Pole Treatment Protocol

Untreated eucalyptus in Portugal fails in 5–10 years — attacked by subterranean termites, wood-boring beetles, and fungal rot long before it wears out structurally. There are two reasonable treatment protocols documented here, and you should choose based on what you’re building.

Eucalyptus Species Selection

For structural poles, select E. globulus (blue gum) or E. grandis (flooded gum). These species have characteristic bending strength of 25+ MPa. Avoid E. camaldulensis for primary rafters — its strength is more variable. Source poles 15–18cm diameter at the butt end for main rafters.

Which Protocol Should I Use?

The Regenerative Default (char + linseed oil — no industrial chemicals) gets you ~15–20 years of above-ground service life in moderate Portuguese conditions. That’s long enough to outlast 4–5 plastic re-skin cycles on a greenhouse, with no borax handling and no soaking trough.

The Extended-Life Protocol (char + borax soak + linseed oil) gets you 30–40 years. Worth doing when pole replacement involves disassembling significant structure.

You’re building…	Protocol	Reason
Gothic Arch Greenhouse	Regenerative Default	Plastic re-skin every 3–4 years already invites touch-up; pole replacement at year ~15 is tractable
Greenhouse BioDome	Regenerative Default	Same logic — frame is exposed, plastic-only skin, easy access to poles
Standard BioDome	Extended-Life Protocol	Permanent dwelling — poles are encased in cob, wattle, and living roof; replacement = major surgery
Earth-Sheltered SolarPod	Extended-Life Protocol	Permanent dwelling, partially buried — no practical way to swap poles later
High-pest sites (lower elevation, coastal Portugal)	Extended-Life Protocol	Subterranean termites and house longhorn beetles are more active at lower elevation

If you’re unsure, the Regenerative Default is the safer first build — you can always do the Extended-Life Protocol on your next one once you’ve experienced the climate at your site.

Building During Fire Season?

Both protocols above start by charring the buried 50cm — and open flame is banned during the Portuguese período crítico (no open flames from YELLOW alert upward). If you’re prepping poles in the dry months, you can’t char. See Protocol C — The No-Burn Foot for torch-free ways to protect the buried section, from a free sacrificial re-foot to granite-pillar standoffs that take the pole out of the ground entirely.

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Protocol A — The Regenerative Default

Five steps, no industrial chemicals. Targets ~15–20 years above-ground service life.

1. Debark Within 48 Hours

   Remove bark immediately after felling while wood is fresh. Use a drawknife or scraper. Bark traps moisture and prevents preservative penetration.

2. Char Bottom 50cm (Deep Char Method)

   Torch the lower 50cm of each pole until the char layer reaches 8–10mm depth. You should see alligator-scale cracking in the char layer. After charring, brush off loose char with a stiff wire brush. The carbonised layer is your primary defence against subsurface organisms — it carries no nutrient value for fungi or insects, and creates a hydrophobic barrier in the most rot-vulnerable zone.

   

   Target 8–10mm char depth with visible alligator-scale cracking

3. Bend into Arch Frame While Green

   Freshly debarked, freshly charred poles are still at high moisture content from the living tree — peak bending flexibility. Assemble the arch frame immediately. Clamp, lash, or bolt poles into their curved positions while they bend easily.

   Joint Shrinkage

   Green eucalyptus shrinks 4–8% radially as it dries. Connections made on wet poles will loosen. Plan to re-tighten all lashings once the frame has set.

4. Dry in Place (2–3 Weeks)

   With the arch frame assembled, allow the structure to dry in position. Wood fibres lock into the curved shape as moisture content drops. Protect from rain with tarps if no roof is in place yet. Check joints and re-tighten lashings as the wood shrinks. Use a moisture meter — target below 15% before sealing.

5. Seal with Hot Linseed Oil

   Heat linseed oil to 60–70°C. Brush on all pole surfaces above the charred zone. The oil hardens over 2–3 days and waterproofs the wood while remaining breathable. Linseed oil also deters insects mildly and resists fungal colonisation. Reapply every 5–10 years — this is the maintenance trade-off you accept for skipping the borax soak.

