Reforestation in the Inland Northwest, including northeastern Oregon, USA, is often limited by a dry climate and soil moisture availability during the summer months. Reduction of competing vegetative cover in forest plantations is a common method for retaining available soil moisture. Several spring and summer site preparation (applied prior to planting) herbicide treatments were evaluated to determine their efficacy in reducing competing cover, thus retaining soil moisture, on three sites in northeastern Oregon. Results varied by site, year, and season of application. In general, sulfometuron (0.14 kg ai ha–1 alone and in various mixtures), imazapyr (0.42 ae kg ha–1), and hexazinone (1.68 kg ai ha–1) resulted in 3 to 17% cover of forbs and grasses in the first-year when applied in spring. Sulfometuron+glyphosate (2.2 kg ha–1) consistently reduced grasses and forbs for the first year when applied in summer, but forbs recovered in the second year on two of three sites. Aminopyralid (0.12 kg ae ha–1)+sulfometuron applied in summer also led to comparable control of forb cover. In the second year after treatment, forb cover in treated plots was similar to levels in nontreated plots, and some species of forbs had increased relative to nontreated plots. Imazapyr (0.21 and 0.42 kg ha–1) at either rate, spring or summer 2007, or at lower rate (0.14 kg ha–1) with glyphosate in summer, provided the best control of shrubs, of which snowberry was the dominant species. Total vegetative cover was similar across all treatments seven and eight years after application, and differences in vegetation were related to site rather than treatment. In the first year after treatment, rates of soil moisture depletion in the 0- to 23-cm depth were correlated with vegetative cover, particularly late season soil moisture, suggesting increased water availability for tree seedling growth.

Concentrations of hexazinone and two metabolites in vegetation were determined for 2 yr after broadcast application of a 10% granular formulation of hexazinone at 2 and 4 kg ai ha−1 rates in August 1986 in a boreal forest. Prewinter concentrations of hexazinone in stems of trembling aspen, Saskatoon berry, and willow ranged from 0.02 to 0.05 μg−1 dry wt at 64 d after treatment (DAT). Absorption of hexazinone accelerated during spring thaw (1987), and residues in foliage of several woody and herbaceous species peaked during May to July. Patterns of accumulation of hexazinone and its metabolites varied with the species. Foliar concentrations diminished significantly by the end of the first growing season in 1987 (372 DAT) and were undetectable or extremely low at the end of the second growing season in 1988 (707 DAT). Based on the highest residue concentrations detected in several plant species, it is estimated that wildlife would ingest a maximum of 16, 28, and 24 mg of hexazinone, metabolite A, and metabolite B, respectively, for every kg of dry matter consumed. Reported LD50 values suggest that application of hexazinone at the 4 kg ai ha−1 rate or less poses no toxicological threat to wildlife.

The growth of loblolly pine seedlings was reduced by interference from bermudagrass, tall fescue, and broomsedge. The order for treatment influence on pine seedling growth rate measurements was tall fescue > bermudagrass > broomsedge > weed free for the 2-yr study. The increase in height of seedlings for the weed free, broomsedge, bermudagrass, and tall fescue was 4.1, 2.9, 1.9, and 1.1 cm/month, respectively; and the average increase in seedling stem diameter was 1.3, 0.5, 0.4, and 0.2 mm/month, respectively.