DISCUSSION, 

 CONCLUSIONS 



Several years of trials have demonstrated that soil fumigation at the Coeur d'Alene 

 and Savenac nurseries is a practical operation from the standpoint of pest control, 

 growth stimulation, and increased survival potential. 



Weed and disease control have been best with methyl bromide-based fumigants and 

 with Vapam, applied at medium to high rates. The dichloropropene-based fumigants were 

 not effective enough to be considered practical. Late August or early September fumi- 

 gation has proven more effective and more practical than spring fumigation. Fumigation 

 at this time of year gives plenty of time to prepare the soil and to fumigate at the 

 optimum temperature and moisture content. The nursery workload is relatively low at 

 this time, so the extra job of fumigation is not a great hardship to the nurseryman. 

 Most important, however, when conditions for sowing are favorable the following spring, 

 the soil can be worked and the seed sown at the earliest possible date to gain the advan- 

 tage of early germination and a long growing season. Spring fumigation proved opera- 

 tionally difficult and resulted in stunted seedlings and no observable disease control. 



Both spring and fall fumigation substantially reduced weed populations during the 

 first growing season, and reduced potential weeds by killing seed to a depth of at least 

 9 inches. IvTien compared to hand weeding, the more effective fumigants provided sub- 

 stantial savings in weed control costs. 



Production was greater in effectively fumigated seedbeds, reflecting primarily the 

 control of pathogenic fungi. Control of white grubs was observed 1 year and may have 

 played a part in the observed effects in other years. Although nematodes were not 

 abundant enough to be considered a problem, reduction of their population by fumigation 

 could have contributed to the good results. Fumigation, by providing more plants per 

 pound of seed, reduced the seed cost by substantial amounts, often exceeding the $300 

 to $400 per acre fumigation expense. 



Soil fumigation with the methyl bromide-based fumigants has generally increased the 

 size of seedling stock of Douglas-fir, western white pine, Engelmann spruce, and 

 ponderosa pine. These effects of fumigation on the size of planting stock are, for the 

 most part, beneficial since they have helped to reduce the time required to produce 

 stock which is suitable for planting. A reduction of 1 year in production time is 

 roughly equivalent to $3 to $4 per thousand seedlings or $3,000 to $4,000 per gross 

 nursery acre. Since fumigation increases the plant density from a given amount of seed, 

 a reduction in sowing rates is required to capitalize on the full effects of fumigation 

 on plant size. 



Survival of 2-0 stock from fumigated seedbeds when "outplanted" into semi-controlled 

 soil-moisture-stress levels was generally equal to or better than that of stock from 

 unfumigated soil. Douglas-fir survival under moderate and severe soil moisture stress 

 was significantly improved by fumigation. In contrast, the survival potential of 

 Coeur d'Alene grown 2-0 ponderosa pine was reduced by fumigation. However, fumigation 

 has made it possible to raise suitable 1-0 ponderosa pine planting stock which survives 

 field planting well and is probably not as severely affected by competition in the 

 nursery beds as is the 2-year-old stock used in testing field survival of stock from 

 the fumigation tests. 



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