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.