Ever gone to the effort of harvesting and making beautiful things with bamboo just to see it crumble away with a borer insect infestation? Many great building and crafting bamboo varieties are exceptionally vulnerable to being eaten out by the Powder Post Beetle (Powder Borer) or Bamboo Weevil after harvest and drying. When I first started harvesting our initial bamboo poles in the mid-nineties at Djanbung Gardens I sought advise regarding ways to treat my bamboo for maximum longevity. I was informed that impregnating the culm walls with either a Copper Sulphate or a Borax solution would do the trick and there were two proven successful methods for doing this, one required immersing the culms in a container of the solution for several weeks and the other involved using sophisticated pressure equipment to force the solution into the wall membranes.
Neither system appealed to me as realistic or practical for home production. The immersion process would require a large container and huge amounts of solution, a logistical challenge for culms that reach 9 meters or more in length and even cut to suitable lengths for building would require a set-up at least 4 meter long. The engineering pressure solution was right out of the question.
Having a basic understanding of plant biology and how they draw up liquid from the soil through their stems as they photosynthesise and transpire, I wondered if I could use this biological function to treat freshly harvested bamboo. Like cut flowers in a vase that continue to photosynthesise and use up the water in the vase, would bamboo culms draw up a treatment solution? I researched the properties of Copper Sulphate and Borax and found that Copper Sulphate was more readily water-soluble. Both substances are natural chemicals, Copper Sulphate, also known as Bluestone, is used by organic farmers as a fungicide (not to be confused with Copper Arsenic that was commonly used for treating timber and is a very toxic substance)
The challenge was how to get the freshly harvested culms to stand in a container of the solution as soon as possible after harvest to ensure that photosynthesis would continue to function effectively. The culms of Bambusa oldhamii can be over 10m long. The first year I harvested several smaller culms from my oldhamii clumps and a quite a few culms from the much smaller species, Textillis gracillis (Weavers Bamboo), and stood them in buckets of copper sulphate solution on the veranda, tying the culms upright to the veranda post. Sure enough, the following day, most of the solution had been taken up by the bamboo, and the walls of the culms had changed colour with a bluish tinge from the copper sulphate so I knew the transpiration had worked. As a control, I also kept some untreated culms harvested on the same day as the treated ones to see if there’d be a difference. Within around six months the untreated culms were being eaten out by the powder borer beetles and the treated culms showed no signs of infestation.
The next challenge was how to treat longer culms too big for tying to the veranda post, so before the next bamboo harvest season I got help building a pole frame to lean the culms against. This worked for a few years until the Oldhamii culms started to reach full height and were too long for the pole frame. Since then we’ve built a bamboo scaffold each year for treating the new bamboo harvest.
Harvest and Treatment
When to harvest. It’s recommended to harvest bamboo in the dry season when the sugar levels are at their lowest. Clumping bamboos are generally tropical and subtropical species that shoot and put on their growth during the wet season in summer and autumn, with the dry season extending through winter and spring. Here in the southern hemisphere subtropics we start the bamboo harvest with the waning moon in August and finish in late November. It’s also recommended to cut the bamboo during the waning moon.
Culms should be at least 3 years of age before harvest, so with the Oldhamii we mark the culms each year by scratching Roman numerals on one of the bottom nodes. The first year a new culm emerges as a shoot it grows to full length within a couple of months. In its second year it puts out its branches and leaves, and in the third year it begins to harden. The culms will begin senescence or deterioration after 6 or 7 years of age, so ideally harvest will occur when the culms are 3-7 years of age.
We harvest the bamboo culms cutting as close to the ground as possible, and once 4 or 5 large culms have been cut we immediately erect them on the scaffold with the base in the solution for treatment. The time from cutting to placing in the solution should be no longer than around 20 minutes, before the capillary vessels start to seal and leaves begin to wilt – they need to be still alive and photosynthesising.
Standing large culms up can be tricky and needs a good team of people. Once they’re leaning on the scaffold they need to be tied and secured immediately so they won’t fall. Wind can play havoc, and unfortunately the dry season is often our windiest time of year.
The copper sulphate treatment solution.
Copper sulphate solution is 1:10, so for 500gr of Copper sulphate we add 5 litres water and stir until it’s fully dissolved. I advise standing the culms into an empty container then adding the solution once the culms have been securely tied to prevent tipping the container. I’ve found that a large B. oldhamii culm will take up 1000-1500ml of solution over a 24 hour period. After 24 hours the leaves have wilted, photosynthesis has ceased and no further solution is taken up.
Caution: It’s important to use only plastic or other non-metal buckets and containers, as the Copper sulphate will corrode metals. Copper sulphate must be handled carefully and with caution to avoid all contact with the skin and eyes and to avoid inhalation of the powder.
Disposing of leftover solution. We add any leftover solution to the next batch so it’s not until the end of the harvest season we’ll have surplus solution to deal with. During treatment you’ll find some of the copper sulphate will begin to crystallize overnight in the bucket and will need to be stirred or agitated to dissolve again, before adding to your next mix. At the end of the harvest season, any left over solution must be safely and appropriately dealt with or used – do not just throw it out. Copper sulphate can be safely utilised as a natural fungicide on fruit trees for problems with canker, greasy spot, melanose or Phytophthora; or on tomato plants if there’s problems with blight, nailhead rust, bacterial spot, leaf mold, Anthracnose, Stemphyllum leaf spot or Septoria; and it can be used to kill slime and algae on pavers. Please research first and use it correctly and as advised. While copper sulphate is an approved fungicide for certified organic growers, it needs to be applied with caution to avoid toxic concentrations in soil or water. If there’s no need to use leftover solution in the garden, just leave it in a container to evaporate off over a few days, then retrieve and store the remaining bluestone crystals left behind for use another day.
Post-treatment and curing
After the bamboo has been treated, take the culms down and remove the tops and branches so they’re ready for curing. Curing should be in the shade, don’t leave bamboo lying in the sun as it will dry out unevenly and be more prone to splitting. Ideally bamboo culms should be stored where they’re protected from sun and rain. We’ve built a simple rack using star pickets and bamboo with a tarpaulin roof and shade cloth sides to store our treated culms until we want to use them.
Will transpiration work with Borax?
I met a bamboo engineer from Central America earlier this year who claimed the transpiration method was useless. After further discussion he said the trials he was involved in used a borax solution, so I suggested it might worth while repeating the trials using copper sulphate. I haven’t tried out Borax with the transpiration method but have used borax/boric acid solution for immersing small pieces of bamboo in a bucket for crafting projects. Large poles have been successfully treated using Borax/Boric Acid solution by drilling or punching holes through the node walls (leave the bottom one intact), then standing the culms upright and filling the inside hollow with the solution allowing 4-7 days for it to penetrate into the wall.
There are many other methods for treating bamboo, traditional systems using heat (fire) to dry out the saps, or soaking in water for extended periods of time to leech out the sugary sap, and bamboo enthusiasts are constantly trialling new process and variations with varying degrees of success. I’m always interesting in knowing what methods others have tried that have worked or failed for different types of bamboo.
Robyn Francis has been growing, harvesting, crafting and building with bamboo for 20 years and been gleaning tips and inspiration through her travels in traditional bamboo cultures of Asia and Latin America and working with other bamboo enthusiasts in Australia. She shares her passion for this versatile and amazing plant through her permaculture courses and the annual bamboo internships and workshop at Djanbung Gardens in the Australian sub-tropics.