I grew up in Marlborough, just as the wine industry was beginning to gather momentum. Back then the landscape was less the monoculture it is now – apricot and cherry orchards, berry fruit and vegetable crops as well as sheep farms were still commonplace in the landscape. The region’s dry climate also meant that walnut trees thrived – I’m not talking about commercial plantations but individual trees or small groups thereof, many planted by early settlers, dotted about on farms and long-abandoned homesteads on the backroads and well, roadsides in general. Many an autumn Sunday drive concluded with a stop-off at a known spot where the grass beneath an easily accessible walnut tree would be carefully searched and the soggy-jacketed nuts gleaned from their hiding places. Stained hands were a small price to pay for the treasures contained within!
When my family left Marlborough for the West Coast, we were gifted a seedling walnut tree in a yoghurt pot by some friends. After a couple of years, we settled on our own piece of land and the tree was planted. The wet, wild coast is not an ideal place for walnuts but this tree has thrived and 26 years later, is a towering specimen. It produced a basketful of nuts in 2013, but was sadly hit by Cyclone Ita in 2014 and now grows at a 40° angle, propped up by a sturdy support. The humid climate means blight is commonplace, but our beloved tree battles on. It has never cropped as well since the cyclone, but even this year, when it looked like there was nothing, Mum found a couple of nuts lurking in the grass at the end of summer. I think it’s the kind words and occasional hug she bestows on it as she goes about tasks in the garden – the complete opposite of naturalist John Ray and his ghastly utterance. Ever the joker, my Dad popped one of these walnuts in my shoe as I was leaving one day and I still carry it in my car as a good-luck token.
I categorise walnuts in the love/hate food group, along with liquorice, capers, olives and coriander. Many people just find them too bitter or simply don’t like the flavour – I wonder how much of this is due to only being exposed to nasty rancid supermarket nuts of dubious age and provenance? You really can’t compare a properly cured, freshly-shelled new season walnut to the translucent supermarket pre-shelled nut that has been imported from goodness knows where goodness knows how long ago.
Walnuts: a short family history
Walnuts, belonging to the Juglandaceae, are a centuries-old food source, with wild species found right throughout Asia, the Americas and the Middle East - they can claim to be the most widely distributed tree nut and enjoy a widespread following to this day. Juglans regia, the Persian, English or Carpathian walnut is the species we are most familiar with commercially. Although Juglans is the type genus for this family, it also includes pecans and hickories (Carya spp.), both important food crop species in their own right.
The four main Juglans species you are most likely to encounter in New Zealand are J. regia (as previously mentioned); J. nigra, the eastern black walnut, native to the eastern USA; J. cinerea, commonly known as butternut and again native to the eastern USA and J. ailantifolia (syn. J. sieboldiana, also J. ailantifolia var. cordiformus or heartnut), the Japanese walnut, native to Japan. The latter has become an invasive species due to its ability to readily self-seed and as such is prohibited for importation into New Zealand and is subject to Regional Pest Management Plan controls. Black walnuts also produce edible kernels with a distinctive flavour, but they are extremely hard to extract from the shell, which is much thicker and harder than that of the English walnut. Specialised equipment is required for extraction on a commercial scale. The kernel is also a lot smaller – more work for less reward? Despite this, black walnuts have an almost cult following and are interesting in the fact that a sizeable portion of the crop in North America is wild-harvested. It is also valued for its strong, dark timber. In New Zealand, J. nigra is more likely to be grown for landscaping – all walnuts make wonderful shade trees.
3.8 million tonnes of in-shell walnuts were produced worldwide in 2017, with China leading production, followed by the USA and Iran. The walnut is an accessory fruit, producing a drupe-like nut. The green (later dark brown) hull is an involucre, or modified bract, enclosing the hard endocarp (shell) with the single seed that we eat contained within. Walnut hulls can be used to make a brown dye for wool and fabric. Walnut timber is durable and prized by woodworkers for its lustre and optical reflectance – being widely used for flooring, musical instruments, gunstocks and decorative features. Intricate Chinese carved walnuts, or hediao, are the result of a craft passed down through generations since the Ming Dynasty (1368-1644) and are used as good luck tokens and keepsakes.
