Diagnostic Characteristics
The distinguishing characteristics of SORGHUM HALEPENSE are the ribbed leaf sheath, the conspicuous midrib, the large, purplish panicle and the extensive rhizome system. PANICUM BULBOSUM, which has been confused with SORGHUM HALEPENSE, can be recognized by its short, knotty rhizomes and bulbous swellings at the base of the culms.
Habitat
In the United States SORGHUM HALEPENSE grows in disturbed lands. As a weed, this plant is found in ditches, cultivated fields and wastelands throughout Arizona, commonly growing below 5,000 feet elevation in irrigated croplands (Gould 1951).
SORGHUM HALEPENSE is adapted to a large range of conditions. An ideal environment for Johnson grass is the subtropics: warm and humid with summer rainfall. Most of the ecotypes are frost sensitive; the aerial portions are killed during winter freezes. Temperatures below 13 C tend to inhibit flowering (Holm et al. 1977). Cold tolerant ecotypes have been found growing in northern United States and southern Canada (Warwick and Black 1983). Rhizomes are also intolerant of high temperatures; rhizomes exposed to 50 C to 60 C (122 F to 140 F) temperatures for several days died (Warwick and Black 1983).
Johnson grass is adapted to a wide variety of soil types, however fertile porous soils support larger plants than poorly drained clay soils (Warwick and Black 1983). Soils with a pH of between 5 and 7.5 are ideal for Johnson grass. Experiments using flooding as a control measure revealed that aeration is not critical for survival of rhizomes or seeds (Horowitz 1972a).
Most land-types, especially disturbed and flooded bottom lands, are susceptible to this tenacious and invasive weed (McWhorter 1981). The massive creeping rhizome system and the abundance of dormant seeds protect this plant from severe conditions. Rhizomes have been found 120 cm deep in cultivated soil and five year old seeds displayed 50% viability (Warwick and Black 1983), making complete eradication of this species very difficult. Once the environmental conditions improve these dormant structures are available to resume growth and invade adjacent land. A single Johnson grass plant produces 200 to 300 feet of rhizomes in one month and 10 bushels of seed can be produced on one acre in a single growing season (McWhorter 1981).
Ecology
GENERAL LIFE-CYCLE: The life-cycle of SORGHUM HALEPENSE is conducive to its survival in a wide range of environmental conditions (Holm et al. 1977, Monaghan 1979, Warwick and Black 1983). Growth from the apical or axillary nodes on the primary rhizomes which have survived the winter begins as temperatures increase in the spring. The secondary structure, the annual above- and below-ground growth, uses the stored carbohydrates from the primary rhizome to get a rapid, early start in growth (Holm et al. 1977). The development of rhizome spurs and secondary tillers begins a month after growth has resumed when approximately six leaves are present. Depending on the climate, flowering begins roughly two months after growth commences and continues throughout the growing season. Most of the year's rhizome growth takes place after flower production (Warwick and Black 1983), however, no causal relationship exists between the two (Horowitz 1972b, Monaghan 1979). The tertiary rhizomes which grow deep into the soil (as deep as 120 cm) survive the winter and become the following season's primary structure (Holm et al. 1977, Warwick and Black 1983). The remains of the primary rhizomes from the previous year's growth decay as the temperature begins to drop (Holm et al. 1977).
Hundreds of seeds are produced on each panicle throughout the summer flowering period (Monaghan 1979, Warwick and Black 1983). Seedlings emerge later and grow slower than rhizome sprouts, but the pattern of development is similar between the two structures (Warwick and Black 1983).
SEXUAL DEVELOPMENT: Self-compatibility, immense seed production, effective dispersal techniques, seed dormancy and seed longevity are features which make SORGHUM HALEPENSE a prolific weed. Most members of the genus SORGHUM are self-compatible (Warwick and Black 1983). Johnson grass plants growing less than 130 m apart will cross-fertilize. However, less than 5% of fertilized plants are the result of crossing, even in fields where plants are closely spaced (Warwick and Black 1983). The self-compatibility insures seed production throughout the growing season.
Plants begin to flower approximately two months after growth commences. The exact flowering time depends on temperature, plant vigor and photoperiod (Horowitz 1972b, Warwick and Black 1983). Johnson grass is a short-day plant (Ghersa et al. 1985). Experiments in Mississippi resulted in flowering of all treatments, ranging from 8 to 16 hours of light. However, seedhead formation was inhibited in the 16 hour treatment and reduced in the 14 hour treatment. The 10.5 and 12 hour photoperiod treatments resulted in the greatest amount of seed production (Warwick and Black 1983). Temperatures below 13 C to 15 C (55 F to 59 F) inhibit floral production (Warwick and Black 1983). In Arizona flowering begins in April and continues through November (Kearney and Peebles 1951).
