[2] For brevity, the single designation poison-ivy is used under this heading to apply equally to all poison-ivies and to poison sumac.
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.