The potential for using boron pressure-treatment prior to over-treatment with conventional, heavy-duty wood preservatives to limit internal decay in-service was investigated in two field tests on Douglas-fir utility poles. Pole sections were pressure-treated with disodium octaborate tetrahydrate (DOT) and then over-treated with either pentachlorophenol or copper naphthenate in oil. Alternatively, poles were treated with a mixture of DOT and ammoniacal copper zinc arsenate in a single treatment. The pole sections were installed in the field and monitored for changes in boron content over time. Boron levels immediately after treatment were extremely high in the outer 25 mm of poles in both tests and declined sharply away from the surface. Field exposure for one to five-years resulted in some boron losses near groundline, but overall levels near pole surfaces remained well above the fungal protection threshold. Diffusion did occur further inward, but there was still a steep boron gradient from the pole surface five-years after treatment. Results are discussed in the context of Douglas-fir heartwood resistance to boron diffusion compared with other wood species and the potential for long-term retention of chemical within the heartwood.

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