Homegrown Ground Durable Posts.

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11/28/2023

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Published by fodderfarm on 02/20/2024

A brief history

It was not until the early 20^th century that chemical treatment of posts became more common place. Prior to that certain species of trees were sort after due to their natural durability in the ground. In many countries, forests were managed as coppiced wood lots to provide ongoing resources for building and fencing etc. But in these places livestock containment often took other forms such as hedgerows, laid hedges, stone walls or ditch walls. Clear-felling of land during settlement times allowed for more use of post and rail fences and later strained wire fences. The act of straining fences obviously puts more tension on the posts at each end, so they not only had to withstand rot but also be strong enough to bear the straining weight.

The production of untreated ground durable posts has gained interest in recent times due to the requirement that organic and biodynamic properties use nonchemical treated posts and many vineyards are choosing to move this way as well.

‘The standard treated timber posts on most New Zealand farms have been pressure-injected with copper chromium arsenate (CCA). This wood-preserving cocktail is highly toxic. Copper and chromium are preservatives, but they are also heavy metal contaminants which can leach into soil. Arsenic is the ingredient of greatest concern; it can leach into groundwater supplies, and chronic exposure is associated with various cancers. Arsenic has been detected above established safe levels in soil tests in Marlborough vineyards that use CCA posts. It is also toxic to insects, frogs and other wildlife.’ https://www.biogro.co.nz/producer-profiles/woodshield

Natural durability

The lifespans of untreated ground durable wood posts ranges from 10 to 50 years or more depending on the species and ground conditions. In a wet environment such as marshy land or stream edges the wood will decay faster than posts in areas with good drainage, dry sandy soils provide the best longevity. But temperature also plays a part with posts lasting longer in cold or hot arid climates due to the lower fungal activity. Most of New Zealand provides conditions where the soil remains damp with temperatures which support fungal life, this will lessen the life of untreated posts.

Studies have been carried out in New Zealand on the natural durability of many trees by Scion in Rotorua. They used the Australian durability standard which puts the durability into four classes.

Class 1 means that the wood will survive for 25 years or more in the ground.
Class 2 means the wood will survive 15 to 25 years in the ground.
Class 3 means the wood will survive 5 to 15 years in the ground.
Class 4 means the wood will survive less than 5 years in the ground.

The tests carried out by Scion used 50mm and 20mm stakes, but their durability can be translated to larger posts as seen in the chart below. Their observations are based on the ground and climate conditions of their Rotorua site and may vary in other areas, so these classifications are only a guide.

Durability classification

Other factors that influence the durability are.

The classifications only apply to heartwood, with all sapwood having poor resistance to decay and insect attack.
The inner core of heartwood around the pith generally has lower durability than the rest of the heartwood.
Durability is also influenced by the size or diameter of the post, the larger the piece size, the longer it will last.
The age of the tree also generally influences natural durability with mature stands producing more durable timber than semi-mature trees.[2]

When using large branches or small trunks for fences it is best to use those with a minimum Small End Diameter (SED) of about 8 cm, higher SED are preferable. Sapwood degradation can cause the post to loosen in the ground in time and the staples can become lose. Posts with less than 2cm of sap wood are best.

Seasoned Verses Green Wood

Seasoning is the process of drying wood to remove excess water. Untreated posts need not be seasoned or dried before using. In time they will lose and gain water until they are in balance with the moisture content of the soil and air. However, there are advantages to seasoning posts before using. Seasoned posts are lighter to handle, and generally hold staples better. Many post suppliers say that seasoned posts do not check or crack as readily as unseasoned posts, but, as seen in the document below, those who use green posts claim the ‘splits and checks are largely eliminated at the ground line’ where they matter.

PRESERVATIVE TREATMENT OF FENCING POSTS.

