Aerial spraying improved tree growth by 1.7 to 2.5 times in Douglas-firs affected by varnishleaf ceanothus, suggesting an effective herbicidal approach.
Well-stocked stands of young Douglas-firs are often found under dense overstories of varnishleaf ceanothus, but tree growth is retarded by shrub shade and competition for soil moisture. Aerial spraying effectively released Douglas-firs, and their growth was 1.7 to 2.5 times that of comparable trees under unsprayed ceanothus. A similar but less dramatic response was obtained by basal spraying varnishleaf ceanothus on small plots. A herbicidal treatment is suggested for releasing Douglas-firs from varnishleaf as soon as the trees are well established on such sites.
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Gratkowski et al. (1974) studied this question.
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