Herbaceous weed control in young conifer plantations with formulations of nitrogen and simazine

Abstract

Weed control and 2nd-year survival and growth of newly planted Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) and noble fir (Abiesprocera Rehd.) seedlings were measured after application of herbicide and fertilizer in a replicated complete factorial experiment with four levels of simazine (0, 2.2, 4.4, 8.8 kg/ha), three levels of nitrogen (0, 110, 220 kg/ha), two types of nitrogen (urea prill; urea + trimamino-s-triazine (TST) prill), and two kinds of formulations (cogranular prill of simazine + nitrogen; nitrogen prill followed by liquid simazine). For the first growing season, total weed and grass control increased with increasing simazine rates. Total weed control was better when urea + TST, rather than urea alone, was applied in conjunction with simazine. Formulation and nitrogen rate were not significant. After plot treatment with 1.1 kg/ha of liquid hexazinone at the beginning of the second growing season, Douglas-fir survival decreased as rate of urea alone increased; survival decreased with little or no weed control and remained constant or increased with good weed control as rate of urea + TST increased. Noble fir height and diameter and Douglas-fir diameter declined with poor weed control but increased at least to the levels of untreated seedlings with good weed control. Noble fir diameter responded positively to added nitrogen. Although simazine may be toxic to 1st-year conifers, this study suggests that more complete weed control in conjunction with fertilization may benefit young conifer plantations.