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NZFFA Guide Sheet No. 3: Eucalypts for Timber

Introduction

The eucalypts (gum trees) offer some excellent hardwood timbers that have properties distinctly different from, and complimentary to, radiata pine. In particular eucalypt timbers are:

  • stronger
  • harder 
  • heavier 
  • more durable 
  • attractive in appearance

However, there are over 600 species of Eucalyptus. Many are poor timbers, and even amongst the good timbers there can be problems in the sawing, drying and processing.

An understanding of the timbers behaviour is essential for successful sawmilling. Leaning trees are prone to splitting and twisting due to growth stresses.

The different species and provenances (regional seed sources) vary markedly in:

  • site requirements and site tolerances
  • growth rates 
  • form 
  • timber properties 
  • ease of sawing and processing 
  • susceptibility to insect pests and diseases 
  • coppicing ability

By comparison, radiata pine seems relatively even and tolerant of a wide range of site and climatic conditions.

In considering timber properties, one group of eucalypts is clearly superior. The Stringybark Eucalypts offer several advantages:

  • They are the easiest eucalypts to saw and process with only limited growth stresses and few problems in drying. 
  • They have hard, strong, durable and attractive timbers that are inter-changeable and useful for a wide range of uses from fence posts to structural timbers, flooring and furniture. For “on-farm” uses they are about 70% stronger and more durable than C. macrocarpa
  • They grow well on low fertility, dry sites. 
  • They have few insect pests.

Some may be able to be milled on shorter rotations (i.e. milled at smaller diameters than many other eucalypts). On the other hand, they do have some disadvantages:

  • Most of them require fairly warm sites (northern slopes) and many are frost tender. 
  • They prefer free draining soils and are prone to disease on wet sites. 
  • Form can be variable with a tendency to throw double leaders.

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Eucalypt species

E. muelleriana. This produces probably the best timber of the group. It is a vigorous tree growing to a large size, but the form can vary with some trees forked. It has performed well on a wide range of mainly warmer sites throughout the North Island including heavy Northland clays and coastal sand dunes and turns up quite often in some of the early eucalypt plantings. However it is considered quite frost tender, but grows in cool areas where there is sufficient air drainage. There are some markedly better provenances round, though without hard research data to support them. Check with local farm foresters.

E. pilularis. Though not strictly a stringybark, it has almost identical timber to E. muelleriana. Another vigorous tree of variable form that grows to large sizes. Similar site requirements to E. muelleriana, frost tender and prefers coastal sites.

E. globoidea/eugenioides. Two closely related, easily confused species, generally smaller and less vigorous trees than E. muelleriana with somewhat different site preferences. To describe them as tolerating harder sites may be an over-simplification. Frost tolerance and form varies with provenance, but at best they are impressive trees.

E. laevopinea has not been widely grown in New Zealand to date but appears to combine vigour, fairly good form, large size, good wood properties and much better frost tolerance than the above species.

There are a number of other promising stringybarks such as E. agglomerata, E. baxteri and the cold tolerant E. youmanii and E. caliginosa. These, along with a few more species have been included in research trials commenced in 2003.

The Ash Eucalypts are species from cooler, higher rainfall and often higher altitude areas of south eastern Australia. They were so named on account of a perceived similarity between their timber and European ash. They are common and grow well over much of New Zealand favouring moderate to well drained sites with adequate rainfall and often cooler temperatures. Their timber properties are not as good as the stringybarks, in particular there can be problems with collapse during drying, and internal checking, but they will grow on cooler sites. Generally they have few insect problems but can suffer root rots and fungal leaf infections. The common species are:

E. fastigata has the best timber of the ash eucalypts, being the easiest to process. It is a fast growing, large tree that seems to be able to carry larger volumes and basal areas per hectare than any other eucalypt. Form can be variable when young, but this is related to seed source. There are few insect problems, but leafspot fungi can appear under warm, humid conditions. Frost tolerance varies with provenance from about -10°C to about -14°C in some cases.

E. obliqua is probably the second choice amongst the ashes. It is a variable species with a wide geographical spread. The best provenances are fast growing, very large trees of good form. It is amongst the most site tolerant of the ash group. The timber can be subject to gum veins.

E. fraxinoides is vigorous but of poorer form, better known as a shelter species but it mills well. It needs good drainage and will not tolerate wet feet.

E. regnans/E delegatensis are other common ash eucalypts with merit. They are fast growing but need cooler, inland sites with good drainage. E. regnans looks magnificent and E. delegatensis is very frost hardy, but both have considerable defects in solid wood during drying. Veneer is certainly a favoured option. Both species have health problems (leaf spot) on humid sites.

E. nitens is the most site tolerant of the timber eucalypts handling wet and moderately dry sites, severe frosts and wind while maintaining very fast growth rates and excellent form. These attributes combined with its good pulping properties have made it the most widely grown eucalypt in New Zealand. However, it does not like warm, humid conditions and does have a lot of insect problems. For solid wood uses are limited because of collapse and high shrinkages, but young plantation material is being used for framing in Australia. For decorative end-uses it requires quartersawing, and it does produce good veneer that can dramatically improve the performance of radiata pine based laminated veneer lumber.

Although many eucalypts related to E. nitens have been grown over the years, including a host of ornamentals, none have really gained favour as timber species. However, recently there has been recognition of the merits of E. maidenii. This species, planted quite widely up to about the 1950s, produces a very high density, high strength timber that deserves more attention.

The Eastern Blue Gums produce very attractive, red timbers well suited for flooring, furniture, decorative and structural uses as well as having good ground durability.

