Application. Herbaceous weed cover in conifer plantations can be effectively controlled not only by application of hexazinone herbicide but also by simultaneous application of hexazinone and the slow-release nitrogen fertilizer triamino-s-triazine.Reducing herbaceous competition with hexazinone increased noble fir survival by about 100% and Douglas-fir and noble fir diameters by 38% and 25%, respectively, on a wet coastal site and two successively drier interior sites in Oregon. Early and simultaneous treatment may prove to be cost effective if, as has been previously shown, such early advantage persists and if the slow-release nitrogen gives added advantage.Abstract. In order to determine if herbicide efficacy is affected by nitrogen fertilizer, the influence of different nitrogen fertilizers applied in different combinations with hexazinone formulations were evaluated on herbaceous weed communities. Field studies comparing three application methods in conifer plantations showed greatest reduction in total weed cover with a co-granular formulation of hexazinone and triamino-s-triazine.Slightly less control was achieved with separate applications of liquid hexazinone and triamino-s-triazine granules, and poorest control with granular urea followed by liquid hexazinone. Weed control increased with an increase in hexazinone rate. Statistical analysis of the effect on conifers showed that the highest hexazinone rate significantly increased survival of noble fir (Abies procera Rehd.) and stem diameter of both noble fir and Douglas-fir (Pseudotsuga menziesii (Mirb.) France), and that the highest nitrogen rate significantly decreased survival of both species but did not affect stem diameter. Survival of noble fir and diameter of both noble fir and Douglas-fir were significantly increased where a co-granular formulation of hexazinone and triamino-s-triazine granules was used.