Seed production is prolific. Depending on the ecotype, anywhere from 37 to 352 seeds can form on a panicle (Warwick and Black 1983). Seeds form only in the sessile spikelets, with a maximum of one seed per spikelet (Warwick and Black 1983). In Israel, a single plant produced 28,000 seeds (84 g) in one growing season, and in Mississippi the average plant produced 1.1 kg of seeds per season (Horowitz 1972b, Warwick and Black 1983).
Shattering of the seeds from the spikelets enhances seed dispersal (Holm et al. 1977). Seeds that land in a water source may be carried, possibly quite far, to new sites. Other modes of dispersal include wind, livestock and contaminated machinery, grain or hay. Seeds pass unharmed through birds and cattle (Holm et al. 1977, Warwick and Black 1983).
Seed dormancy and longevity provide a source of seed in the soil which is ready to germinate when unfavorable conditions subside. The seed dormancy properties are dependent on the ecotype (Monaghan 1979). Due to the mechanical reduction of water penetration by the tannins in the seed coat, seeds will not germinate naturally when first wetted (Warwick and Black 1983). Less than 10% of seeds from 43 ecotypes germinated at 20 C (68 F) (Monaghan 1979). However, when the seed coat is removed or if the seed is treated with sulfuric acid and then exposed to temperatures greater than 20 C, the seed will germinate (Monaghan 1979, Warwick and Black 1983). Five months in dry storage at room temperature overcomes most dormancy (Monaghan 1979). The longevity of seeds varies depending on the environmental conditions in which the seed was stored. Seeds stored in the laboratory under dry conditions remained viable for over seven years. Studies in California showed a 50% viability in seeds stored for five years, however another study resulted in only 2% viability in seeds which remained in the soil for six years (Warwick and Black 1983). Seeds buried in the soil for two and a half years displayed a 60% to 75% viability (Warwick and Black 1983).
Most seeds do not germinate the year they are produced, but germinate readily the following year (Holm et al. 1977). The soil type and seed depth influence germination; the deeper a seed is buried and the greater the clay content of the soil the less likely a seed will be to germinate (Warwick and Black 1983). The majority of seedlings arise from seeds in the top 7 cm of the soil, but seeds as deep as 15 cm can germinate (Warwick and Black 1983). Seeds require temperatures 10 C higher to germinate than do rhizomes to sprout (Horowitz 1972b). The light conditions affect the temperature requirements for germination. The highest germination rate is in seeds pre-chilled for 20 days at 10 C (50 F) and then exposed to alternating temperatures of 24 C and 40 C (75 F and 104 F) with continuous light (Warwick and Black 1983).
VEGETATIVE DEVELOPMENT: The prolific seed production ability of Johnson grass accounts for the dispersion of this invasive weed, but the immense, rapidly growing rhizome system gives SORGHUM HALEPENSE its competitive edge against other species. The extensively creeping rhizomes, which regenerate easily when cut into small pieces, are capable of growing or remaining dormant in a wide range of environmental conditions; these qualities make it exceptionally difficult to eradicate SORGHUM HALEPENSE.
The existence of physiological dormancy in Johnson grass' rhizomes is controversial (Monaghan 1979). However, apical dominance in the rhizome system prevents axillary buds from sprouting. Nodes furthest from the apex have a reduction in apical suppression (Hull 1970). The importance of this in the survival of the plant should not be overlooked. At all times, including during favorable conditions, the majority of rhizome buds will not be growing, thus the application of herbicides will have little to no effect on the viability of these inactive regions.
Primary rhizomes provide stored energy for an initial rapid growth period in the spring. No causal relationship between tillering, flowering and rhizome formation is found (Horowitz 1972b, Monaghan 1979). New rhizomes may grow either during or after tiller formation and either before or after flower production (Horowitz 1972b), although ninety percent of the year's rhizome production occurs after flowering (Warwick and Black 1983). A decrease in the percent of the plant's total fresh weight in shoots and roots and an increase in the percentage of the plant's fresh weight as rhizomes occurs from April through August (Warwick and Black 1983).
Rhizome production can be immense depending on soil type, light and temperature. Sixty to 90 m of rhizomes were produced per plant in one growing season in Mississippi (Warwick and Black 1983). A 600-fold increase in the rhizome length occurred in 120 days starting with a 7.5 cm section (McWhorter 1981).