De-barking

While most people say leaving the bark on will provide a habitat for insects, which can increase the risk of damage, and keep the moisture in, speeding up rot, there are many who disagree. Firstly, it would depend on the species of tree and its type of bark. The black locust, for example, if not debarked within the first few days of being cut will ’set’ its bark and become almost impossible to peel. Traditionally in many places in the USA, black locust and the Osage Orange were used green and still barked. In the UK sweet chestnut is usually debarked but some will leave the bark on if splitting into ¼ rounds.

Often posts were debarked when green but if storing the posts in a dry place some would leave the bark on to slow the drying process and reduce checking in the timber. The bark would often dry and fall off by itself. As stated above the trials done by scion only used the heartwood so the effects of leaving the bark or sap wood on have not been studied.

The eucalyptus post in the foreground is green and still barked.

Durable Species.

In-ground natural durability classification for the heartwood of NZ-grown species (50 min square stakes)

Very Durable – Class 1

Black locust – Robinia pseudoacacia

One of the most durable timbers you can grow for posts, black locust has also unfortunately become considered as a pest species in some areas due to its readiness to spread from seed or suckering. However, plantings can be managed to reduce this from happening. While the leaves and stems are considered toxic, black locust is used in fodder systems throughout the world and is said to have a nutritional value similar to lucerne. The browsing animals usually self-limit how much they consume, horses are considered the exception here, so it is best to restrict their access. The seeds are edible and often used as poultry feed which will further reduce spread if the trees are planted within a poultry area.

Seedling trees are often more spreading in form than the timber cultivars that have been bred in Hungary. Planting the trees close together (approx. 1m spacings) to force upward growth and trimming all lower branches will help ensure straighter growth. The trees can later be thinned, coppiced or pollarded as needed. Root cuttings can be selected from straighter trees. Depending on growth conditions trees can be harvested from 10 years but durability increases with age and 25 to 30 years is thought to be optimal.

The nitrogen fixing and mineral accumulating properties of the black locust means they can be planted in food forest, hedgerow or silviculture systems.

Sugar Gum – Eucalyptus cladocalyx

The most ground durable of the eucalyptus the sugar gum is best grown in very dry areas with a low risk of frosts. It is more wind tolerant than many eucalypts but does better on sheltered sites. Some varieties grow straighter than others but pruning from a young age will produce better timber. Harvest time for posts is generally 15 to 20 years.

It’s uses include heavy marine and external building applications, poles and posts, railway sleepers, crossarms, outdoor and indoor joinery. Other benefits are as an excellent source of nectar.

Silver pine – Manoao colensoi

Grows in rich soil in a wet climate, usually in older, poorly drained areas with leached infertile soils, and in peaty or acid swamps. The silver pine or Manoao is native to New Zealand and grows mainly on the Central Plateau and the South Island’s western side.

It is easily grown from seed or can be propagated via hardwood cuttings. It is a slow growing, attractive small tree reaching approx. 15 m.

Mostly used for boatbuilding and as marine piles, mine props, sleepers and fence posts. While the posts can last over 40 years without rotting, they have been said to snap after some time.

Totara – Podocarpus totara

Often found as original fence posts across the North Island, totara grows best in free-draining soils with good shelter, but can cope with dry, stony, and exposed sites.

The trees can be prone to multi leaders so selecting a single leader while young and removing the rest will produce a straighter tree. Can be propagated from seed or cuttings, though seedlings may take a long time to establish. The heartwood is also slow to develop, and trees are better harvested after 50 years or longer.

Its straight grain means it readily splits into slabs or posts and is extremely durable with some posts still standing 100 years later. The heart timber is also used for house piles, wharf piles and sheathing of boats. It is noted that older fence posts can snap under sudden strain.

Black Locust

Durable – Class 2

Eucalyptus spp.

E. Amygdalina – Black Peppermint.

Widespread in the Tasmania apart from the south-west, growing in dry forest. Prefers a well-drained soil and is relatively drought tolerant and frost resistant.

