Summary (from the scientific paper Dixon et al., 2005)
The selective herbicide clopyralid is often used to control competing Cirsium arvense (creeping thistle) in newly planted woodlands. When applied as an overall spray at different dates in the spring (at 0.2 kg a.e. ha-1) to 10 tree species (below) it did not reduce survival and had little effect on growth:
- Fraxinus excelsior (ash)
- Prunus avium (cherry)
- Quercus robur (oak)
- Acer pseudoplatanus (sycamore)
- Populus x canadensis cv ‘Ghoy’ (poplar)
- Pseudotsuga menziesii (Douglas fir)
- Pinus nigra ssp. laricio (Corsican pine)
- Larix kaempferi (Japanese larch)
- Picea abies (Norway spruce)
- Picea sitchensis (Sitka spruce).
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. ha-1 followed by 0.2 kg a.e. ha-1 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 a.i. ha-1; fluazifop-p-butyl at 0.38 kg a.i. ha-1; propaquizafop at 0.15 kg a.i. ha-1) 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 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.
- When creating new woodlands on good quality farmland, creeping thistle can be very damaging to young trees. Herbicides are often used as this species cannot generally be controlled by mulches, cutting, or shallow cultivation. However, as creeping thistle grows so tall, directed applications of contact herbicides are not practical.
- Clopyralid (as Dow Shield) can control creeping thistle very effectively, when applied as a sequential dose of 0.5 litres per hectare to extending shoots in May, followed by 1 litre per hectare 3 weeks later.
- Sequential doses as described above are generally safe to apply over actively growing trees.
- Where grasses occur in mixture with thistles, cycloxydim (e.g. as 2.25 l/ha Laser), fluazifop-p-butyl (e.g. as 3 l/ha Fusilade Max) or propaquizafop (e.g.as 1.5 l/ha Falcon) are also safe to apply over trees.
- Practical guidance on use of clopyralid, cycloxydim, fluazifop-p-butyl and propaquizafop has been issued in Forestry Commission Field Books (e.g. Willoughby and Dewar, 1995) and a Technical Paper (Willoughby and Clay, 1999). Practical guidance on tree tolerance will be included in a future revised version of Forestry Commission Field Book 8.
Dixon, F.L., Clay, D.V. and Willoughby, I. (2005 in press). The tolerance of young trees to applications of clopyralid alone and in mixture with other foliar acting herbicides. Forestry 78 (4) 353-364.
Willoughby, I. and Clay, D. (1999). Herbicide update. Forestry Commission Technical Paper 28. Forestry Commission , Edinburgh.
Willoughby, I. and Dewar, J. (1995). The use of herbicides in the forest. Forestry Commission Field Book 8. HMSO, London.
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