The depth of the rhizomes depends on the heaviness of the soil (Monaghan 1979). Cultivated soils and soils with a low percentage of clay are more likely to have deep rhizomes than high clay- content soils; rhizomes have been found 120 cm deep in cultivated soils, but the majority of rhizomes are found in the top 20 cm of soil (Holm et al. 1977). Longer rhizome segments (20 cm long) started growing 20 days earlier and had more rapid growth than segments half their size. Both depth and size of the rhizome are important in determining growth patterns. Usually, the bigger the segment the deeper it can be located in the soil while still successfully producing shoots (Warwick and Black 1983).
Since carbohydrate production depends on photosynthesis it follows that a larger amount of rhizomes may be produced at greater light intensities (Monaghan 1979). Temperature has a major effect on rhizome production, due to both photosynthetic rates and to physiological factors at temperature extremes. Variations in the effects of temperature on rhizome growth exist between ecotypes (Holm et al. 1977). In Israel, the minimum temperature for rhizome production was 15 C and the maximum was 30 C (Horowitz 1972b). Similar temperature requirements were found by Hull (1970) in Rhode Island, the minimum temperature was 15 C, however the optimum temperature was 30 C (the maximum temperature was not given).
Most ecotypes have rhizomes that cannot tolerate freezing temperatures or hot drying conditions (Monaghan 1979, Warwick and Black 1983). Unlike most perennial grasses which store carbohydrates in the form of fructosans, Johnson grass stores starch and sucrose; this may partially explain the lack of cold tolerance in Johnson grass as compared to other perennial plants (Monaghan 1979). In the laboratory, rhizome buds were killed by exposures to -3 C (27 F) for 24 hours and in the field 20 cm deep rhizomes were killed at temperatures below -9 C (16 F) (Warwick and Black 1983). Adaptation and the formation of new ecotypes account for the geographic spread of Johnson grass in northern U.S. and southern Canada. From 1959 until 1977 Johnson grass died during the cold winters of Canada; in 1977 the first vegetative structure survived the winter from a newly evolved cold tolerant ecotype (Alex et al. 1979).
Most Johnson grass is intolerant of high temperatures. Three days of 55 C (131 F) kills most rhizome buds. Heat and drying has a synergistic effect; seven days of desiccation at 30 C (86 F) results in rhizome death (Warwick and Black 1983). Six days of drying at 14 C to 54 C (57 F to 130 F) results in a 78% weight loss and a cessation of sprouting of the nodal buds (Horowitz 1972a). Johnson grass exhibits greater damage from desiccation than do CYNODON DACTYLON or CYPERUS ROTUNDUS.
CARBOHYDRATE CYCLE: The temporal pattern of the levels of fructose, glucose, sucrose and starch in the rhizomes are similar, although the individual concentrations are unrelated (Horowitz 1972d, McWhorter 1974). Sucrose and starch are the primary forms of carbohydrate storage in the rhizomes, and glucose is the principle reducing sugar in the developing leaves (McWhorter 1974). The concentration of total soluble carbohydrates in the rhizomes decreases during the first three weeks after germination or sprouting, reaching the lowest concentration of the year between 10 and 30 days after emergence; this occurs with a concurrent increase in glucose level in the developing leaves (McWhorter 1974). An increase in the rhizome carbohydrate level begins following the development of the first few leaves and peaks during flower maturation; this is followed by a sharp decrease in the carbohydrate level and then a gradual accumulation of carbohydrates in the late fall (McWhorter 1974). Rhizomes reach a maximum weight following high carbohydrate levels during the period of seed maturation (Oyer et al. 1959). The annual double cycle of the rhizome carbohydrate level, high in early winter and early summer and low in early spring and fall, results in a stored energy supply during the winter that allows for early spring growth followed by a replenishment of carbohydrates during the summer for fall rhizome growth (Horowitz 1972d).
Reproduction
SEXUAL DEVELOPMENT: Self-compatibility, immense seed production, effective dispersal techniques, seed dormancy and seed longevity are features which make SORGHUM HALEPENSE a prolific weed. Most members of the genus SORGHUM are self-compatible (Warwick and Black 1983). Johnson grass plants growing less than 130 m apart will cross-fertilize. However, less than 5% of fertilized plants are the result of crossing, even in fields where plants are closely spaced (Warwick and Black 1983). The self- compatibility insures seed production throughout the growing season.
VEGETATIVE DEVELOPMENT: The prolific seed production ability of Johnson grass accounts for the dispersion of this invasive weed, but the immense, rapidly growing rhizome system gives SORGHUM HALEPENSE its competitive edge against other species. The extensively creeping rhizomes, which regenerate easily when cut into small pieces, are capable of growing or remaining dormant in a wide range of environmental conditions; these qualities make it exceptionally difficult to eradicate SORGHUM HALEPENSE. (Also see section on Ecology.)