Relation of Temperature to the Selective Herbicidal Effects of 2,4- Dichlorophenoxyacetic Acid

1. Plants of Barbarea verna and Plantago lanceolata which were established in 4-inch clay pots in a warm greenhouse (65⚬-75⚬F.) were moved to greenhouses in which temperatures of 32⚬-40⚬, 50⚬-65⚬, 65⚬-75⚬, and 75⚬-90⚬ F. were maintained, and separate lots of twenty-four plants each were spray-treated with 0, 500, 1000, and 1500 p.p.m. concentrations of 2,4-dichlorophenoxyacetic acid 1 week later. Carbowax 1500 was included in the acid sprays. 2. Treatment at the two higher temperature ranges resulted in rapid killing (18-21 days). All three concentrations of the acid were about equal in their effectiveness under the conditions of the experiments. At 50⚬-60⚬, however, the time required to kill plants with the acid treatment was extended for 11-15 days longer, while at 32⚬-40⚬ the treated plants were still living at the end of 50 days following spray applications. 3. The data indicate that the acid was present in lethal amounts, either within the tissues or on the surface of plants, treated in the greenhous...