What You’re Trading Away

The Regenerative Default skips the borax bath, which means the above-ground portion of each pole has no internal preservative. Linseed oil reapplications are the substitute. If you let the linseed sealing lapse for several years in a row, expect insect damage to start appearing earlier than 15 years. Walk the structure annually and recoat any weathered sections.

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Protocol B — The Extended-Life Protocol

Six steps, adds a borax soak. Targets 30–40 years above-ground service life. The sequencing matters — char goes before borax, so the heat-opened cellular structure of the charred zone receives the borax solution at depth.

1. Debark Within 48 Hours

   As above.

2. Char Bottom 50cm (Deep Char Method) — Before the Borax Soak

   As above. Charring before the borax soak (rather than after) opens up the cellular structure of the most rot-vulnerable section, so when the poles go into the bath the borax solution penetrates significantly deeper into that zone.

3. Borax Soak (72+ Hours)

   Submerge poles completely in borax solution. They float, so weight them down with stones or a weighted board. Minimum 72 hours; longer (up to 2 weeks) is better for deep penetration. Long arch poles (Gothic Arch, Earth-Sheltered SolarPod rafters) benefit from 5+ days.

4. Bend into Arch Frame While Wet

   Poles coming out of the borax soak are saturated and at peak flexibility. Assemble the arch frame immediately — clamp, lash, or bolt poles into their curved positions while they bend easily.

   Joint Shrinkage

   Wet eucalyptus shrinks 4–8% radially as it dries. Connections made on wet poles will loosen. Plan to re-tighten all lashings once the frame has set.

5. Dry in Place (2–3 Weeks)

   As in Protocol A.

6. Seal with Hot Linseed Oil

   As in Protocol A. With borax inside the wood and linseed sealing the outside, reapplication can stretch to every 8–10 years.

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Protocol C — The No-Burn Foot (When Fire Is Banned)

Both protocols above open with the same move: charring the buried 50cm. That char is the buried-section defence — a carbonised, nutrient-free, hydrophobic skin that lets a blue-gum foot survive in the ground long enough to reach the 15–20 year pole life the Regenerative Default promises. But open flame is exactly what you can’t use during the Portuguese período crítico. The fire-ban season runs through the dry months, and our own fire-safety alert levels call for no open flames from YELLOW upward. A propane torch charring pole-butts on a dry hillside in August is the precise activity the ban exists to stop — and heat-shrink rot sleeves are out for the same reason. If you’re prepping poles in fire season, you need a buried-foot defence that never strikes a match.

This is also where blue gum is weakest. E. globulus is fast-grown, mostly sapwood, with little of the durable heartwood some eucalypts carry; in ground contact it rates around durability class 3, and the field tests are blunt — buried blue gum gets attacked, heartwood and sapwood alike. Untreated, a blue-gum foot in damp winter soil is a 4–8 year proposition. The char was buying most of that life back. Take the fire away and you have to buy it back another way.

The Key Insight — Wood Rots at the Groundline, Not the Tip

Decay fungi need three things at once: moisture, oxygen, and food. A handspan below grade the soil is too airless; the wood above grade dries out between rains. The two only overlap in a narrow band — maybe 15cm tall — straddling the soil surface. That band is where the vast majority of posts rot through and snap off, while the deep end comes out of the hole years later still sound. Every option below is just a different way to defend that one 15cm ring.

Four no-burn options, cheapest first.

Tier 0 — Gravel drainage + sacrificial over-length foot (€0)

Keep the gravel bed and collar exactly as specified — a foot that drains and breathes already outlasts one packed in wet clay or, worse, sealed in concrete. Then stop fighting the rot and plan around it: cut arch poles 30–40cm over-length and bury the extra. When the groundline gives out around year 8, pull the pole, saw off the punky foot, and re-set the now-shorter pole in the same hole. The gothic arch flexes mostly in its top third, so it shrugs off losing a hand’s width at the butt. One pole, two lives, no materials, no chemicals — the purest expression of the Regenerative Default.

