Poplar - Kawa, Populus deltoides × yunnanensis
Poplar is normally grown for erosion control in pasture. Poplars can reduce slips and provide shade, shelter and fodder for stock while having little impact on pasture growth. Poplar may also be grown for timber, either within a pastoral setting or as plantation forests.
Kawa is not very frost tolerant and favours moist sheltered sites such as valley floors, gullies and terraces. It is widely planted in northern and western New Zealand.
Commercial return: Medium
RainfallHigh rainfall; Moderately low rainfall;
AltitudeModerate altitude; Low altitude;
Soil drainageModerately free draining; Poor drainage;
Soil depthDeep; Moderate depth;
Poplars are commonly sold as bare stakes and poles, and sometimes as rooted cuttings. Stakes and rooted cuttings are much cheaper than poles and rooted cuttings in particular have a more reliable strike rate in areas with dry summers or on exposed hillsides.
Poplar poles are generally planted into pasture which requires no site preparation. Poplar stakes and rooted cuttings may require some vegetation control so that grasses and weeds are not competing for soil nutrients and moisture.
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.
Most regional councils operate nurseries. The Hawkes Bay Regional Council offered stakes and poles in 2019 at prices of: 1 Metre, $2.20; 2 Metre $5.40; 3 Metre $10.00. Protectors were 1.1m Netlon $3.40; 1.7m Netlon $4.60; 1.7m Dynex pole protector $7.20. Delivery was extra.
The best time to plant poplar is from late June to early August. Plant across the lower half of exposed windy slopes, and plant only the sheltered, moist sites on the upper part of slopes.
All poles, stakes and cuttings planted in paddocks must be protected from stock. Cattle should be kept out for at least one and preferably two or three years. Do not plant poles where there are goats or deer.
Avoid any ridge tops exposed to prevailing winds. When planting on very steep slopes plant the poles away from steep banks with thin soils, and angled out from the vertical. This will prevent stock on the uphill side from eating the growing tips.
Planting techniques are described in this fact sheet How to care for poplar and willow poles.
Planting techniques are shown in the following short videos:
- Planting Poplars & Willows: Choosing the Best Poles
- Planting Poplars & Willows: Successful Planting
- Managing Poplars & Willows: Re-Ramming Recently Planted Poles
When planting poles for soil conservation, space poles at between 25 and 100 stems per hectare (sph). Steep slopes require closer planting as described in this short video: Planting Poplars & Willows: Successful Planting.
When growing for timber, plant poplar as rooted cuttings at up to 1,000 sph. Trees may later be thinned down to a final spacing of 250-300 sph, similar to a radiata regime.
Spray or clear weeds around stakes and rooted cuttings during the first summer, if necessary more than once. Be careful not to spray the poplars themselves. Glyphosate is the safest, and can be sprayed on wet sites, but must not come into contact with green stems. As they grow poplars are unlikely to shade out weeds, so problem weeds such as gorse and blackberry should be sprayed out before planting.
All poles, stakes and cuttings planted in paddocks must be protected from stock, usually with sleeves to prevent rubbing. Cattle should be kept out for at least one and preferably two or three years. Do not plant poles where there are goats or deer.
Kawa is not very palatable to possums but these should be controlled so they do not break out leaders and branches. Young kawa wands may be palatable to hares and rabbits. They will slice off trees near ground level at a 45 degree angle, killing them.
Pests and diseases
Kawa is rust resistance in most situations. Insect damage is generally of little importance and usually fungal diseases are not fatal.
Poplars are enormously versatile. They readily coppice (i.e. the stumps resprout after harvesting), and so may be regularly harvested for biofuels or for taking up nutrients in riparian strips. Their leaves and small branches are palatable, so the trees may be pollarded for fodder. They have extensive root mats that interlock to bind erodible soils. Grown to maturity they produce quality timber.
Grown for supplementary feed, pollarded trees may be cropped on a 3-4 year cycle once they are more than 5 years old. Pollarding is described in this Poplar fodder trial. Another study, Poplar and willow as supplementary fodder sources suggests that the nutritional value of intact poplar leaves in summer and autumn is usually similar to that of pasture during the same period, particularly during drought, and they are rich in trace elements such as zinc.
The leaves are palatable to livestock and prunings may be a useful feed source during drought, so pruning should be scheduled for summer. Form prune poplar late in summer to encourage one dominant leader, and if grown for timber, prune the trees every couple of years to prevent lower branches from getting too big. Guidelines are here »
This video may also be useful: Managing Poplars & Willows: High-Pruning Poplars »
On highly unstable sites trees should not be thinned at all. Otherwise to maintain grass growth it is generally safe to fell every second tree to a final spacing of 20 to 24 metres on slopes, and 10 to 12 metres on stream banks, between years 10 and 15. Grown at higher stockings for timber, poplars may be thinned on the same schedule as radiata pine, i.e. to around 600 stems per hectare at ages 4-5 and to around 250-300 stems per hectare at ages 9-10.
When to harvest
Poplars grown for erosion control can be harvested for timber if they are cut and replanted in rotation to maintain soil protection. One suggestion is to log selectively, taking every fourth tree every 10 years and letting the others grow on so that the total root system stays intact.
Poplars grow quickly and on good sites can produce useful timber in 15 years. For high grade peeler logs and saw logs they should be grown on for another 10 to 20 years.
If the poplars have been pollarded for supplementary feed and remain productive, they may be grown to almost any age.
Mature poplars can provide high quality veneers, appearance grade timbers and useful farm timbers. See Tree Grower article Wood properties and use of poplar and willow.
Markets and demand
Kawa grows rapidly at rates of 1 to 2.5 m a year depending on rainfall and soil fertility. After age 25 the trees keep growing but may be regarded as mature, reaching heights of 30 to 35 metres.
This paper offers some advice on monitoring growth.
Timber return on investment
In New Zealand poplar is primarily grown for erosion control, but even so it can be managed and harvested for timber. On good sites it grows faster than radiata pine, it can reach a marketable size in 20 years or less, and it can attract better prices than radiata pine as export logs. It is one of the few timber species which can support grazable pasture throughout its rotation.
Plant & Food Research publication Trees for the farm: A decision support tool for farmers suggests the typical costs (in 2015) of establishing, managing and harvesting poplar grown for erosion control at a stocking of 60 stems per ha and harvested at age 20 would be around $6,000 per ha spread over the period.
Return on investment will depend on what you are growing the trees for, the price of planting stock, how you manage the trees and when you harvest.
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 Kawa poplar is an exotic hardwood, tables A2.2 and A2.4 apply, suggesting Kawa poplar stores carbon roughly three times faster than indigenous forest and about 80% as fast as radiata pine.