Robinia for trellised crops – hops, kiwis, orchards

Ferdinand Terlinden

1 day ago

7 min read

Robinia series · Choosing by use

Heights, diameters and anchoring by crop — and the only calculation that matters: the likelihood of having to reopen a row.

A trellised crop — grown on a framework of wires strung between posts — ties up that framework for as long as the planting lasts: 5 to 12 years for soft fruit, 20 to 30 years for orchards or kiwis, and several decades for a hop garden. Robinia (Robinia pseudoacacia) — also known as black locust, and often sold as acacia — reaches durability class 1–2 (EN 350) and use class 4 (EN 335), in ground contact and without treatment: around 30 years in the ground, against 15 years for sweet chestnut and 10 years for oak.

Our sawmill in Teiu, Hunedoara County (Romania), produces posts up to 13 m long, and up to 40 cm in diameter in standard lengths: enough to equip a hop garden with wirework at 7 m, a kiwi pergola, an orchard under hail netting or a fruit wall. The two maximums do not come together on the same piece — beyond 9 m, the available diameters come down. Our timber is FSC® certified (C153588); supply as FSC® 100 % is available on request when you order.

~30 years in the ground, untreated

Indicative service life of a robinia post in ground contact, against around 15 years for sweet chestnut and 10 years for oak.

Hop garden framework on robinia posts at dawn, grid of tensioned wires before the season — Out of season, a hop garden is reduced to its framework: the bines grow back every spring, while the posts stay in the ground for the whole life of the planting.

What does a perennial crop demand of its framework?

Three demands add up over the whole life of the planting: duration, since a post that lasts 10 years under a crop planned for 25 means rework halfway through; load, which is permanent and increasing; and wind, which produces the peak forces. And the cost of a broken post is not the cost of the post: it is the labour of reopening a row in mid-season, plus one or two reduced harvests on the plants affected.

Hop gardens: why height is the real issue

A hop garden — a plot planted with hops — is grown tall: the crop guides of the French chambers of agriculture (Chambres d'agriculture) recommend at least 7 m above ground, with heights recorded in France ranging from 3.60 m to 7 m. The framework combines outer poles around 27 cm in diameter, leaning outwards (at around 75°), and vertical intermediate poles of around 20 cm, set 70 cm to 1 m deep and held by anchors.

Each string carries 30 to 40 kg of bine at the end of the season, with 2,800 to 3,200 plants per hectare. Height governs everything else: an 8 m pole set 1 m deep works on a 7 m lever arm. And a hop garden stays in production for several decades — the planting outlives the framework.

Hop garden trained on tall acacia posts, training strings tensioned before the bines climb — A hop garden is grown at least 7 m above ground: at the end of the season, each string carries 30 to 40 kg of bine, and height governs the rest of the design.

Kiwi: the crop that loads the framework most

Actinidia (kiwi) is grown on a pergola — a horizontal roof of wires about 1.80 m above the ground — or on a T-bar, a post and its crossarm. The pergola gives the highest yields, 20 to 45 t/ha for green kiwi: those tonnes hang from the framework, together with the woody frame of the vine and the foliage, and the load peaks in autumn. The critical points are therefore the end posts and their anchorages — screw anchor or deadman.

Trellised orchards and hail netting: the wind decides

A bare trellised orchard — apples or pears grown as a fruit wall — needs only posts 2 to 2.50 m above ground, spaced no more than 6 m apart. Everything changes with hail netting, which turns the row into a sail and transfers the wind load of the whole plot to the posts and anchors: it is the wind, not the fruit, that sizes the framework.

These frameworks are built with posts of around 5 m, spaced no more than 8 m apart for apples and 10 m for plums, at €16,000 to €20,000 per hectare, netting included. End posts and edge posts lean outwards and are anchored. Folding the netting away outside the hail season removes the winter wind load.

Robinia posts and tree stakes in a young trellised orchard, rows equipped for trellising — In a trellised orchard, the end post leans outwards and works against its anchor: that is where the row stands or falls, never in the middle.

Soft fruit: light frameworks, replaced too often

Raspberries, blackcurrants, blueberries and blackberries are trellised low: wires at 1.60–2 m, rows 2 to 3.50 m apart, trellising at €2,200 to €2,400 per hectare. This is where replacement is most frequent, because the temptation of the small, cheap stake is strongest: a 6 to 8 cm round full of sapwood — the pale, perishable ring of the wood — lasts 3 to 10 years in the ground, whatever the species.

Which diameters, which lengths, what depth?

The basic rule: bury about one third of the length, more for end posts. The orders of magnitude below are based on the usual heights above ground.