Tier 1 — The membrane boot (€3–6/pole) — the workhorse

Wrap the 15cm groundline band in a waterproof membrane so moisture and oxygen can’t reach the wood where they’d otherwise meet. Off-the-shelf, this is a self-adhesive post wrap — RotGuard (SBS-modified bitumen) or peel-and-stick Postsaver Pro-Wrap, both carrying 25-year rot warranties on durable timber. The improvised equivalent is a strip of EPDM pond liner — you may already have it on hand from the soaking trough — glued and taped into a sleeve. The wrap runs from ~5cm above the finished soil line to ~10cm below it, sealed snug top and bottom so it sheds rather than funnels water. The self-adhesive versions need no flame at all — peel, wrap, overlap.

Never Wrap a Green Pole

A blue-gum pole goes into the hole wet. A tight membrane on wet wood traps that moisture against the grain and rots it from the inside — the exact failure you’re trying to prevent. Wrap at the 2–3 week dry-down visit (the same visit as the hot-linseed sealing): scrape back the top of the gravel collar on each foot, wrap the now-dry groundline band, and re-backfill. It becomes one more task on the cure day. Done this way, a wrapped blue-gum foot runs 15–25 years — meeting or beating the char, with no fire.

Tier 2 — Boron under the boot (€5–9/pole)

Belt and braces: treat the foot chemically before wrapping it. Brush or soak the bottom 50cm in borate solution (the same disodium-borate mix from the Borax Soak Reference below, with a little glycol added to help it diffuse into the wood). Boron is lethal to decay fungi and insects and is about the most benign preservative there is. Its one weakness — it’s water-soluble and leaches straight out of anything in bare ground contact — is precisely why it pairs so well with the membrane: the wrap keeps the boron in. This is essentially the Extended-Life borax defence minus the five-day trough and minus the fire, and it pushes the foot to 20–30 years.

Tier 3 — Don’t bury wood at all: stand it on stone or steel (€0–30/pole) — the outside-the-box answer

The most durable wooden post is one that never touches soil. Bury something that can’t rot and lash the pole to it above grade, so the wood lives entirely in the dry, breathing air where blue gum is perfectly durable. Two ways to do it:

• Granite pillars (≈€0–20/pole — the regenerative pick). If you have the 1.5m × 10×10cm granite pillars on hand — and in the granite country of the Montanhas Mágicas, stone is the one material that’s genuinely free — set each into the posthole on the same gravel drainage bed, buried 55–60cm (a little deeper than the 40cm wood holes, because the pillar’s job is to take the arch’s inward kick, and its ≈40kg mass plus tamped backfill or a small concrete collar is what resists that thrust). That leaves ~90cm standing proud. Lash the eucalyptus pole to the proud face with galvanized wire at two or three points, butt resting a few centimetres above the soil, never in it. Two details earn their keep: slip a scrap of EPDM between pole and stone at each lashing point so the contact line can’t trap water against the wood, and seal the pole’s end-grain butt with linseed. Granite is geologically inert — no rot, no rust, no leaching, ever — so the pillar is effectively immortal and matches the gabion stone you’re already laying. The foot stops being the failure point: you’re limited by the arch itself, 30+ years.
• Galvanized steel shoe (≈€15–30/pole — the engineered alternative). The same idea in metal: a short length of pipe or a U-bracket welded to a rebar foot, bedded in a small concrete plug over the gravel, with the pole butt sitting on a steel plate a few centimetres clear of the soil and bolted or lashed in. Galvanized steel lasts decades in well-drained gravel. It costs money and welding where the rest of this build wants neither, and the stone does the same job for less — but it’s the off-the-shelf route if granite isn’t to hand.

The Bonus — Re-Poling Becomes a One-Hour Job

Both standoffs overturn the “pole replacement means lifting the whole tunnel” warning in the build guides. When an arch eventually tires, you unlash it from the pillar (or lift it out of the shoe) and stand a fresh pole in its place — no excavating buried stubs, no jacking up the end of the structure. For a building you mean to keep, this is the foot that ages best.

