Eucalyptus - Blackbutt, Eucalyptus pilularis
Blackbutt (“pilularis”) is a fast-growing, sub-tropical eucalypt planted for timber only in warmer North Island localities such as Northland and the Bay of Plenty. It tolerates mild frosts once established and does best in free draining moderately fertile soils at altitudes of less than 200 metres. It will grow for 500 years reaching 70 m high and 3-4 m in diameter.
Commercial return: High
RainfallModerately high rainfall; Moderately low rainfall;
Soil depthDeep; Moderate depth;
Soil drainageFree draining; Moderately free draining;
Prepare grass sites by spot spraying a one metre circle for each planting site. Use glyphosate herbicide with spray dye a few days before planting as this will not leave chemical residue in the soil. Prepare spots using straight lines and accurate spacings between spots so that trees can be easily found later. Cut down woody weeds and flatten these on the ground so they decay and don't support weed growth.
Container grown stock is available from some commercial nurseries; Price per tree should range from $0.80 - $1.50 for 1 year-old contract-grown forestry stock (2020 prices).
Blackbutt requires a warm, sunny site with moderately fertile soil, good soil drainage and an annual rainfall of 1,000 – 1,500 mm spread over the year. The seedlings are frost tender so unless the site is completely frost free, plant in the spring after the last winter frost.
Blackbutt seedlings are not shade tolerant so should be planted in an open site. Use wetted water-absorbing gel in the planting holes to ensure the seedlings don’t dry out before they establish roots. Freshly planted seedlings can be desiccated by severe equinox winds.
Plant as soon as possible after you receive the plants. Make sure the root plugs are moist and plant trees with a slow release fertilizer tablet or put a trowel of high nitrogen fertiliser such as DAP or urea in a spade slit above the seedling.
Dig a hole twice the size of the plant container, leaving some soft soil at the bottom. Tease out (straighten and trim) any pot-bound roots before firming the soil around them, ensuring there are no air cavities.
As there can be a lot of variability in planting stock, use a high initial stocking to ensure a wide choice of final crop trees with a good selection of well-formed fast-growing stems. An initial stocking of 2000 stems per hectare (2m x 2.5m) will allow the trees to self-prune with a small defect core and provide a high selection ratio. Trees from genetically improved seedlines can be planted at 600 - 1000 stems per hectare and pruned.
- Prevent weeds from competing with the Blackbutt seedlings for at least two years after planting.
- Protect seedlings from browsing by stock and wild animals.
Control competing weeds in the first season by manual clearing or by carefully spraying with a residual herbicide. Be aware that eucalypts are very sensitive to herbicides. Spray in calm conditions a couple of months after planting, after the bulk of spring weed germination has occurred. Do not use triazine group chemicals (terbuthylazine, etc.) on light soils such as sands. Use glyphosate with a shield to protect the young trees and make sure no herbicide gets on foliage or green stems. Haloxyfop is safe to use around eucalypts for controlling grasses.
Spray-releasing can also be used during the second season to increase the rate of growth. However, this is not recommended because trees become more prone to toppling in strong winds (windthrow).
Eucalypts are damaged if grazed by stock, goats and deer so fence the area to keep them out. A single hotwire electric fence will deter most cattle, but it is best to construct a full post and wire fence.
During the first summer eucalypt seedlings are highly palatable to possums which should be controlled with poison, trapping or shooting. Young eucalypt seedlings are also palatable hares and rabbits. They will slice off trees near ground level at a 45 degree angle, killing them.
Pests and diseases
Blackbutt currently has few insect pests In New Zealand. On permanently or seasonally wet sites blackbutt is susceptible to root diseases. The species grows best on well drained soils where rainfall is well distributed over the year.
Blackbutt is grown for timber, carbon storage or firewood. Grown for timber, aim for large logs rather than attempt to restrict branch growth. Grown for carbon, note that a reasonably high stocking of slim trees can hold more tonnes/ha than a low stocking of fat ones. Likewise, grown for firewood, slim trees (up to 30 cm diameter, age 7 - 15) will be easier to fell and handle.
Suitability for steep slopes: Blackbutt does not like strong winds, or bony thin soils with low fertility. However, provided exposure and erosion is not too severe, blackbutt is suitable for erosion control in warmer locations. Blackbutt coppices, so is suitable for permanent erosion control on steep slopes. See Report: Trees for steep slopes - Eucalyptus »
A series of short videos are available on growing eucalypts for timber.
