Introduced. Annual. Propagates by seeds. Time of bloom: June to September. Seed-time: August to October. Range: Locally in most of the states. Habitat: Wherever the clovers or alfalfa are extensively grown.
The farmer who sees his newly seeded clover or alfalfa field partly or wholly in the grip of this parasite gets a realizing sense of the value of clean seed. For the Dodder is there because it was sown there with the crop. If the plant is allowed to ripen fruit, the ground will be made foul and unfitted for similar crops for at least eight years, the dormant vitality of the seed being retained for a period of five to seven years.
The parasite awakens late - nearly a month after the green-leaved plants on which it must depend have started growth; it germinates in the soil but draws from it no sustenance. The seedling looks like a bit of yellowish red hair, two to four inches long, with a slight knob, or swelling, at one end, swaying pliantly about, searching for a host plant to which it may attach itself. If no such plant is within reach the seedling falls to the ground and dies; if there is such a plant, the parasite quickly twines about it, develops tiny, wart-like suckers at the point of contact, breaks connection with the earth, and thereafter "sponges its living," drawing from the host plant the food assimilated by the green leaves for its own growth. Consequently the Dodder needs no leaves and has none, the whole plant being a mere coarse, yellowish red thread, branching very freely and the branches behaving as did the original filament, reaching out for living support, embracing it, and then often parting from the main stalk and becoming separate plants; so that the growth from a single seed may cover a considerable extent of ground, binding the herbage into a tangled mass and sucking out its life. The parasite itself dies at the point where its growth started, when its hosts are killed, but the many spreading branches continue their existence. Even a broken bit of stalk, dropped where it can seize on a host, promptly takes hold and starts a new center. (Fig. 226.)
The small flowers are whitish or faintly tinged with pink, sessile, massed in dense clusters; calyx five-lobed or occasionally only four-lobed, acute; corolla lobes spreading, bell-shaped, with as many stamens as lobes set between their points and ex-serted; ovary two-celled and style two-parted. Scales within the corolla tube large, incurved, and toothed all round. Capsules small, globular, two-celled, four-seeded, but often only two or three seeds are developed. Seeds very small, rounded, oval, grayish or yellowish brown; they are the most dangerous, the most to be dreaded of all the impurities of clover seeds. No seed should ever be harvested from a Dodder-infested clover field and such seed should be unsalable at any price. Neither should such a crop be harvested and fed as hay, for the seeds, uninjured and viable, often pass the digestive tracts of animals, and may be spread on other fields in the manure, not to speak of the seeds that would be scattered wherever the hay was handled. Baled hay is one of the sources of Dodder distribution.
Fig. 226. - Clover Dodder (Cus-cuta Epithymum). X 1/2. Capsule and seed very much enlarged.
Sow clean seed. Infestation is often in patches where a single seed or but a few have germinated. In such a case, cut the infested plants close to the ground, before any seeds have ripened if possible, pile them on the spot where they grew, let them dry for a day or two, cover with straw, shavings, or some light rubbish, soak with kerosene oil, and burn, being careful to get every thread, cutting beyond the apparent limit of damage. Then stir the surface soil of the patch lightly with rake or hoe, making a small trench about the edge, cover a couple of inches deep with oil-soaked chaff or rubbish, and again burn, in order to destroy any seeds that may have matured and fallen to the soil. Or the soil of the patch may be well sprinkled with crude carbolic acid. If a whole field is infested, it will be best to plow the crop under; but it must be done before the seeds ripen, indeed before they form, else the land will be made unfit for occupation by clover or alfalfa for seven or eight years. Or, the field may be pastured off by cattle or sheep before the seed ripens; but in that case the animals must not be moved about, for bits of stalks may adhere in the clefts of their hoofs, or unsuspected seeds may be in the droppings. If seeds have been allowed to mature, the crop should be mowed, dried, and burned on the spot, for Dodder-infested crops should not be carried about because of the danger of infection. Cultivated crops should then be grown for some years before the land is again used for clover or alfalfa.
Spraying with Arsenite of soda has been found in some cases to be satisfactory, a solution of one pound of the poison to ten gallons of water being used. The clover and the alfalfa were also much injured, seemingly killed by the treatment, but recovered and made new growth from the roots after having been relieved from the strangler's grip. A twenty-per-cent solution of Iron sulfate has also been found to be effective on alfalfa fields, killing the parasite and apparently destroying the crop; but, as with the arsenical treatment, new growth sprang from the roots.