Farm Forestry New Zealand

Poplar and willow arboretum at Aspendale

Allan Wilkinson, New Zealand Tree Grower November 2020.

Aspendale is a private arboretum established by Allan and Lois Wilkinson, near sea level and close to the coast on 10 hectares of flat peatlands at Manakau, near Levin north of Wellington. Planting began in 2003 and include a poplar and willow collection representing the most disease and pest-tolerant clones selected from the national collection currently administered by Plant and Food Research. The aim was to provide a national resource of named breeding material for public and private breeding programmes to develop new clones for soil conservation, windbreaks, agroforestry, effluent treatment, biomass production, riparian planting and amenity.

Poplar and willow planting

In the first two years we planted one metre stakes of each selected clone for them to grow to flowering age and size. Initially we planted 93 poplar clones of 14 species and various hybrids, 61 tree willow clones of five species and hybrids and 62 shrub willow clones of 23 species and hybrids. Early tree spacing was seven by seven metres so that we could fit in as many clones as possible. Planting on the peat was easy and the hope was that the trees would help dry and stabilise the land as their roots spread.

The growth of the poplar clones on this land has been interesting to observe. With hindsight wider spacing would have resulted in more stable trees with flowering branches lower down and more accessibility for breeding material. Unlimited water supply gave excellent growth rates but also led to crown breakage and toppling, something which would not normally occur on drier soils and river gravels.

Suckering varieties of Populus trichocarpa from California have not toppled but have suffered crown breakage in gales. The rapid tree growth also caused shrinkage and collapse of the peat, revealing buried stumps and other woody remnants of the pre-existing kahikatea and totara forest.

Aspen poplar P. tremuloides is suckering happily in its corner. A single Moroccan clone of P. alba has been the fastest-growing poplar planted reaching 77 cm in diameter at breast height after 17 years. P. glauca from Nepal is the slowest, with just a very short growing period, producing leaves in December which it keeps until mid-winter. P. nigra Blanc de Garonne is also showing good growth reaching 76 cm after 17 years. Another showing good growth over the same period is kawa with a diameter of 60 cm.

Poplar and willow pests

The poplar and willow clones have also been observed over the last 17 years for the appearance of new races of the poplar leaf rusts Melampsora larici-populina and M. medusae, the willow rusts M. coleosporoides and M. epitea and the anthracnose diseases caused by Marssonina brunnea and M. salicicola. To date no serious epidemics have occurred on clones in the arboretum although the timing and intensity of attacks have varied with seasonal weather conditions.

The willow collection thrived until 2014, when we first found the giant willow aphid on the cultivar S. melanostachys. Since its arrival, most of the tree willows have been severely affected with crown dieback and some toppling as the trees fall over. Additionally, over the last five years all the shrub willows have died from a combination of the aphid, heavy shading and lack of monitoring.

The best tree willow clones at this stage appear to be a single clone of the American black willow Salix nigra, one clone of Pacific willow S. lasiandra, and a hybrid of S. matsudana and S. pentandra which we are using as a shelter belt because of its narrow crown. New willow clones with resistance to the aphid are being developed by Trevor Jones at Plant and Food Research in Palmerston North.

Amenity planting

As part of the initial development and to help with drainage, we created two ponds in very wet areas. Once the poplar and willow collections were established, we turned our attention to trialling other tree species on the peat and establishing shrubberies and ornamental planting around the ponds and along the mounds thrown up along the various deep drains throughout the property. We planted one paddock a year. Species grown included deciduous and evergreen forest trees and shrubs, including cultivars of camellia, magnolia, hydrangea and rhododendron.

Our section of New Zealand natives is mainly from the seedlings we find dispersed by birds. They include some kahikatea which would have been from the original forest on our land. Others which are abundant include kowhai, totara, titoki, lacebark, lemonwood, mahoe, pittosporum, coprosma, manuka, kanuka, cabbage trees and flax. It is delightful to see the birdlife that these attract. Under shelter, we have brought in specimen trees of rimu, miro and red beech. Our one attempt with pohutukawa was unsuccessful.

