Poison-ivy[2] can be killed either by mechanical means, such as digging, or by applying chemical herbicides. The best modern herbicides are so deadly to plants that a single treatment kills all the leaves and 90 percent or more of the roots. This is the easiest and least dangerous way to destroy these poisonous pests. There are places, however, where chemicals cannot be used, as in hedges and shrubbery where the poison-ivy is closely mixed with the valuable plants. In such situations, hand-pulling is the only satisfactory method. At times also chemicals and spraying equipment are not available, and then the mattock, scythe, or plow must be resorted to. Even with these simple tools some timing and handling methods are better than others.

CHEMICAL WEED KILLERS

By using care poison-ivy can be destroyed with chemicals without danger of poisoning to the operator. Except in very heavy growth, one may stand at a distance from the plants and apply the herbicide without touching the leaves. Most chemicals are applied as a spray solution, and many sprayers can be equipped with extension nozzles 2 feet or more in length. The greatest danger of ivy poisoning occurs from careless handling of gloves, shoes, and clothing after finishing the work. Garments should be worn that can either be destroyed after use or washed thoroughly in hot soda solution and soapsuds.

The chemicals thus far found most satisfactory on poison-ivy are, in approximate order of effectiveness, ammonium sulfamate, 2,4-D (2,4-dichlorophenoxyacetic acid), ammonium thiocyanate, borax, carbon disulfide, coal-tar creosote oil, fuel oil and similar petroleum distillates, sodium chlorate, and sodium arsenite. Less effective chemicals include common salt, kerosene, gasoline, calcium cyanamide, ammonium sulfate, and iron sulfate.

Chemicals other than borax are best applied as a spray to the foliage. Borax is scattered dry on the soil around the plants. Any field or garden spray machine or even a sprinkling can is useful for applying the liquids, but a common knapsack sprayer holding about 2 gallons is convenient and does not waste the solution, application should be as a fine spray under moderate pressure rather than a driving mist, since the object is to wet the leaves of the poison-ivy and avoid wetting the leaves of desirable plants.

Most chemicals used on poison-ivy are injurious to the foliage of all kinds of plants and must be applied with caution if the surrounding vegetation is valuable. During the early part of the growing season the leaves of the poisonous plants usually tend to stand conspicuously apart from those of adjacent plants and can be treated separately if sprayed with care. Later the leaves become intermingled and injury to adjacent species is unavoidable. Chemicals other than oil are not injurious to the thick bark of an old tree, and poison-ivy clinging to the trunk can be safely sprayed with them.

Chemical sprays can be used at any time when poison-ivy is in full leaf, June and July being perhaps the best months. Ordinarily treatments should begin not later than August 15, as poison-ivy then begins to go dormant and sprays are ineffective.

Best results are obtained both with sprays and with dry chemicals when the soil is moist but not wet at time of treatment. Borax can well be applied just before a rain, but sprayed chemicals are likely to be washed off if rain falls within 24 hours. One gallon of spray solution is sufficient to cover all the leaves on 1 square rod (272 square feet) of dense poison-ivy, or 2 to 4 square rods of a scattered stand.

Sprays give best results when applied in early morning or late afternoon when the air is cool and moist. Spraying in the middle of a dry, hot day is likely to be disappointing unless the plants are in deep shade.

No method of chemical eradication can be depended upon to kill all the plants in a stand of poison-ivy with one application. Three to four light retreatments made as soon as the new leaves are fully expanded are always necessary (1) to destroy plants missed the first time, (2) to treat new growth from the old roots, and (3) to destroy seedlings. At least three and sometimes four treatments at intervals of 2 to 8 weeks are necessary before all plants are dead. These followup treatments do not require much time or material, but neglect of them may easily lead to serious reinfestation.

Poison-ivy has an annoying habit of “playing possum.” Plants believed dead sometimes revive after many months. Thus an area under treatment must be watched closely for at least a year if complete eradication is to be assured.

Dead foliage and steins remaining after the plants have been killed with chemicals are only slightly poisonous, but to avoid difficulty the dead stems should be cut off and burned.

Ammonium sulfamate.—This chemical, under a proprietory name, has come on the market recently as a weed killer and has proved especially useful against poison-ivy. Ammonium sulfamate is not dangerous to use under ordinary conditions, is not harmful to the soil, and is not flammable. It is slowly corrosive to metals, and spray equipment should be washed thoroughly with a solution of lime or soda immediately after use. Like salt, ammonium sulfamate absorbs moisture from the air and must be stored in a dry place.

Ammonium sulfamate is dissolved in water at the rate of 12 ounces per gallon and sprayed on the leaves until they are wet but not dripping. The foliage begins to wilt within 21 hours, but the action is slow, and a week or more may elapse before the leaves die. Some new growth may be expected after 4 to 8 weeks, and this should be sprayed as soon as the new leaves are fully expanded. Two or occasionally three such retreatments are needed before the kill is complete.

