Five-Year Performance of Three Conifer Stock Types on Fine Sandy Loam Soils Treated With Hexazinone

In May 1987, hexazinone (Velpar® L) was applied by helicopter at 0, 1, 2, and 4 kg active ingredient (a.i.) in 45 L of total solution per ha to a sandy loam site approximately 74 km northwest of Thunder Bay, Ontario. Container (FH408 paper pot)jack pine (Pinus banksiana Lamb.) and container (FH408 paper pot) and bareroot (1½ + 1½) black spruce (Picea mariana [Mill.] B.S.P.) were hot-planted 1 month and deferred-planted 12 months after herbicide application. Survival, height, and diameter growth were monitored annually, through the fall of 1991. Jack pine container stock planted 1 month after hexazinone treatment at 2 and 4 kg a.i./ha suffered 12% greater mortality than trees planted 1 yr later. However, the benefits of early establishment and herbaceous weed control offset these early losses and hot-planted areas supported equal volumes at age 4, and 2.3-fold more volume per ha than deferred-planted areas 5 growing seasons after herbicide treatment. Black spruce container and bareroot stock exhibited high tolerance to hexazinone throughout the range of rates tested. Overall, a 1 yr delay in planting resulted in stem and stand volumes that were less than half of those observed in hot-planted areas. Growth response was positively related to the level of herbaceous weed control achieved, with 4 kg treated areas supporting volumes 2- to 4-fold greater than those on untreated areas. The data illustrate significant growth advantages associated with early crop establishment and herbaceous weed control. North. J. Appl. For. 16(2):72-81.