EFFECTS OF CHEMICAL WEED CONTROL ON THE GROWTH PATTERNS OF CONIFER TRANSPLANTS

Summary. Studies were undertaken to determine the tolerance of three conifer species to repeated applications of several herbicides over a 2-year period and the degree of weed control required to obtain optimal growth.
The results indicated that: (1) 60% or more weed control was required to obtain optimal growth of Picea glauca (Moench) Voss and P. pungens Engelm., while Pinus sylvestris L. showed optimal growth with average weed control as low as 40%; (2) weed competition did not affect the percentage survival of any of the three species under irrigated nursery conditions; (3) absence of visual injury symptoms was not an indication of the tolerance of a species to a herbicide; (4) severe weed competition caused an overall reduction in growth with significant reductions in stem diameter and in height and fresh weight of shoots; (5) herbicidal toxicity caused reductions in height, fresh weight or stem diameter but not necessarily of all three; (6) initial survival was affected only by those herbicides that were highly toxic at the doses applied; and (7) significant differences in growth, due either to weed competition or to herbicidal toxicity were apparent, in most instances, after the 2nd year's growth.
The treatments that gave 70% or more control each year with no apparent growth retardation over the 2-year period of the three species tested were: single yearly applications of neburon 4 lb/ac or linuron 2 lb/ac, two applications each year of DCPA (dimethyl 2,3,5,6-tetrachloroterephthalate) 10 lb/ac and directed application of sodium pentachlorophenate 15 lb/ac.