2,4-D (2,4-dichlorophenoxyacetic acid).—This chemical under the abbreviated name 2,4-D, or DCP, is a recent addition to the list of effective weed killers and may become an important weapon for use against poison-ivy. It should not be confused with DDT or DD, two other new agricultural chemicals, the first an insecticide and the second used as a soil fumigant. Under wartime restrictions the use of 2,4-D was very limited, but its prospective value is so great that those interested in the control of poison-ivy should be informed about it.

The chemical 2,4-D is one of the so-called “plant hormones,” or “growth regulators.” In minute quantities it stimulates some of the plant functions, but in slightly larger dosage it is toxic. A solution consisting of one-seventh ounce in 1 gallon of water sprayed on poison-ivy foliage kills all leaves hit and may kill the roots. In the concentration used in weed killing 2,4-D is not believed to be harmful to animal life, but until this point can be thoroughly clarified the chemical should be used with reasonable caution to avoid poisoning. 2,4-D is not flammable and is not corrosive to spray machinery. If the sprayer is used also for insecticides and fungicides it must be washed very thoroughly to remove the slightest trace of the 2,4-D. This is very difficult, however; it is safer to use another sprayer.

To make a spray solution 2,4-D must be combined with other chemicals, as it is not soluble in water. A number of substances, including polyethelene glycols and naphthenic soaps, have been used for this purpose, but the best solvent has not yet been determined. Commercial weed killers containing 2,4-D are of varying composition.

Ammonium thiocyanate.—This byproduct of the coal-gas industry is one of the newer herbicides. The crude commercial chemical is a pink or reddish salt, very soluble in water. It does not keep well, tends to disintegrate into a red, sludgy mass, and is difficult and disagreeable to handle. A purer form in clean white crystals is available and is well worth the slight extra cost. In either form ammonium thiocyanate is injurious to the skin and to leather articles, and in time it corrodes metals. The spray solution is made by dissolving 1 to 1½ pounds of ammonium thiocyanate in 1 gallon of water.

Most annual, plants and some perennials are killed by a single application of ammonium thiocyanate. Poison-ivy usually requires three or four treatments. Ammonium thiocyanate has a high content of nitrogen, much of which remains in the soil and eventually becomes a fertilizer. Thus an area treated with this chemical is likely to produce a profuse growth of annual weeds a month or two after treatment.

Borax.—Common borax scattered on the soil is deadly to the roots of many plants. Spread dry at the rate of 10 to 15 pounds per square rod, it kills poison-ivy slowly over a period of 6 to 8 weeks. A second somewhat lighter treatment 3 to 5 months later is usually necessary to kill all plants completely.

Borax is easy to handle, nonpoisonous, and nonflammable, readily obtainable, and not expensive. It is one of the best chemicals to use around stone walls, rocky ledges, fences, and other inaccessible places.

Plants differ greatly in their resistance to borax. Some species, as Virginia creeper and deep-rooted trees, do not seem to be injured, but others, notably dogwood, are very sensitive to it. Until more complete information is available as to the effect of borax on other plants, it should be employed on poison-ivy only where possible damage to surrounding vegetation is not objectionable.

Carbon disulfide.—The fumes of this pungent, volatile liquid are very poisonous to plants, and the chemical is widely used in the West to kill deep-rooted perennial weeds. When poison-oak, particularly, is growing in a mixture with wild morning-glory (bind-weed) or similar noxious weeds, both plants may often be killed at one operation by treating the soil with carbon disulfide. Holes ordinarily 6 inches deep and 18 inches apart in all directions are punched in the soil with a 1-inch iron bar, and 2 ounces of carbon disulfide are poured in each hole. The openings are closed immediately with a wooden club. This dosage equals 20 pounds of the chemical per square rod. Carbon disulfide is likely to be less effective on shallow-rooted than on deep-rooted plants and is therefore of limited value against poison-ivy, which is surface-rooted.

Coal-tar creosote oil.—In the fractional distillation of coal, many oils and oillike substances are obtained, some of which are very toxic to plants. The most common and usually the cheapest is the creosote used for wood preservation. This substance has great penetrating power and is an excellent herbicide on woody plants like poison-ivy. Ordinarily creosote oil is too expensive to use alone and is therefore mixed with some kind of petroleum oil. A mixture of 75 percent fuel oil and 25 percent creosote oil is almost as effective as straight creosote and is made at one-third the cost. Two applications of this mixture at an interval of about 6 weeks usually kill a high percentage of poison-ivy. Creosote is not selective and kills all plants alike.

