Tolerance of broadleaved tree and shrub seedlings to preemergence herbicides

Control of competing vegetation is essential for the successful establishment of tree seedlings in nurseries and direct-sown woodland; this usually requires potentially expensive hand weeding or post-sowing preemergence herbicides. In order to identify suitable herbicides, two container experiments tested the response of 12 broadleaved tree and shrub species to napropamide and pendimethalin applied preemergence. Most species tolerated rates adequate for controlling many annual weed species although Rhamnus cathartica L. (buckthorn) and Alnus glutinosa (L.) Gaertn. (alder) were damaged by all rates of napropamide. A study of application date of napropamide and pendimethalin applied post-sowing to Fraxinus excelsior L. (ash) in containers showed that pendimethalin was damaging if applied when seeds were germinating or seedlings emerging, but napropamide was tolerated at all growth stages. A field experiment tested the tolerance of ten species sown in seedbeds to napropamide alone and in mixture with pendimethalin. Results generally confirmed the indications of tolerance from the container experiments. Applications of 2 kg a.i. ha(-1) napropamide plus 2 kg a.i. ha(-1) pendimethalin appeared to be safe on Corylus avellana L. (hazel), Fagus sylvatica L. (beech), and F. excelsior, provided tree seeds were sown to the correct depth and at least 2 weeks elapsed between herbicide treatment and tree seed germination. The mixture of 2 kg a.i. ha(-1) pendimethalin plus 1.0 kg a.i. ha(-1) of napropamide was tolerated by Acer pseudoplatanus L. (sycamore) and Crataegus monogyna Jacq. (hawthorn). Applications of 1.0 kg a.i ha(-1) napropamide alone were moderately tolerated by Carpinus betulus L. (hornbeam) and Cornus sanguinea L. (dogwood).