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

Abstract

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 greenhouse and held at a relatively low temperature (32⚬-40⚬). 4. Spray treatments with 1000 p.p.m. concentration of the sodium salt of the acid were about equal to the acid in effectiveness in causing death of Barbarea verna, Brassica juncea, Convolvulus arvensis, Plantago lanceolata, P. major, and Sisymbrium officinale. The salt likewise was found to be present in or on plants treated at low temperature (50⚬-60⚬), since upon removal to a warmer temperature they died rather quickly. 5. The ammonium as well as the sodium salt of the acid was effective in killing Hydrocotyle rotundifolia, Prunella vulgaris, and Stellaria media. Death of plants of Hydrocotyle and Prunella was hastened by removal from a greenhouse at 60⚬ to one maintained at 75⚬-80⚬. 6. Field experiments have demonstrated that dormant Plantago lanceolata in frozen ground, having some living leaves, when treated with the acid or its sodium salt (applied in spray solution of 1000 p.p.m. at 5 gallons to 1000 sq. ft.) is killed when the plants are placed under temperature conditions favorable for growth.