They can suffer serious growth stresses making them difficult to saw, but they dry and process well. Unfortunately, they suffer from a wide range of insect pests with three or four serious pests having arrived over the last 20 years. They cannot really be recommended until these are under control or resistant strains have been identified. The two main species are:

E. botryoides varies with provenance from a scrubby coastal species to a tall forest tree of moderate form. It tolerates saline winds but is prone to crown breakage. On good sites with moderate or better fertility it can be very vigorous and it will stand quite wet sites.

E. saligna is a taller, better form tree but needs moderate to good fertility and does not like saline winds. The provenances vary considerably. Other species in this group include the swamp species E. robusta and E. grandis, which is a large tree of good form widely grown in South America, but has been varible in growth and form in NZ.

Other Species

There are numerous other timber species that have been grown successfully in New Zealand and some notable options are:

E. microcorys needs warm, frost free sites, but is rated the best quality hardwood of eastern Australia. It produces an extremely durable, hard, strong, attractive, light brown coloured timber that mills very well. It is slower growing but will handle heavy Northland clays through to coastal sand dunes.

Corymbia maculata (was E. maculata) is a good milling species for warmer sites on a range of soil types.

E. bosistoana produces one of the heaviest, hardest, most durable timbers imaginable. It will handle seasonally wet sites and significant frosting. E. melliodora is the dry site equivalent. Both are excellent nectar trees.

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Eucalypt Establishment

In different areas and environments, eucalypts have been successfully established almost every month of the year under a variety of regimes. However, we would recommend the following for stringybarks:

  1. Site selection, plant on best site available, preferably with shelter and good air drainage. 
  2. Cultivation, this is critical to successful establishment, minimum treatment is spade cultivation, but it is better if the soil can be broken up. 
  3. Releasing of eucalypts from weed competition is vital, but needs great care. They are sensitive to many of the commonly used chemicals, especially when used at higher rates. Gardoprim, Gallant and Versatill have all been used successfully, but species, temperature and soil type all affect sensitivity. Avoid getting chemical on the trees, some cover with shields. At higher temperatures (above 18°C ) and on light or sandy soils, soil active herbicides are likely to affect eucalypts. Some growers use glyphosate (Roundup), but this needs great care, calm conditions, low nozzle pressure and preferably a fan nozzle. Releasing young trees from weeds by hand can be successful, but requires good timing and repeating before weeds recolonise and smother trees. root competition means slower growth rates than with chemical released trees. 
  4. Use side-slot container grown stock 20-30 high. 
  5. Cut off the bottom 20-25% of the root bundle if the container had a solid bottom. 
  6. Plant with slow release fertiliser tabs or put a trowel of high nitrogen fertiliser such as DAP or urea in a spade slit 10-20 cm from the seedling, about one month after planting. 
  7. Spring planting is probably safest starting in August in the north through September on warmer, drier sites to October, November and even December oncooler, moister sites. Other eucalypts can be established as above but many growers prefer to plant the more frost hardy species as bare rooted stock in July or August.
  8. Interplanting with pines is cheaper than solid eucalypt planting. Alternating rows of eucalypts and pines 4-5 metres apart with a total initial stocking of 1000-1200 per hectare may work. Faster growing eucalypts (i.e. most species on appropriate sites) should be planted a year after the pines. Just remember which species you are trying to grow and be committed to eucalypt management. Managing a mixture gets more difficult as the trees get older. However, the more eucalypts that are planted, the more good eucalypts there are to select crop trees from. High stocking, between 1600 and 2000 trees per hectare is much more expensive but allows the best selection of only the best crop trees. However, timely thinning is essential to bring the stocking down to an appropriate final crop. 
  9. Avoid grazing with stock until trees are tall enough to damage to the leader, eg sheep, 2-3m and cattle 3-4m.

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Eucalypt Silviculture

Many eucalypts will shed their smaller branches naturally, but pruning from about age 2 to 3 years is recommended. Pruning should be done in the driest part of the year, mid to late summer,  and preferably before the branches exceed 2-3 cm in diameter. Pruning gauges can be useful. Double leaders need to be removed to a single leader, the earlier the better. Annual visits are recommended.

Thinning is critical and should be done progressively as crowns touch until the trees are 12-15 metres high and pruned to 6-7 metres. At this stage thinning can be aimed at getting a final crop of 100-350 trees per hectare, depending on the target log diameter and rotation length, with trees reasonably evenly spaced. Production thinning can produce low quality sawlogs.  At this stage check your trees. Often there will be only 3-5 branches in the next 2 or 3 metres of trunk and pruning to 8-9 metres is an easy option. If there is a double leader within a couple of metres of the pruned trunk, it may help to top one leader, if this can be done safely.

If you are using contractors for silviculture, you may choose to follow a structured regime:

Year 3 - form prune 300-400 potential crop trees per ha.
Year 5 - prune the best 200 trees per ha. to three metres and thin to about 300-400 trees per ha.
Year 8 - prune 150 crop trees to six metres.
Year 12 - prune to nine metres when mean crop height reaches 25 metres and thin to 150 trees per ha.

On a good site eucalypts should get to 70cm diameter within 30-35 years. It might be noted that in South America, eucalypts are grown for solid wood on rotations as short as 16 years. Shorter rotations may be possible, and stringybarks may suit a 25 year rotation.

Every locality is different and there is no substitute for local experience. We recommend that you join and actively participate in your local branch of the New Zealand Farm Forestry Association  P O Box 1122, Wellington to find out what has succeeded and what has failed in your area. Join the NZ Farm Forestry Association Eucalypt Action Group.

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