On eradication of woody plants with herbicides in fields and pastures
Authors: Mukula Jaakko 
Published: July 2018

On the initiative of the Department of Plant Husbandry of Agricultural Research Institute, experiments for destroying woody plants on fields and pastures were conducted in 1948—1949 with the following chemicals: Artificial hormones (sodium salt of 2,4-D, morpholine of 2,4-D, triethanolamine of 2,4-D, ethyl ester of 2,4-D, butyl ester of 2,4-D, and sodium salt of 2M-4K), potassium chlorate, and ammonium salt of dinitro-ortochresol. The substances Were applied in aqueous solutions, or as emulsions. Three different methods Were used: spray application to foliage, absorption through a cut branch, and application to soil. The following thicket-forming woody plants, common in Finland, were investigated: grey alder, Alnus incana (L.) Willd., willow, Salix sp., birch, Betula sp., mountain ash, Sorbus aucuparia L., and aspen, Populus tremula L. Following conclusions have been drawn from the experiments; It is possible, and, probably with the exception of aspen, even advisable to destroy small sprouts, 0.5—1.5 m. in height, of these woody plants by spraying their foliage with artificial hormones early in summer, especially if mechanical clearing machines cannot be used. Of the experimented substances, esters of 2,4-D proved the most effective in spray applications, but satisfactory results were even secured with other artificial hormones. For different species of woody plants the necessary concentration of the solution varies from 0.1 to 0.4 % of the active substance (p. 6). For dense thickets, 0.5—1.5 m. in height, the amount of solution used Was 1250—2000 l. per ha., applied by means of knapsack type of pressure sprayers. All branches must be sprayed. It is difficult to make the treatment effective enough, if only one application is made. Therefore it is important to conduct a new control spraying after 2—3 weeks. Treatment is most effective, if given in Warm, sunny weather. The best results are secured with spraying towards the end of June or at the beginning of July. If treated early in spring, many new sprouts develop on stems and branches; if treated late in summer, artificial hormones do not kill leaves effectively enough. The leaves of young sprouts and suckers die more readily than those of old tall trees and shrubs. It is possible to 2prevent the regeneration of alder almost completely by sprayings in one summer, if sprouts do not grow from the stumps of very tall trees. Willow, birch, mountain ash, and aspen often show slight regeneration in the following summer. The results obtained with sprayings with artificial hormones confirm with slight exceptions the results of earlier investigations (1, 4,5, 6, 12, 23, 24). It is possible to kill the leaves of sprouts by sprayings with chlorate and chresol, but such treatment does not prevent the development of new sprouts. Aqueous solutions of the investigated types of artificial hormones can be effectively applied to alders through a cut branch (fig. 9). Water-insoluble artificial hormones cannot be thus absorbed. Solutions of low concentrations are absorbed more easily than those of high concentrations. The amount of active substance used, however, is for the latter type greater than for the former. Relatively tall alders, and their roots, can thus be killed. But tall willows, birches, mountain ashes, and aspens are not killed, if the substance is applied to one branch only. The necessary amount of substance is even for alder so great that this method cannot be recommended for general practice. Solutions absorbed through cut branches are chiefly transported to different parts of the plant in the xylem. Because of the transpiration stream their translocation downwards in the xylem is very difficult. If transpiration stream, however, is prevented by cutting the stem above the absorbing branch, the substance travels vigorously towards the roots, at least in July and August. Based on this, it has been suggested that high thickets of sprouts (and suckers) as well as tall trees and shrubs, from whose stumps thickets develop, should be destroyed by applying artificial hormones to the stumps of cut plants. Thus it would not be necessary to pay attention to the transpiration stream, and active substance need not be wasted for destroying the above-ground parts of these plants. Possibilities of using this method must be further investigated. Also solutions of chlorate and chresol can be absorbed through cut branches into alders. It remains to be investigated whether this absorption is effective. Applications of the above-mentioned artificial hormones, chlorate, and chresol to the soil do not secure satisfactory results with regard to destruction of woody plants, at reasonable costs.