Herbicides for grass control in forestry: pot experiments on efficacy and crop tolerance.
Imazapyr applied as an overall spray to perennial grasses in summer effectively controlled Calamagrostis epigejos and Molinia caerulea. Additional surfactant sometimes enhanced activity. Summer applications separately to foliage or roots were both effective against Molinia caerulea. Propyzamide as an overall spray in summer or autumn severely reduced growth of Deschampsia cespitosa and, to a lesser extent, Holcus mollis. Applications to the foliage only were more toxic than root drenches. Activity was not increased by spray additives. A mixture of terbuthylazine + atrazine applied overall in May or July generally controlled perennial grasses more effectively than atrazine alone. May and July applications of the mixture consistently damaged larch (Larix leptolepis), and May treatment reduced the growth of lodgepole pine (Pinus contorta var. latifolia). Douglas fir (Pseudotsuga taxifolia [P. menziesii]) and sitka spruce (Picea sitchensis) were however unaffected. There was much less damage to larch from root drenches in May compared with overall spraying. Application to beech (Fagus sylvatica) in summer caused severe damage. Treatment of newly-planted broadleaved trees with high doses of the mixture in March damaged alder (Alnus rubra), ash (Fraxinus excelsior), birch (Betula pendula), cherry (Prunus avium) and sycamore (Acer pseudoplatanus) but not beech and oak (Quercus robur). The results suggest that some conifers may tolerate overall spraying of terbuthylazine + atrazine in summer but that directed spraying is necessary with others such as larch. The tolerance of broadleaved trees to this mixture also requires further investigation.