usually egg-shaped, smooth, and less than 1 inch long -- the lower leaves with hair stalks and the upper without stalks (Figure 33b). The flowers are small, with five white, deeply notched petals.
Chickweed starts to germinate in autumn, grows throughout the winter, develops seeds from April until early summer, then dies. Occasionally, some chickweed may continue to grow during the summer, but most of the plants die during late spring.
Figure 33. - Chickweed, Stellaria media. (A) trailing stem with roots at joints. (B) pair of opposite egg-shaped leaves.
Wild garlic has two kinds of underground bulbs. The soft-shelled bulbs germinate in autumn, while the hard-shelled bulbs remain dormant until succeeding years. Some bulbs remain alive in the soil for at least five years.
Figure 34. - Wild garlic, Allium vineale. (A) underground bulb. (B) head of aerial bulblets. (C) leaf base surrounding stem.
Henbit is a winter annual that reproduces by seeds and stems rooting at the lower joints. The stems are 4 to 16 inches tall, square, and branch close to the ground. The plants are erect and have very few or no hairs. The leaves are opposite (two leaves at each stem joint), almost circular, with the edges having rounded teeth or lobes (Figure 35a). The flowers are pink to purple, two-lipped, and arranged in whorls at the base of the leaves (Figure 35b).
The growth habits of henbit are like those of chickweed.
Figure 35. - Henbit, Lamium spp. (A) pair of opposite somewhat circular leaves with rounded teeth or lobes. (B) flower whorl at base of leaves.
Buckhorn plantain is a perennial that reproduces by seed and new shoots from the roots. The stems are usually 8 to 16 inches high, leafless, and bear a short, dense flower spike at the top from 1 to 3 inches long. The narrow leaves arise from the base of the flower stems; they are 4 to 8 inches long, lance-shaped with several prominent veins running lengthwise, and arranged in a basal rosette at the surface of the soil (Figure 36).
Buckhorn plantain produces mature seed from June to September.
Figure 36. - Buckhorn plantain, Plantago lanceolata. Note leafless stem with short, dense flower spike at top and narrow, prominently veined leaves arising from basal rosette.
Broad-leaved plantain is a perennial that reproduces by seeds and new shoots from the roots. The stems are usually 4 to 12 inches high and leafless, with a long, slender flower spike at the top which is from 2 to 10 inches long (Figure 37a). The leaves arising from the base of the stem are broad, somewhat egg-shaped, with several prominent veins, and are arranged in a basal rosette at the surface of the soil (Figure 37b).
Broad-leaved plantain produces seed from June to September.
Rugel’s plantain (Plantago rugelii) looks very much like P. major; because control measures are the same for both species, they are treated as broad-leaved plantain in this publication.
Figure 37. - Broad-leaved plantain, Plantago major. (A) stem with long slender flower spike. (B) broad, somewhat egg-shaped leaves arising from basal rosette.
Yellow nutsedge is a perennial that reproduces by seed, rhizomes, and tubers (nutlets) (Figure 38). The stems are yellow-green, solid, triangular, and grass-like. Long, slender rhizomes (A) are produced that terminate in tubers (B) which produce new plants.
Yellow nutsedge can be found in home lawns or in wet soil areas.
Figure 38 Yellow nutsedge
Figure 39 Wild violet
Dandelion is a perennial that reproduces by seeds and new shoots from the roots (Figure 40). The stems contain a milky juice and arise from a long, thick, fleshy taproot. The leaves vary in shape, are usually oblong, and more or less tapering in outline. They vary from having no teeth to coarse teeth, usually are covered with short soft hairs, and are arranged in a basal rosette. The yellow flowers are arranged in a single head on a long hollow stalk.
Dandelions produce most of their flowers in May and June.
Figure 40. - Dandelion, Taraxacum officinalis. (A) upper section of long taproot. (B) coarsely toothed leaves from basal rosette. (C) flower head on long, hollow stem.
Red sorrel is a perennial that reproduces by seeds and creeping roots (Figure 41). Low stems, usually less than 18 inches high, are scattered or in mats and are produced from creeping roots. Upper leaves may be somewhat long and narrow with the lower leaves shaped somewhat like an arrowhead, but with two small lobes at the base.
The plants produce seeds from late April to September. The seeds are small, triangular, reddish brown, and glossy.
Figure 41. - Red Sorrel, Rumex acetosella L. (A) plant showing creeping root system. (B) leaves with their characteristic lobes.