If planted at a high stocking, pruning is not required because trees will self-prune and any poorly formed trees will be thinned out.
Form pruning: Blackbutt tends to have good form with few double leaders. However, where double leaders emerge, these should be reduced to a single leader as soon as possible so the tree can straighten up. If planted at a low stocking, form prune dominant branches and double leaders annually to minimise branch size and encourage a single dominant leader, and clearwood prune the trees annually to minimise defect core diameter. Pruning can begin once the trees are well-established and above the height of any competing weed vegetation – e.g. from around 2 metres tall.
Form pruning guidelines are available here.
See video on Form pruning eucalypts »
Clearwood pruning: The aim of clearwood pruning is to minimise the defect core and produce wood free of knots. Pruning "lifts" must be regular and undertaken at a frequency that minimises branch size and diameter over stubs (DOS). Ideally, only final crop trees would be clear-pruned. The smaller more horizontal branches on lower stems of Blackbutt established within a highly-stocked stand (i.e. 1600 stems per hectare or more) will generally self-prune when they are shaded out. Larger, upward pointing branches should be removed as soon as possible as they will reduce height growth and increase defect core diameter.
Clearwood pruning guidelines are available here.
See video on Pruning eucalypts for clearwood »
Suitable pruning tools include loppers and a pruning hand-saw, a battery-operated reciprocating saw, or battery-operated loppers. Form pruning can be undertaken by using a pole pruner.
For timber production, thinning is critical and should be undertaken progressively to reduce the risk of windthrow in residual trees.
For clearwood pruned trees, once pruned to full height (e.g. 6-8 m), thinning should commence once the crowns touch and before lower branches begin to die. Thinning should aim for a final crop stocking of around 200-350 stems per hectare of evenly-spaced trees, depending on the target log diameter and rotation length. Thinning can commence before pruning lifts are completed and care needs to be taken to thin before the height to diameter ratio gets too high. Trees left at a tight stocking for too long become tall and skinny.
See a short video on thinning eucalypts.
When to harvest
Every locality is different, but on a good site blackbutt should get to 50 cm diameter within 30 years. Small sawlogs can be grown on rotations as short as 16 years.
Blackbutt is readily sawn, has fewer growth stress problems than most other eucalypts, and no checking occurs with drying. The wood is a pale yellow-brown, hard, strong, tough, moderately heavy and durable. It is coarse textured; fairly straight grained and the sapwood is resistant to borer attack.
Blackbutt is naturally durable and can be used untreated for structural applications, cladding and exterior decking. Because of its decorative appearance blackbutt is used for flooring, joinery, panelling, furniture, and veneer. Blackbutt can also be used for in-ground applications such as fence posts.
When correctly sawn and seasoned, eucalypt timber can have attractive appearance, along with high strength and stiffness. Most eucalypts have a compression core and tensioned outer wood. When cut longitudinally the ends will bow out. Quartersawing gives the best results in terms of dimensional stability, and to get straight quarter sawn boards a straightening cut is required.
Markets and demand
There is good demand for blackbutt timber because it is hard, strong and suitable for applications requiring durability. Thus the species competes with high-value imported hardwoods and can achieve very good prices in the market, provided grades are good.
A handful of mills around the country have developed a small marketplace for blackbutt timber.
Blackbutt is relatively fast growing. Diameter growth is approximately 1 cm per year and height growth 1-1½ metres a year for the first 20 years. Mean tree height after 8 years is about 12 m, after 22 years is about 30 m and after 50 years is typically about 45 m.
Practical hints for measuring trees offers some advice on monitoring growth.
Carbon sequestration rate over time, and relevant Look-up Tables
If the land is registered under the Emissions Trading Scheme and the trees are planted so as to comply, i.e. at least one hectare, with tree crown cover of more than 30 percent in each hectare and an average width of tree crown cover of at least 30 metres, then they should earn carbon credits. The relevant look-up tables for determining how quickly they store carbon and earn credits, and guides to how to use the tables, may be downloaded here.
As blackbutt is an exotic hardwood tables A2.2 and A2.4 apply, suggesting it stores carbon roughly three times faster than indigenous forest and about 80% as fast as radiata pine.
Timber return on investment
Analysis suggests possible internal rates of return of around 8% pa excluding land costs and carbon revenue, but the result is very dependent on end-use options and log prices. Improved log prices will significantly lift IRR. If logging of old growth tropical rainforests declines as a result of environmental concerns, demand for durable hardwoods such as blackbutt would increase, potentially increasing returns.