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Résumé :
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The selective herbicide clopyralid is often used to control competing Cirsium arvense in newly planted woodlands. When applied as an overall spray at different dates in the spring (at 0.2 kg acid equivalent (a.e.) ha1) to 10 tree species (Fraxinus excelsior, Prunus avium, Quercus robur, Acer pseudoplatanus, Populus x canadensis cv. Ghoy', Pseudotsuga menziesii, Pinus nigra ssp. laricio, Larix kaempferi, Picea abies and Picea sitchensis) it did not reduce survival, and had little effect on growth. However, some species showed distortion of the youngest sprayed leaves or needles for several weeks after treatment, particularly F. excelsior, L. kaempferi and P. x canadensis. Sequential applications of clopyralid (first at 0.1 kg a.e. ha1 followed by 0.2 kg a.e. ha1 after 3 weeks), which are often required to control C. arvense, did not lead to increased leaf damage or growth reduction. Mixtures of clopyralid with selective graminicides (cycloxydim at 0.45 kg active ingredient (a.i.) ha1; fluazifop-p-butyl at 0.38 kg a.i. ha1 and propaquizafop at 0.15 kg a.i. ha1) did not cause significant adverse effects on survival or growth of any species. If herbicides are required to control mixed stands of susceptible problem weeds such as C. arvense and grasses which are overtopping young trees, these herbicide mixtures, applied as overall sprays, are less likely to cause damage to trees than attempts to use directed applications of broad-spectrum foliar-acting herbicides.
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