Crop	Line post (Ø cm)	End or edge post (Ø cm) · depth
Hop garden, wirework at 7 m	Ø 18–22 · 8 to 9 m	Ø 25–30 · 8 to 9 m · 0.80–1.00 m
Kiwi, pergola at 1.80 m	Ø 10–14 · 3.00 m	Ø 16–20 · 3.50 m · 0.80–1.00 m
Trellised orchard, no netting	Ø 8–12 · 3.00 m	Ø 14–16 · 3.50 m · 0.80 m
Orchard under hail netting	Ø 14–18 · 5.00 m	Ø 20–25 · 5.00 m · 1.00–1.20 m
Soft fruit, wires at 1.80 m	Ø 8–10 · 2.50 m	Ø 12–14 · 3.00 m · 0.60–0.80 m

An end post leans 20 to 30° outwards and works against its anchor, never on its own. A post set in concrete gets a sloping concrete collar, never a cup that holds water. And robinia goes in without any base treatment: its durability is in the heartwood.

Robinia, steel or concrete: what each does best

Prestressed concrete is consistent, available in large runs and often the cheapest per unit: it is the standard equipment for Italian kiwi pergolas. Its weakness is impact — it breaks cleanly and damages machinery. Galvanised steel wins on handling, a 5 m post weighing about 25 kg against 80 to 100 kg in wood; on the other hand, it rusts at ground level and bends after an impact. Robinia flexes and can be repaired, and its density holds staples and coach screws for the long term — provided they are driven with a pneumatic nailer or pre-drilled.

Where robinia reaches its limit: long straight lengths

Robinia is a crooked-growing tree: it rarely grows straight over a great height. Logs capable of yielding 8 or 9 m straight and sound are rare, and the price reflects it — that is the real constraint in a hop garden, explained in “The price of robinia explained”. There are three ways to work around it: accept a slight bow on the intermediate poles; reserve robinia for the end posts; or mix in steel intermediates.

Key points

· The right criterion is how long the post lasts in the ground, matched to the life of the planting: around 30 years for robinia.

· A hop garden is grown at least 7 m above ground, on leaning 27 cm outer poles and 20 cm intermediates.

· A kiwi pergola carries 20 to 45 t/ha of fruit: end posts and anchorages decide whether it stands.

· Under hail netting, the wind decides: 5 m posts, no more than 8 m apart for apples.

· Long straight lengths of robinia are rare and expensive: that is its real limit in a hop garden.

Frequently asked questions

What post height for a hop garden, and can robinia manage 7 metres?

At least 7 m above ground: that is the height recommended by the crop guides of the French chambers of agriculture, with heights recorded in France ranging from 3.60 m to 7 m. With 70 cm to 1 m in the ground, you need posts of 8 to 9 m. Our sawmill produces robinia up to 13 m long and 40 cm in diameter: 7 m above ground is achievable. The caveat is commercial, not mechanical — straight 8 m logs are increasingly scarce and the price reflects it.

What end-post diameter for a kiwi planting?

Allow 16 to 20 cm in diameter for end posts, against 10 to 14 cm within the row. A pergola carries 20 to 45 t/ha of fruit for green kiwi, plus the woody frame of the vine and the foliage, and the end post alone takes the tension of the wires. It leans 20 to 30° outwards, against a screw anchor or a deadman.

Is robinia suitable for hail-netting frameworks?

Yes, provided the design is based on wind. Deployed netting behaves like a sail and transfers the force to the posts and anchors: standard frameworks use posts of around 5 m, spaced no more than 8 m apart for apples. In robinia, allow 14 to 18 cm in diameter within the row, 20 to 25 cm at the edges, 1 m to 1.20 m of burial depth and anchored end posts.

How deep should a trellised-orchard post be set?

The rule of burying one third of the length is covered in “Robinia fencing and cleft palings – choosing by use”. What is specific to orchards is the height above ground: a 2.50 m post, and even more a 4 m post under hail netting, works as a cantilever — overturning force is what governs. Allow 0.80 m for bare trellising and 1 m to 1.20 m under netting.

Can trellis wire be stapled directly into robinia?

Not by hand. Robinia is too dense for a staple to go in with a hammer or a manual stapler: the point bends or glances off. You need a sufficiently powerful pneumatic staple gun, professional trellising equipment — failing that, pre-drill and fit screw-in wire clips. In return, once in place, a staple holds far better than in softwood, where it works loose after a few seasons under tension. Two precautions. Robinia is acidic (pH around 4.5) and corrodes ordinary steel: staples, wire clips and coach screws must be stainless steel. And because the wood is dense, pre-drilling is essential — see “Screws and fixings for robinia”.