SPATIAL AND TEMPORAL FACTORS IN WEED INTERFERENCE WITH NEWLY PLANTED APPLE-TREES

Temporal and spatial combinations of tree-row weed suppression treatme nts mere evaluated during 5 years in a New York apple (Malus domestica Borkh, cv. Imperial Gala on Malling 26 rootstocks) orchard planted in Apr, 1991, and provided with trickle irrigation. Twenty-eight factori al treatment combinations [0, 2, 4, and 6 m(2) weed-free areas (WFAs); and May, June, July, August, May + June, June + July, May + June + Ju ly, and June + July + August weed-free times (WFTs)] were maintained f rom 1991 to 1995 by postemergence paraquat herbicide applications in t ree-row strips, Trunk cross-sectional area (TCA) growth and yield were monitored annually, and few differences were observed as WFA increase d from 2 to 4 to 6 m(2) per tree, However, WFT substantially influence d TCA, fruit production, and yield efficiency. Early summer WFTs incre ased TCA during the first two growing seasons, compared with late summ er treatments, When trees came into production in 1993-94, yields incr eased as the duration of WFT increased, but where similar periods of W FT had been established later during the growing season, annual yield, cumulative yield efficiency, and the ratio of crop value to weed-cont rol costs were all reduced, Groundcover species distribution was evalu ated each gear in September, and graminaceous weeds were more prevalen t in the early and midsummer WFTs, while herbaceous broadleaf weeds do minated in the August treatments, A quadratic model regressing cumulat ive yield efficiency on WFTs grouped into 30-, 60-, and 90-day categor ies showed that efficiency peaked between 60 and 90 days of WFT. It ap peared that timing of weed suppression may be as important as the area of suppression beneath trees in comparable apple orchards, that early summer weed control was especially important for newly planted trees, and that drip irrigation allowed reductions in the area and amount of tree-row herbicide applications, without significant losses in apple tree growth or crop value.