Eucalyptus - Mountain ash, Eucalyptus regnans
Mountain Ash (“regnans”) is a fast-growing, mostly smooth-barked cool climate eucalypt commonly planted in New Zealand for timber, and for hardwood chips for fine papers. It tolerates frosts and windy, erodible hill country conditions, but requires free-draining soils and does best in deep soils at altitudes of less than 600 metres. It will grow for 500 years reaching 100 m high and 2.5 m in diameter and on good sites will produce sawlogs in less than 20 years.
Commercial return: Medium
AltitudeModerate altitude; Low altitude;
RainfallHigh rainfall; Moderately high rainfall; Moderately low rainfall;
Soil depthDeep; Moderate depth;
Soil drainageFree draining; Moderately free draining;
WindStrong wind; Moderate wind;
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).
Mountain ash dislikes excessively cold sites, drought and wet feet, preferring fertile soil with good drainage and an annual rainfall of 1,000 mm. Unless the site is frost free, plant in the spring after the worst frosts have passed. Mountain ash seedlings are not shade tolerant so should be planted in an open site. If you are planting in a low rainfall area use wetted water-absorbing gel in the planting holes to ensure they 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 (2 x 2.5 m) 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 Mountain ash 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 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.
Young eucalypt seedlings are palatable hares and rabbits. They will slice off trees near ground level at a 45 degree angle, killing them.
During the first summer eucalypt seedlings are highly palatable to possums which should be controlled with poison, trapping or shooting.
Pests and diseases
Mountain ash currently has few insect pests In New Zealand. On permanently or seasonally wet sites it is susceptible to root diseases. Mountain ash requires a cool climate and foliage fungi can also cause problems where there is high summer rainfall or warm humid conditions. The species grows best on well drained soils where rainfall is well distributed over the year.
Mountain ash is usually grown for timber, carbon storage or firewood. Grown for timber, aim for large logs rather than attempt to restrict branch growth. Grown for carbon, a reasonably high stocking of slim trees can hold more tonnes per hectare 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. Note that mountain ash forests are particularly susceptible to bushfires.
Suitability for steep slopes: Mountain ash tolerates strong winds but not bony thin soils, making it less suitable for erosion control than other eucalypt species. Mountain ash does coppice, 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.
Prune in the driest part of the year, mid to late summer, and preferably before branches exceed 2-3 cm in diameter. Double leaders need to be reduced to a single leader, the earlier the better. Annual visits are recommended.
Form pruning: Mountain ash 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 Mountain ash 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 mountain ash should get to 70 cm diameter within 30-35 years. Small sawlogs can be grown on rotations as short as 16 years.
Mountain ash is a low density hardwood timber with a decorative straight grain. Long, clear lengths without knots or degrade hold the greatest value. The wood is strong and stiff and rather coarse in texture. Drying degrade (checking, collapse) can occur. However, mountain ash is dimensionally stable once dried. The timber is soft, easy to work and suitable for steam-bending. It is not durable so is used for internal appearance applications such as flooring, panelling and furniture. Mountain ash is suitable for general construction but requires boron treatment for framing to meet the durability requirements of the Building Code.
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 stability and low surface checking, but to get straight quarter sawn boards, straightening cuts are required. Gum veins can be present. Despite these issues, when correctly sawn and seasoned, mountain ash timber has an attractive appearance and good grades, which can yield good returns.
Markets and demand
Despite strong demand for imported hardwoods there is only a limited market for locally grown mountain ash because of its low durability and low surface hardness.The wood has limited applications and flooring made from it tends to be lower value than from harder, more durable eucalypt species such as blackbutt.
A handful of mills around the country have developed a small marketplace for eucalyptus timber. Pulp and chip markets provide an outlet for arisings from logging, provided the trees are not too distant from the market.
Mountain ash is fast growing. Diameter growth is approximately 1-2 cm per year and height growth 1-2 metres a year for the first 20 years. Mean tree height after 8 years is about 15 m, after 22 years is about 33 m and after 50 years is typically about 65 m. However, while on suitable sites mountain ash can match or out-perform radiata pine in both height and diameter, they need plenty of room and wide spacings for large diameter logs. Consequently if grown for large trees the buttlog yield per ha of the final crop is generally less than radiata.
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.
Mature mountain ash forests store more carbon than any other known species, and the Field Measurement Approach of the Emissions Trading Scheme (applicable for forests of 100 ha or more) might well give higher carbon storage figures than the look-up tables (A2.2 and A2.4). The tables suggest mountain ash stores carbon about 80% as fast as radiata pine.
Timber return on investment
A 2010 study looked at timber recovery and economics of short-rotation small-diameter mountain ash forestry and found that harvesting and milling 18 year old mountain ash with diameters of 25-43 cm gave a net present value of over $10,000 per ha with an internal rate of return of 16%. However the study noted that markets would have to be developed before this could be achieved in practice.