Fuel oil (Diesel and similar oils).—Crude petroleum is refined into many kinds of oil, and those corresponding to fuel oil. Diesel oil, stovetop oil, and orchard-heating oil are more injurious to plants than crude oil itself or the higher distillates, such as gasoline and kerosene. Fuel oils are widely used for destroying vegetation, particularly by highway departments in States having oil resources. When sprayed on poison-ivy, fuel oil brings about a slow but fairly sure death of the plants. Two or three applications may be necessary before the roots are killed. Fuel oil, like creosote, must be used with care around valuable trees and shrubs because it penetrates the bark and kills or injures all species. As used for poison-ivy spraying there is no injury to the soil. The oil-soaked vegetation is flammable.

Sodium chlorate.—Sodium chlorate is the standard herbicide for killing deep-rooted perennial weeds like bindweed, but when used as a spray, it is a dangerous fire hazard, and when applied dry, it sterilizes the soil for 2 to 4 years. Where sterilization of the soil is desirable or not objectionable, one application of the dry chemical, evenly at the rate of 3 to 4 pounds per square rod, is sufficient for a complete kill.

If used as a foliage spray on poison-ivy, the chemical is dissolved in water at the rate of 8 to 12 ounces per gallon. The leaves are killed within a few hours, but the plants are likely to make a strong new growth and may require as many as six treatments before they cease to produce new leaves.

Sodium arsenite.—Sodium arsenite is one of the most powerful plant poisons known and is widely used for sterilizing soil on railroads, roadways, and other places where no vegetation of any kind is desired. It is relatively inexpensive. In normal times sodium arsenite is obtainable in both powder and liquid form. In weak solution (4 to 6 ounces per gallon of water) the chemical can be used as a spray to destroy poison-ivy. Like sodium chlorate spray, it kills the leaves but not the roots, and five or six treatments may be required. The solution kills the leaves of all plants impartially and will also kill the young tender bark of shrubs and trees. This may be either an advantage or a disadvantage, depending upon circumstances.

ERADICATION BY MECHANICAL MEANS

Poison-ivy can be grubbed out by hand quite readily early in spring and late in fall. When the ground is soft after rains the roots come out in long pieces. Grubbing when the soil is dry and hard is almost futile, since the roots break off in the ground, leaving large pieces that later sprout vigorously. Eradication by grubbing is permanent if well done. Because of his close contact with the plants, the person doing the grubbing should have a high degree of immunity to ivy-poisoning. Many people are not so immune as they believe, and it is common sense to wear leather gloves with gauntlets and a shirt with long sleeves. If care is taken to prevent the poison-ivy from touching the face, and if the clothes, including the gloves, are burned or thoroughly washed after use, severe poisoning may be avoided.

Poison-ivy vines climbing on trees should be severed at the base and as much of the vine as possible pulled away from the tree. Often the roots of the tree and weed are so intertwined that grubbing is impossible without injury to the tree. Close mowing of the poison-ivy shoots at frequent intervals is the only remedy. Roots and stems removed in grubbing should be burned or otherwise destroyed, since the dry material is almost as poisonous as the fresh. Care must be taken to keep out of the smoke.

Old plants of poison-ivy produce an abundance of seeds, and these are freely disseminated, especially by birds. A poison-ivy seedling 2 months old usually has a root that one mowing will not kill. Seedling plants at the end of the first year have well-established underground runners that only grubbing or chemicals will kill. Seedlings are a constant threat as long as old poison-ivy is in the neighborhood.

Plowing is of little value in combating poison-ivy unless followed by persistent stirring with a cultivator or harrow, to keep all roots loosened from the soil. Otherwise plowing merely propagates the weed.

Mowing with a scythe or sickle is a common method of attacking poison-ivy, but it has little effect on the roots unless frequently repeated. The number of cuttings required to kill depends upon the age of the plants and the size of the roots, but is seldom less than four. New shoots should be cut as soon as the first leaf on each stem is full size. Cutting either earlier or later delays eradication.

Weed burners are implements resembling oversize blowtorches. Two types are manufactured, one producing a short, hot, blue flame and the other a long smoky-yellow flame. Both are useful for destroying foliage. Neither is likely to kill perennial plants like poison-ivy at one operation, even though the flame is held on the weeds for a minute or more until leaves and stems are completely incinerated. The roots are not injured by this, and new leafy shoots soon develop. A better plan is to “sear” the plants by passing the flame slowly over the vines just long enough to sear or wilt the leaves, but not long enough to burn them. The roots appear to be injured more by “searing” than completely burning the leaves, and the operation is quicker and cheaper. Three to four searings when the leaves reach full size usually kill the roots.

? U. S. Government Printing Office: 1945—652512

For sale by the Superintendent of Documents, U. S. Government Printing Office, Washington 25, D. C.