Dalapon (2,2-dichloropropionic acid), injurious to Douglas fir (Pseudotsuga menziesii (Mirb.) Franco) and grand fir (Abies grandis (Dougl.) Lindl.) when used alone for weed control, was used with safety and even with beneficial effects when combined with atrazine [2-chloro-4-(ethylamine)-6-(isopropylamino)-s-triazine] and 2,4-D, [(2,4-dichlorophenoxy) acetic acid] at rates of up to 3.36 and 4.48 kg/ha, respectively. Harmful effects of dalapon were masked at rates up to three times those used for adequate control of grasses. This suggests a type of selectivity previously unreported. Interpretation of herbicide effects and interactions was made possible with an analytical model that separated effects of vegetation from those of herbicides on a causal basis. This technique included two-dimensional adjustment of the base of reference for a four-dimensional response-surface equation. This surface fit observed data as well as a single multiple-regression equation, which included all variables. This approach, however, estimated herbicide parameters with appropriate consideration of environmental data and permitted interpretation with respect to cause.