Imazapyr application to Rhododendron ponticum: Speed of action and effects on other vegetation

Abstract

Imazapyr as Arsenal 50 is recommended for pre-planting control of woody and herbaceous weeds in forestry and has been shown to be effective for Rhododendron ponticum control. Its speed of action on R. ponticum was investigated by cutting off foliage at different times after spraying. Where shoots were removed 2 or more days after spraying there was no regrowth from cut stumps. Where cutting was done 6-12 h after spraying there was some effect on the rate of regrowth, but final growth was unaffected. This result suggests that imazapyr is translocated fairly rapidly in R. ponticum and, where required, clearance of bushes may be feasible within a short time of spraying. The actual time interval requires investigation in field conditions. The potential for damage from root transfer of imazapyr from treated R. ponticum or Picea sitchensis was investigated between pairs of either R. ponticum or P. sitchensis planted in the same pot, or between one R. ponticum and one P. Sitchensis planted together. Foliage was separated before treatment and soil covered prior to the foliage of one of the pair being sprayed. This ensured that the only contact between the two plants could be from the roots. There was no evidence of any damage from root transfer of imazapyr from treated plants to untreated plants at any time up to 10 months after treatment. The effect on tree growth from treated plant debris being incorporated into planting soil was also investigated. Plant material from Betula pendula, Rubus fruticosus and R. ponticum was sprayed with two doses of imazapyr in August, then incorporated into tanks of soil in the spring prior to planting with Fraxinus excelsior and P. sitchensis. There were no visible effects on plant growth in the year of planting but, in the second year, small reductions in growth were recorded, particularly of P. sitchensis. The results suggest that there is unlikely to be any long-term reduction in the growth of newly planted trees from treated plant debris. However, sites where a large quantity of treated material is present should be monitored for possible long-term effects.