This section is from the book "A Manual Of Weeds", by Ada E. Georgia. Also available from Amazon: A Manual Of Weeds.
Introduced. Perennial. Propagates by seeds and by rootstocks.
Time of bloom: Early June to July.
Seed-time: July to August to September; when cut for hay crop, both blooming and seeding time may be retarded.
Range: Southern part of United States to latitude of Tennessee, westward to California, and along Pacific Coast to Oregon and Washington.
Habitat: Fields, meadows, waste places.
About 1830 there came to Governor Means, of South Carolina, a message from the Sultan of Turkey, requesting that an instructor in the art of raising cotton be sent to the Ottoman Empire. Two or three years later, when the instructor returned, he brought with him the seeds of a number of plants that seemed to him to be of economic value, and among them was this grass. An Alabama planter, Colonel William Johnson, while on a visit to South Carolina, became interested in the new plant, obtained a quantity of seed, and raised it extensively on his plantation in the fertile bottom lands of the Alabama River. Since then it has spread over about half of the United States, and but for the fact that it is a tropical plant, likely to be winterkilled where the ground freezes to any depth, it might have possessed the land to a much greater extent. And, once established, it is almost impossible to control it because of its deep-running, branching rootstocks. Added to the difficulty of control is the fact that, like all the Sorghums, the plant occasionally develops a poisonous quality, due to the presence of hydrocyanic acid. Complaints of the deaths of cattle and horses from this cause come mostly from the Pacific Coast, where the growth of the grass on irrigated ground is especially rank. In India, where the plant is much used as fodder for cattle, it has been noted that deaths frequently occur when, because of the failure of rain, plants that have reached a good size become wilted. When a rainfall comes, the poisonous principle disappears; just what condition develops it is not known.
Culms large and stout, about a half-inch thick at base, and ordinarily five or six feet tall but may reach a height of eight or nine feet; pith filled with sugary juice. Sheaths smooth; leaves a foot or more long, about an inch wide, smooth, and flat. Panicles very large and loose, the branches whorled and spreading, naked at base; spikelets in groups of three, the central one sessile and fertile, sometimes bearing an awn, usually bent, the glume purplish, covered with fine appressed hair; the two lateral spikelets have pedicels and are staminate or empty. So rapid a grower is the grass that two, three, even four, heavy crops of hay may be harvested yearly, if cut before it blooms; the hay is much relished by all kinds of stock and is very fattening; even the rootstocks are tender and sweet, and hogs eat them eagerly; were it not so aggressive it would be a most valued plant. (Fig. 4.)
With a view toward finding some means of eradication, J. S. Cates, of the Bureau of Plant Industry at Washington, was employed by the Government to make a special study of the plant, and the results of his experiments and conclusions are embodied in Farmers' Bulletin 279 of the U. S. Department of Agriculture. He states that the rootstocks are of three kinds, which he classifies as primary, secondary, and tertiary.
"Primary rootstocks embrace all the rootstocks alive in the ground at the beginning of the growing season in the spring.
"Secondary rootstocks are those which arise from the primaries, come to the surface and there form crowns, thus producing new plants.
"A tertiary rootstock is one starting later in the season, about flowering time, from the base of the crown of this new plant.
"These tertiary rootstocks, when the ground is soft, and especially when a large top is allowed to develop, grow to a large diameter and penetrate to a great depth, sometimes as much as four feet and normally from fifteen to thirty inches; at other times, when the soil is compact, and especially when the plant above ground is not allowed to develop by reason of mowing or grazing, or both, the tertiary rootstocks grow to but small diameter and run along just under the surface, cropping out at intervals to form new plants. Our observations indicate that the primary rootstoeks (i.e., those that were in the ground at the beginning of the growing season in spring) all decay in the fall, after the growing season is over. Their strength has been taken up in the formation of secondary rootstoeks and above-ground growth. In other words, the old rootstocks do not live over a second winter. Only the new ones (secondaries and tertiaries) do this. Under our classification, secondary and tertiary rootstocks become primary rootstocks at the beginning of the next season after their formation, and they, in their turn, send out secondary growth to reach the surface; the plant formed at the surface then sends out from the base of its crown, about the time it blossoms, the large, deep-burrowing tertiary rootstocks which, in the soft land of the cultivated cotton and corn fields, cause so much mischief the following year. The longer the plants are allowed to stand after blossoming, the larger and deeper these tertiary stems become."
Fig. 4. - Johnson-grass (Sorghum halepense). X 1/8.
Acting on this study of the rootstock habits of Johnson-grass, Mr. Cates advises the turning of infested land into meadow or pasture and keeping it so persistently mown or grazed as to allow it no opportunity for bloom; then the tertiary growth of rootstocks will be small and near the surface, enabling the farmer to clean out the grass the next year by a little extra care in plowing and cultivating. As a soiling crop the grass may be cut every month from May until November, and this will leave little energy to be given to the formation of the deeper rootstocks.
Professor Killebrew, of the Tennessee Agricultural Experiment Station, says that the way to keep the grass in subjection is to plow the land and allow hogs to pasture on the juicy rootstocks, which they like better than artichokes. Rotation with winter grains, such as oats, barley, or rye, is practiced in many sections, wheat being too late in maturing. The ground is plowed in late summer and as many of the rootstocks are harrowed out as possible; then the grain is sown in early fall and harvested in the spring, before blooming time for the grass, after which three crops of Johnson-grass hay may be cut during the summer.
Professor Spillman, Agrostologist of the Bureau of Plant Industry, believes the best plan to be the sowing of infested land with alfalfa, after harrowing out as many of the rootstocks as possible, early in the fall, in order to allow the alfalfa to get a good start before winter. The next season cut promptly, whenever the grass is tall enough to make a fair crop of hay. This treatment encourages the clover and discourages the grass, which will finally be crowded out. Professor Spillman succeeded in cleansing a plot of Johnson-grass in one year, without loss of the use of the ground, by a system of fall plowing, with a turning plow capable of turning every inch of the sod, harrowing thoroughly for the purpose of loosening the soil, and then removing the rootstocks with an implement called a root-digger, or grass-hoe. This method is discussed in detail in Bulletin 72 of the Bureau of Plant Industry.