E. botryoides – Southern mahogany.

Fast growing, best in moist sites and can tolerate fairly poor drainage and low fertility. Warm areas, tolerant of wind including salt winds. Suitable for steep slopes, and is a coppicing species.

E. cornuta – Yate.

Naturally occurring in moist fertile soils, yate is a hardy tree tolerating drought, moderate frost, a range of soils and coastal areas.

E. globoidea – white stringybark.

A smaller tree and slower growing suitable for harder sites. Prefers warmer areas, tolerates infertile soils and mild frosts. Easier to mill than most Eucalyptus.

E. muelleriana – yellow stringybark.

A medium to large tree from southeast NSW and eastern Victoria, considered the best milling eucalypt of all. A vigorous tree that is prone to double leaders and is frost tender.

E. pilularis – blackbutt.

From the coastal forests of NSW where it grows very large. Does best in a warm site with reasonably fertile soil, average rainfall and good drainage. Seedlings do not tolerate frosts, but trees will stand some exposure to salt winds. The form is variable and rather poor in some areas.

E. radiata – Narrow-leafed Peppermint Gum

Prefers well-drained soils but can tolerate a range of soil types, including clay and sandy soils. Is reasonably drought tolerant once established and is one of the more cold hardy eucalyptus.

E. saligna – Sydney Blue Gum.

A tall tree from the eastern strip of NSW and southern Queensland. Usually has a good form, requires reasonably fertile soils that are deep. Best in sheltered areas as can be prone to crown breakage in high winds and does not tolerate salt winds

Fuscospora spp.

Synonyms – Fagus, Nothofagus.

F. truncata – Hard Beech

A tall tree with a spreading crown that develops a buttressed trunk with age. Hardy and grows in a wide range of conditions. Prefers well drained, fertile loamy soil but tolerates dry soils, coastal sites and moderate frosts.

F. cliffortioides – Mountain Beech

A graceful medium sized tree, that tolerates hard, infertile, poorly drained sites in alpine areas.

F. fusca – Red Beech

Prefers deep, well-drained, fertile soils often found on lower to mid slopes and river terraces. It is usually intolerant of unfavourable conditions and is better suited to cooler climates.

Quercus robur – European oak

A broadly spreading, slow growing and long-lived tree. Very hardy, tolerating a range of sites including heavy clay soils and exposure. Appletons have trialled interplanting oaks with Alnus cordata to encourage more upright growth in the oaks.[ii]

Castanea sativa – Sweet chestnut

A hardy deciduous long-lived tree that does best in sheltered spots, in fertile free draining soils. Coppices well and is a traditional greenwood species.

Moderately Durable – Class 3

These have been included due to their rapid growth and/or multipurpose uses.

Gleditsia triacanthos – Honey Locust

A hardy deciduous tree that thrives in the heat and dry but tolerates a variety of soil and climate conditions. Fixes nitrogen and the edible seed pods are valued for stock fodder.

Acacia melanoxylon – Blackwood

A fast-growing, spreading tree that prefers sheltered sites with good rainfall and tolerates wet soil. It does have a suckering habit but is great for erosion protection. Produces flowers in late winter for bees. Hardy, but handles only light frosts when young.

Cupressus macrocarpa – Macrocarpa

A fast-growing dense shelter tree that does best in fertile, sheltered, well-drained sites. It is salt tolerant and cold hardy.

Uncategorised but Traditionally Used in Other Countries.

Maclura pomifera – Osage orange

One of the most popular ground durable trees in USA. Very hardy, does best in moist well-drained soils, but tolerates both dry and wet conditions, poor soils, drought, heat, cold and wind.

Morus spp. – Mulberry

Said to be quite rot-resistant the mulberry is a hardy deciduous tree, depending on species. It does best in fertile free draining soil but will tolerate most conditions including drought and salt but not wet soils. Mulberry produces high quality leaf fodder and delicious berries.

Here is another excerpt from the 1925 PRESERVATIVE TREATMENT OF FENCING-POSTS. By A. R. ENTRICAN.