Walnuts require a temperate climate, with about 600–800 hours of chilling below 7°C, temperatures that do not exceed about 38°C and low humidity, consequently the tree is cultivated commercially throughout southern Australia.
Traditional small-scale orchards are found in Gippsland, in the central regions and the north east of Victoria; in the southern highlands and central tablelands of New South Wales, in the Adelaide Hills and Riverland of South Australia and in south west Western Australia.
Large walnut plantations are located on the east coast of Tasmania, in the Goulburn Valley of Victoria, in the mid-Murray Valley, South West Western Australia and in the Riverina of New South Wales.
Historically, it was recommended to grow walnuts on deep, well-drained soils, with a soil pH greater than 6.0. However, commercial production in Australia now occurs on a wide range of soil types, from deep alluvial soils of river valleys to clay soils of the riverine plains. Good site preparation, advanced agronomy and variety development have overcome limitations of some of the ‘new’ environments in which walnuts are now growing in Australia.
Walnut is a deciduous tree that requires distinct cold and warm seasons. It grows best in a temperate climate, with about 600–800 hours of chilling below 7°C each year and maximum temperatures that do not exceed about 38°C. Development of new production systems since the 1990s has seen the expansion of the walnut industry into environments once thought too hostile.
In a rain-fed system, in the Great Dividing Range, good production can be achieved on annual rainfall of 1,000mm, however back-up irrigation is advisable to ensure the crop has water at the critical development and production stages. Under irrigation on the riverine plains, the water use target for mature walnuts is 12 ML/ha per season.
Showery weather in spring causes blight and excessive heat periods in summer can cause sunburn, both resulting in decreased nut yield. Frost in mid to late spring, which is the flowering period, can affect nut set and yield for the season.
There are hundreds of recognised varieties of the species Juglans regia that have been developed around the world, especially in France and the United States of America (California). Variety development has been based on many characteristics including leaf-out time, maturity, shell thickness, hardness of the shell, kernel colour, flavour and disease resistance.
Varieties planted in Australia include Chandler, Howard, Tulare, Serr, Vina, Lara and Ashley, most of which are Californian varieties. French varieties planted in Australia include Franquette. New growers should select varieties on the basis of marketing options and suitability to the climate of the orchard location. For example, Chandler and Tulare are grown for their light coloured kernels. Chandler is grown in areas prone to spring frosts as it leafs-out later in the season, as does Franquette.
Walnut is a cross-pollinating plant, therefore at least two varieties must be planted to ensure good nut set. It is recommended that pollinator varieties are planted in each orchard to ensure good pollination.
Variety recommendations should be sought from other growers or specialist horticultural nurseries.
Planting and crop management
Walnut orchards are established by planting selected cultivars that are grafted onto a robust rootstock, such as the American black walnut (Juglans nigra), Californian black walnut (Juglans hindsii) or a walnut hybrid. The planting pattern will be determined by soil type, topography and machinery to be used. In traditional orchards, often where soil profiles are deep, trees were planted to a grid as large as 15 x 15 metres. As production has become more intensive and operations are highly mechanised, trees may be planted closely together (6m rows and 3m intervals) and be managed as hedgerows (the ‘Tatura’ system).
On more shallow soils, the planting row may require ripping and hilling. As with all long-term plantings, soil tests should be carried out several months before planting, so soil pH, soil structure and nutrient levels can be corrected ahead of planting. Planting ideally occurs in late winter or early spring. Mulching and weed control are important at planting time to maximise moisture and nutrient availability to the young trees.
Walnut orchards will require ongoing nutrient management and fertiliser programs can be based on estimation of nutrient removal rates, soil tests and/or leaf tissue analysis.
Depending on location, walnuts may require regular irrigation to maintain sufficient water supply to the tree. Soil moisture monitoring systems, such as tensiometers or capacitance probes, can be used to estimate water requirements effectively and efficiently. Mulching can be used to conserve soil moisture on newly planted trees.
In newly planted orchards, trees are pruned manually to train the trees into the desired shape, as well as to maintain healthy growth on the tree and keep the canopy open with good airflow. As the orchard matures, pruning may be done manually or mechanically depending on the layout and size of the orchard. Mechanical pruning may commence in a hedgerow orchard after 5–7 years, when the trees are strong enough to be managed with pruning saws.
Pruning is required every 3–4years, depending on growth rates of the trees. Ideally, canopy management should aim to intercept 70–80% of sunlight falling to the ground and maximum tree height should not exceed 80% of the row width. It is important that the tree canopies do not touch across the rows. Trees pruned to the central leader system or pyramid-shaped trees have better light interception, even as the trees age.
Detailed information on the management of walnut orchards is available for purchase to members of the Australian Walnut Industry Association.
Weeds, pests, and diseases
Weeds compete with newly planted trees for water and nutrients, therefore weed control before planting is important, as is mulching of the rows after planting. In a well-maintained mature walnut orchard, the inter-row area is mown regularly to assist with access to the trees and planting rows are mulched or sprayed with herbicide.
Australian walnut growers face very few pests and diseases, unlike their overseas counterparts. Growers will spray their orchards in spring as the nuts start to develop, to prevent walnut blight. Individual trees may also succumb to phytophthora and require treatment. Sunburn damage can be lessened by spraying the nuts with Kaolin (a white clay) for sun protection.
Cockatoos are a significant pest for walnut growers in some regions, as they may strip new buds and will damage the green fruit on the tree or remove it entirely, from the time the fruit appears through to maturity when the fruit opens and the nuts are developed. Control measures include gas guns, kites, balloons and deterrent sprays, all of which must be used in rotation as the birds will become conditioned to just one approach. Depending on local and state regulations, growers may be permitted to use firearms for scaring birds or for culling birds attacking the crop.