Durability of Douglas-Fir Hop Poles Treated with Preservatives By Diffusion

Six groups of 20 poles each were observed during 14 years of service in an irrigated hop yard to compare the durability of preservative-treated and untreated round, unseasoned poles of coast-type Douglas-fir with traditional poles of untreated split western redcedar. Diffusion preservative treatment with Osmosalts applied full-length, or double-diffusion treatments by soaking butts in copper sulfate plus sodium chromate, or in sodium fluoride plus copper sulfate, can extend average life of Douglas-fir poles to 15 or more years. Effectiveness of Osmoplastic butt treatment could not be evaluated fairly. Untreated Douglas-fir poles had an average life of only 9 years. Average life of cedar poles might have been 22 years, but could not be predicted reliably because failures were too few when the test ended inadvertently. Untreated and Osmoplastic-treated Douglas-fir poles were seriously damaged above ground by decay, insects, and woodpeckers. Adhering bark probably encouraged their deterioration. Cedar and other treated Douglas-fir poles remained generally sound above ground.

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