WEED CONTROL 



METHODS 



The effects of fumigation on weed control have been evaluated by periodic sample 

 counts of weeds on three or four randomly selected 1-square-foot plots during the first 

 growing season and the early part of the second. Weed counts were followed by hand 

 weeding. When possible, time studies of hand weedings were conducted to relate labor 

 cost to weed density. Greenhouse germination of weeds from soil samples taken at 

 various soil depths provided information on the residual weed-seed population and its 

 control by fumigation. Experimental design and statistical analyses were obtained by 

 standard randomized block layouts and analysis of variance, by paired-sample t-test 

 procedures, or by regression analysis, 



EFFECTS ON WEED POPULATIOMS 



Weed control has been an outstanding, readily visible feature of the benefits of 

 soil fumigation (figure 1), Most of the fumigants consistently resulted in substantial 

 reductions in subsequent weed populations; this was especially apparent during the 

 early part of the first growing season following fumigation (table 2), Fumigants that 

 did not show a positive control of weeds were: (1) Vapam applied 40 gallons per acre 

 and not covered by tarpaulins; (2) Brozone applied 110 pounds per acre; and (3) Vidden-D 

 applied 50 gallons per acre. 



In general, those products containing 1,3 dichloropropene and related hydrocarbons 

 (Vidden-D, M-2441, and M-2467) were not as effective in controlling weeds as were the 

 methyl bromide-based fumigants or Vapam. In the 1963 spring fumigation, which was less 

 influenced by a post-fumigation influx of windblown weed seed than any of the fall 

 fumigations, weed control was roughly proportional to the amount of methyl bromide and 

 3-Bromopropyne (propargyl bromide) in the fumigant, Vapam applied at 40 gallons per 

 acre and sealed with a polyethylene tarpaulin was as effective as the water-sealed 

 material at twice this application rate. 



Periodic weed counts showed that the effects of fumigation on weed populations 

 tended to diminish during the summer. By September 1963, there were no significant 

 differences in weed population which could be attributed to fumigation; nor was there 

 any carry-over effect into the second growing season. Apparently weeds germinating on 

 the treated plots in late summer and the following spring originated from windborne 

 seed , 



4 



