Herbicide Effectiveness in Response to Season of Application and Shrub Physiology

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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 integerrimus var. californicus (Kell.) G. T. Benson.], greenleaf manzanita (Arctostaphylos patula Greene), bearmat (Chamaebatia foliolosa Benth.), snowbrush ceanothus (Ceanothus velutinus Dougl.), and whiteleaf manzanita (Arctostaphylos viscida Parry), 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.