Choosing

Rough buried-foot life for a well-drained gravel posthole on a dry-summer Portuguese hillside — where the long rainless summers are quietly on your side (the groundline sits dry for half the year, which is why these run longer than they would in the wet tropics):

Foot strategy	No fire?	Cost/pole	Expected foot life
Untreated, gravel drainage only	✅	€0	4–8 years
Char the base (Protocols A / B)	❌ banned in season	€0	enables 15–20 yr pole life
Gravel + over-length sacrificial re-foot	✅	€0	8 yr × 2 lives
Self-adhesive membrane boot (dry pole)	✅	€3–6	15–25 years
Boron soak + membrane boot	✅	€5–9	20–30 years
Granite pillar standoff (pole never buried)	✅	€0–20	30+ yrs, trivial re-pole
Galvanized steel standoff shoe	✅	€15–30	30+ yrs, trivial re-pole

For a fire-season build, the recommendation is the membrane boot, applied at the linseed visit — it lands right where the char did on the lifetime curve, costs a few euros, and slots into a day you’re already spending on the frame. If the structure is meant to be permanent, stand the poles on granite pillars and never think about the foot again — it’s the most durable answer here, it costs nothing but the stone you already have, and it’s the most beautiful. Whatever you choose, keep the gravel drainage and the over-length butts: they cost nothing and they’re your free insurance.

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Borax Soak Reference (Protocol B Only)

Skip this section if you’re using the Regenerative Default. Everything below applies only when you’re doing the Extended-Life Protocol.

Building a Soaking Trough (Modular Sheet Metal)

For batch processing 200+ poles, build a portable above-ground trough from sheet metal sections lined with pond liner. No excavation required — the trough sits on simple supports, can be assembled in an hour, and disassembles flat for transport or storage. Size it to fit the longest poles in your design with clearance:

Dimension	Size	Rationale
Length	Longest pole + 0.5m	Must fit your longest poles with clearance (e.g. 12m for 11.6m arch poles)
Width	60-80cm	Depends on sheet size; fits 4-5 poles side-by-side per layer
Depth	30-40cm	Shallower than a trench — above-ground so no depth lost to ground level
Capacity	Length × 0.7m × 0.35m × 1000	e.g. 12m trough ≈ 2,940L; treats 15-20 poles per batch

Construction Steps

1. Source sheet metal: Corrugated roofing sheets, flat galvanized sheet, old signage, or salvaged panels — anything rigid enough to bend into a U and hold water when lined.
2. Bend into U-channels: Bend each sheet into a broad U shape (~60-80cm wide, ~30-40cm deep). Use a brake or fence post for flat sheet; corrugated sheets fold by hand along the ridges.
3. Set up frame: Place supports (sawhorses, timber trestles, or stacked blocks) along the run at ~1.5m spacing. Level them carefully — water finds every low spot.
4. Line up sections: Lay U-channels end-to-end on the frame, overlapping edges by 10-15cm at each joint.
5. Install pond liner: Drape EPDM (1mm) or heavy polyethylene (200+ micron) along the full length inside the channel run. The liner seals all joints between sections — no need for waterproof connections in the metalwork.
6. Strap it down: Run ratchet straps around the frame and channels every ~1m to hold everything tight and prevent the sides from splaying under water weight.

Borax Solution Recipe

Ingredient	Amount per 1,000L	Cost (approx)
Water	1,000 liters	—
Borax (sodium borate)	12.5kg	€19-25
Boric acid	8.3kg	€12-21

This creates approximately a 2% solution — sufficient for insect and fungal protection. Scale proportionally to your trough volume. Dissolve borax and boric acid in hot water first, then add to trough.

Modular sheet metal trough: U-channel sections on a frame, sealed by a continuous pond liner

Batch Processing Workflow

1. Load batch: Slide 15-20 debarked poles into trough using ramps
2. Submerge: Weight down with stones or a board weighted with concrete blocks
3. Cover: Lay tarp or corrugated sheets over trough to reduce evaporation
4. Soak 72+ hours: Flip poles halfway through if any surfaces exposed
5. Remove and bend: Slide poles out and assemble into arch frames immediately while saturated and flexible
6. Repeat: Solution remains effective for 6-8 batches before needing refresh

Reusing the Solution

The borax solution doesn’t “wear out” — it absorbs into the wood but concentration drops slowly. After 6-8 batches, test by drying a sample: white crystalline residue on the surface indicates adequate concentration. Top up with fresh solution as water evaporates.

After the Build

Disassemble the trough, stack the sheet metal sections flat, and roll up the liner — store for the next build or loan to a neighbor. The dilute borax solution is not hazardous — borax is used in garden soil as a trace mineral (boron). Drain it onto open ground or a garden bed.