Forest trees include groves of coast redwood Sequoia sempervirens, reaching a diameter of three centimetres in the outside rows and 45 cm inside rows after 14 years, and dawn redwood Metasequoia glyptostroboides the biggest with a diameter of 47 cm. Other trees which have thrived include black walnut J. nigra, Mexican alder Alnus jorullensis, red alder A. rubra, Mexican oaks grown from acorns from Hackfalls arboretum, swamp cypress Taxodium distichum, silver maple Acer saccharinum, horse chestnut Aesculus hippocastanum and plane Platanus orientalis. Our final area, planted in 2015, is an experimental area of pecan nut trees.

In other areas we have tried to establish a more park-like appearance using flowering trees and shrubs we have appreciated in other woodland gardens. Some magnolia have settled in better than some of the named grafted cultivars, which have tended to die back to the root stock after wet winters. Camellias have proved hardy through all weather. The rhododendrons are established on the only hill. Hydrangeas, smoke bushes and virburnum do well once established, though some of our original choices were shaded out. Grevillea, leucadendron and proteas took time to establish but are doing well now. The cherry walk planted along what was the old dairy farm race by our sons is surviving but is not tidy. Our attempt to establish a lime path has been defeated by the wet soil.

Lessons learned after 17 years

The main lesson is that when you plant an environment you are changing it. During the first five years we had heavy frosts and wet windy springs and had to protect the young planting. However, in the last five years there have been few frosts. This may be due to climate change, but we think it is also the effect of the tall trees, especially since the canopy closed. Some trees such as the river birches have grown much faster with the increased shelter.

We had not anticipated the extent of the effect of tall trees on under-planting and certainly had not planned for changes due to shading or the speed with which some changes take place. Very little of the original shrubbery around the big pond still exists. We now research more carefully and are aware that what we are planting now may also have to change. Our knowledge of plants other than trees is expanding. One bonus has been the increase in native ferns and tree ferns.

The wind and pests do not help

The equinoxial winds in the Horowhenua are problematic. After some high mortality, we had to establish temporary shelterbelts to protect new planting. Even then salt spray damage during gale force westerlies has killed some ornamentals and caused browning of the foliage on the windward side of coast redwoods. Unless there are persistent gales the redwoods recover their green foliage.

Peat is an interesting medium to work with. All planting of small rooted plants on the peat require intensive mulch to prevent the peat from forming a hard, water-repellent crust during summer. Shallow rooting caused by the high water table resulted in severe losses of Hydrangea macrophylla and Japanese maples. Unfortunately, barberry and blackberry cope well.

Lemon tree borer is a major tree pest and has girdled many species including maples, birch, elm, liquidambar as well as poplar and willow. Possums are also a pest and have become much more so since the local regional council reduced its baiting programme. It is not a good idea to go overseas in the spring and leave your arboretum to the mercy of the elements and wildlife.

Successes

Some new clones of poplar have been selected for release after observation at Aspendale. These include the fastigiate poplar Chiba to replace Lombardy poplar, and Plummer a hybrid of narrow-leaved cottonwood from Utah and Yunnan poplar. Chiba has been available for several years.

Plummer has recently been supplied to Appleton’s Nursery in Nelson for multiplication for sale but requires observation and testing on a wider range of sites. Plummer has excellent yellow autumn colour. It is disease and possum-resistant with straight stem and light branching similar to P. trichocarpa PN 471 released in the 1970s.

Two new gold-foliaged clones bred by Trevor Jones at Plant and Food have been planted at Aspendale. These have been developed from crosses between Manawatu gold and P. cathayana, Manawatu gold and Yunnan poplar. Both look promising for future release as ornamental cultivars.

The success of the original coast redwood planting has encouraged us to plant another avenue, this time using a mixture of 12 selected clones with a border of swamp cypress in the wetter parts.

The arboretum has become a focus for our extended family. Although it was originally a retirement occupation, its planning and ever-changing form and even the hardwork have involved us all. Many close friends and all the family have planted their tree and the arboretum is often the first choice for a family get-together. Although its purpose is to support the breeding of poplars and willows for New Zealand, it brings us great pleasure and many hours of exploring the ‘what next’.

For further information or a visit to Aspendale please contact Allan and Lois by email at Takapu101@gmail.com