JARS v58n1 - The Case for Treated Wood

The Case for Treated Wood
Richard F. Kruppa
Volcano, Hawaii


Vireya rhododendron beds can be made from many things including lumber which is available in convenient sizes (i.e., 4 in. x 6 in. by 8 ft. long). Normal construction grade is terribly expensive, but utility grade can be purchased for much less (a 4 in. x 6 in. by 8 ft. costs $10). A 6-in. deep 4 ft. by 8 ft. bed would cost about $30. Unfortunately utility grade is only available treated with chemicals to resist termites.

In the past gardeners tried to use treated lumber for flower beds with disastrous results; their plants died (1). The word went out: "Don't use treated wood with plants." As late as the early 1990s this was certainly true.


Original wood treatments employed arsenic based chemicals, which controlled termites and killed plants. With the growth of environmental consciousness, a treatment using copper naphthenate was developed and replaced arsenic. Wood so treated is easily identified; it takes on a grayish blue cast. It kills plants too. The author used this treated wood to rebuild a lanai in early 1992 (it was the only kind available without arsenic).

In the 1980s, U.S. Borax developed an effective wood treatment using TIMBOR-DPT (disodium octaborate tetrahydrate), an "EPA [Environmental Protection Agency] registered wood preservative…the only ecological hazard associated with borate treated wood relates to the...preservative which has the potential to leach out...under...exposure to water over a prolonged period of time. Borate compounds may (2) be harmful to boron sensitive plants" (3). Wood so treated is called HI-BOR™ (4) and contains 1% TIMBOR-DPT. Solubility of the active ingredient in water is less than 0.1% (1000 ppm).

Sometime after 1994, HI-BOR™ came to Hawaii and is now sold by HPM who does the treatment here on the Big Island (5). Copper naphthenate treated wood is still available on this island as is arsenic treated (called Wolmanized).

The Experiments

1. On Dec. 5, 2002, a R. lochiae x R. chrisitianae (6) was transplanted from a 6 in. to an 8 in. pot using a 50:50 cinder and treated wood shavings mix. A 2-inch layer of shavings was laid on top of the transplanted vireya. The plant was then watered twice daily because of the drought using a cup of water each time. After three weeks new growth was evident. At this writing the vireya is thriving and shows 1.5 inches of new growth. No fertilizer was given to the plant. No deleterious effects were observed at any time.

2. On Dec. 5, 2002, 300 grams of fresh HI-BOR™ shavings were simmered in 4 quarts of water for 3 hours. This yielded 3 quarts of deep yellow extract having a strong wood odor. On Dec. 9, 2002, a R. jasminiflorum vireya (6) was transplanted from a 4 in. to a 6 in. pot using a 50:50 mix of cinder and treated wood shavings. Two inches of treated shavings were placed on top of the transplant. One-third of a cup of the extract was fed to the vireya each evening and then one-half cup of water each morning. After 35 days, the extract was used up. No fertilizer was given to the plant. The jasminiflorum showed signs of budding new growth. Vigorous new growth (over 1 in.) was evident on Feb. 26, 2003, without addition of fertilizer. No deleterious effects were observed at any time.

Treated wood vireya bed.
Treated wood vireya bed.
Photo by Richard F. Kruppa


HI-BOR™ treated wood shavings appear to be safe as part of a mulch mix and for a top layer mulch for vireyas. The lumber appears to be safe for preparing raised beds. The author built six beds with HI-BOR™ utility grade lumber and planted forty-four vireyas in them between Jan. 5, 2003, and Feb. 10, 2003. They were planted with a mulch mixture of fern fronds, ohia leaves, koa shavings, green HI-BOR™ wood shavings, and cinder. Several are covered with HIBOR™ shavings for a top mulch. All plants are thriving as of this revision (Nov. 2003).

The experiments will now continue on a permanent basis and an additional report will be made in June 2004. There are forty-four vireyas in this test. The author is confident of his conclusions but if you are skeptical, do not use HI-BOR™ wood yourself. For those of you who believe the tests were adequate to prove the safety of the new treated wood, HPM is throwing out shavings literally by the ton because most gardeners don't know about them or are shunning them based on history.

1. Private communications with several gardeners.
2. Author's italics.
3. HI-BOR Material Safety Data Sheet effective August 1, 1994. Publisher, U.S. Borax.
4. A trademark of U.S. Borax.
5. Private communications with HPM personnel.
6. Generously donated by Pacific Island Nursery.

Mr. Kruppa, a member of the Hawaii Chapter, is a retired chemical engineer.