Herbicide Effectiveness in Response to Season of Application and Shrub Physiology

Abstract

The selectivity of five foliage-applied herbicides, 2,4-D [(2,4-dichlorophenoxy)acetic acid], dichlorprop [2-(2,4-dichlorophenoxy)propionic acid], glyphosate [N-(phosphonomethyl)glycine], fosamine [ethyl hydrogen (aminocarbonyl)phosphonate], and triclopyr {[(3,5,6-trichloro-2-pyridinyl)oxy] acetic acid}, applied at three phenological stages of development to five Sierran shrub species, deerbrush [Ceanothus integerrimusvar.californicus(Kell.) G. T. Benson.], greenleaf manzanita (Arctostaphylos patulaGreene), bearmat (Chamaebatia foliolosaBenth.), snowbrush ceanothus (Ceanothus velutinusDougl.), and whiteleaf manzanita (Arctostaphylos viscidaParry), was compared. Treatments in the summer and fall were less effective than spring applications. Periods of herbicide susceptibility generally corresponded to times when moisture stress was low and photosynthesis was high.