Farm Forestry New Zealand

Planting

Douglas fir planting stock is available from many commercial nurseries, either bare-rooted or containerised. Price per tree should range from $0.50 - $1.50 for 1 year-old forestry stock (2020 prices). For best results the root collar diameter should be at least 10mm in diameter, and trees need to be two years old to reach this size.

Plant bare-rooted stock in winter. Containerised stock can be planted from autumn to spring.

Seedlings inoculated with suitable mycorrhizae are required for establishing Douglas-fir on new sites. Although reasonably shade tolerant, Douglas fir should only be planted in open sites free from weed competition, with some shelter and air drainage.

The young trees benefit from spring and summer rainfall. Unless there is summer moisture young seedlings will go dormant until the following spring, when they will ‘flush’ with a burst of growth. New shoots are initially very soft and can be damaged by late frosts or strong winds.

Spacing

Plant Douglas fir at stockings of 1250 - 1650 stems per hectare, (3 m x 2.5 m or 2.5 m x 2.5 m spacings) to provide mutual shelter and to control branch growth. The natural form of this species is tall and conical, but can become heavily branched if given space. It will tolerate high stockings without losing vigour.

Releasing

Release spray after planting once weeds begin to regrow. Douglas fir is very sensitive to weed competition, so releasing is recommended for two to three years after planting.

Douglas fir is also sensitive to many common herbicides and there is a limited range of safe products available. One is terbuthylazine, which can be used directly over the seedling trees while they are dormant, i.e. in September just before they have started flushing. Hand releasing may be necessary to control woody weeds.

See Successful establishment of tree seedlings »

Grazing/Browsing

Stock should be fenced out for at least eight years or until rough bark develops.

Young Douglas fir seedlings are palatable to hares and rabbits. They will slice off trees near ground level at a 45 degree angle, killing them.

Possums continue to debark and debud trees until they are at least 10 years old. They should be controlled with poison, trapping or shooting.

Pests and diseases

Douglas-fir is relatively healthy in cooler regions. Warmer, more humid sites are subject to Swiss needle cast fungus, which affects older foliage. An infected tree may carry only the most recent foliage, whereas a healthy tree might have four or more years of canopy to help its growth. Fortunately the younger needles catch more sun and are more effective at photosynthesis, so the loss of the older needles, while it slows the tree, is not fatal. The disease is aggravated by warm winters and wet foliage in spring. Little can be done about it, but its effect can be mitigated by using lower stockings, cooler sites, and good air flow.

See Pests and diseases of Douglas fir »

Douglas-fir is usually planted for framing timber. Because the most important factor influencing quality in framing timbers is branch size, these forests are usually grown at relatively high stockings to minimise branch size.

Douglas-fir seed is easily blown by the wind and the species will invade both open country and established vegetation if it is not controlled by browsing.

Suitability for steep slopes: Douglas fir is relatively wind hardy but does not like thin eroded soils, so may not suit all steepland sites. Strong winds can cause leader loss and crown deformation, however trees tend to recover with little stem deformity resulting.

Douglas fir provides effective erosion control on erodible hill country until it is clearfelled, when there is a window of risk and erosion can occur. Although it will carry a higher basal area per hectare than radiata pine and it is considered to be a reasonably shade tolerant species, it is difficult to grow under a canopy, so is not suitable for selective harvesting.

Pruning

Douglas fir is not usually pruned for clearwood. Higher initial stockings are recommended to control branch size if growing for framing timber.

Consider pruning the largest 100 trees per hectare where these are of good form. Provided they are the most vigorous, they are unlikely to suffer from competition from their unpruned neighbours; and being the most vigorous, if left unpruned they are the ones most likely to grow large branches which would lower their timber value. Stand gaps also result in excessively large branches and the lower branches on these trees could be pruned.

Thinning

If terrain or location rule out production thinning, the current practice is to thin once to 500-600 stems per hectare when the trees reach a mean top height of about 15 m, and leave this as the final crop. Where production thinning makes sense, remove poorly formed trees by an initial waste thinning when the mean top height is 10-12 m, to leave a stocking of 1000-1200 stems/ha. Later, when the trees reach a mean top height of 18-20 m, halve the stocking to 500 stems/ha with a first production thin. Finally, do a second production thin to 350 stems/ha when the trees reach 25-27 m mean top height.

Timber cut from young or small trees is not noticeably inferior to that from mature trees, and so production thinning (selling the thinnings as small sawlogs) or even fairly short clearfell rotations can generate good cash flow on accessible sites.

When to harvest

Douglas-fir is relatively fast growing but because it needs to be grown at high stockings for the first couple of decades it takes time to reach a merchantable size. As a rule of thumb diameter growth averages around 1 cm a year, suggesting rotations of around 35-45 years.

Douglas fir timber is stiff and strong. However it will easily split and splinter, it will not machine to a smooth surface, and it is difficult to get a consistent finish with staining or gluing which makes it less suitable for furniture, panelling and veneers.

The heartwood, which is clearly distinguished by its pinkish appearance, is relatively durable, easily dried and very stable. The strength and stiffness of Douglas fir timber improves as knot size reduces. Clear timber has been used for yacht masts, ladder rails and scaffold planks, where its straightness and stiffness are valued. Small-knot timber will readily pass machine stress grading for framing. Knottier timber is used in larger sections for structural beams and columns. Both heartwood and sapwood require boron treatment for internal structural applications. Douglas fir is not used externally in New Zealand because the heartwood is not sufficiently durable and resists preservative penetration.

The industry has produced a downloadable pdf explaining the properties of Douglas-fir timber and where it may be used.

Markets and demand

Douglas fir logs are usually sold through the same channels as radiata pine logs. Douglas fir has an excellent reputation as a general purpose structural timber on both domestic and export markets. Wharf gate prices for unpruned structural grade Douglas fir logs are approximately the same as current prices paid for pruned radiata pine logs.

Douglas fir starts slowly with modest volume growth for the first thirty years. Height growth starts to accelerate after five years reaching its maximum of about 1 m a year at age 20 to 30, and the trees will achieve top heights of 30-35 metres by age 40. Ring widths in New Zealand grown Douglas fir are between 3 mm and 6 mm giving a diameter growth of 6 – 12 mm a year. The species can grow strongly for up to 200 years.

The industry has developed the online Forecaster Calculator for radiata pine and Douglas fir as a simple forest simulation tool, that predicts tree growth and the log products per hectare that may result at any given clearfell age.

Practical hints for measuring trees offers some advice on monitoring growth.

Douglas-fir is listed on tables A2.2 and A2.4, suggesting that over its first 30 years it stores carbon roughly two-thirds as fast as radiata pine.

Timber return on investment

The economic returns from Douglas-fir are constrained by its long rotation length. An early study suggested an internal rate of return of 5-6% pa but gave no details on assumptions. Clearly a first rotation forest on land eligible for carbon credits could generate an early cash flow which would lift the apparent return, but it would not be a good indication of its long term profitability. The Forecaster Calculator might give a more useful indication of economics.

• The Douglas fir Association is a grouping of growers and processors who may offer support.
• The NZ Farm Forestry Association has produced an authoritative leaflet on Growing Douglas fir. 
• Fir - Information resources