Suitable climates and growing conditions
Walnuts are stately deciduous trees (15-35 m in height at maturity) with large, pinnately compound leaves which are aromatic when crushed. Trees are monoecious (with male and female flowers borne on the same tree) and are wind pollinated. Although walnuts are self-fertile, in most varieties the male flowers open and release pollen well before the female flowers open. Within a large block of the same cultivar, there should be sufficient variation in the timing for flower opening between individual trees to result in a reasonable fruit set, but it is desirable to plant a few different cultivars, including some early, mid and late-flowering types. With walnuts being such large trees, this is not a likely option for the home gardener on a small section – you will have to take your chances with a single tree, but these can still crop satisfactorily going solo – and you never know when or where some wandering wind-blown walnut pollen might drift in from.
As walnuts prefer hot, dry summers and cold winters, the key walnut-growing regions in New Zealand are Hawkes Bay, Marlborough, Canterbury and Central Otago. The New Zealand Walnut Industry Group hosts approximately 55 commercial growers, with orchard sizes ranging from 50 – 4,000 trees per individual property. We are a small player on the world stage, producing mainly for the domestic market, with the largest commercial grower producing 40 tonnes of in-shell nuts per annum.
Depending on the cultivar, walnuts require 400-1500 hours of winter chill to set fruit. With J. regia’s natural range extending from Southern Europe to the Carpathian Mountains in Eastern Europe, the amount of chilling required and the degree of cold tolerance each selection has gone hand-in-hand with the local climate from which they have originated. Hence Spanish selections aren’t particularly cold-tolerant and have short chilling requirements. Strains from the cooler Carpathian Mountain regions are the opposite, tolerating cold temperatures and requiring a longer period of winter chilling. Most of the English walnut strains you will find in New Zealand are well-adapted to our temperate climate and will get through the winter with few issues. Mature walnut wood will cope with temperatures as low as -30°C (an extreme highly unlikely here in New Zealand) but tender shoot tips can be damaged by -7 to -9°C touches of frost, especially if preceded by warmer weather. Frosts below -1.1°C during flowering (September to October) will likely result in damage to the catkins and new growth. Choose a later blooming cultivar to avoid this if you live in a region prone to late frosts, and a lower-chill variety if based in the Far North.
Walnuts can be grown easily from seed, either as fresh nuts or from nuts subjected to a period of cold stratification (two to three months at 2-4°C) and then gently cracked and sown on their sides in the sand as temperatures begin to rise in the spring. I’ve successfully produced seedlings by poking nuts into a tub outside in the autumn, they germinate in the spring, and grow all summer and I then transplant them into individual pots the following winter when dormant. A seedling-grown tree will have the added uncertainty of genetic variation (it won’t necessarily be the same as the parent tree) as well as taking approximately 10-15 years to produce fruit.
You can use your seedling walnuts as rootstocks for grafting, taking scion wood from a known cultivar and grafting it onto the more vigorous rootstock by whip and tongue graft or budding. Walnut grafting is a bit tricky, as callus formation at the graft union is particularly slow but scion bud development is swift and you need specialised equipment to keep the temperature and humidity constant for the former to take place. Budding has its own challenges; the rootstocks are prone to bleeding a large quantity of sap, which can wash out the new bud. Bench grafting, while the rootstock is dormant, is a more viable option, but I say leave it to the experts – buy a grafted tree/s of the variety or varieties you are keen on from a reputable nursery and save yourself the bother. Grafted trees also have the advantage of producing a crop within three to five years of planting. A mature tree can produce around 100 kg of nuts and will crop well into old age (75-100 years!).
Site selection and planting
Walnuts and pears, you plant for your heirs – 17th century English proverb
The main caveat with walnut trees is – you need space! Most J. regia trees will reach in excess of 20-25 m in height at maturity, some well over 30 metres. Horizontal spans average about 15 m and don’t think you can close-plant other species nearby - walnuts are allelopathic, with all parts of the tree producing juglone, an organic chemical that can inhibit the growth of other plant species, effectively creating a ‘dead zone’ around the walnut tree, extending at least to the dripline (the outer edge of the tree’s canopy, enclosing the area directly below the tree).
The spacing of trees in commercial plantings will vary depending on the rootstocks used, but in the home orchard if planting more than one tree, take into account the growth form and habits of your selected cultivars and allow about 10-15 metres between trees. Remember that walnuts end up as very large trees, so keep them a good distance from buildings. They also have large root systems that could invade and cause issues below the surface (pipework, concrete and pavers etc.). Don’t expect to be able to move your tree at a later date either, walnuts have a deep taproot and don’t take kindly to shifts.
Walnuts are tolerant of a range of soil conditions but well-drained, deep, reasonably fertile alluvial types are ideal. Avoid clay soils – the walnuts’ deep taproots make them very susceptible to waterlogging, relatively short periods of which (>24 hours) can result in tree death. Plant in late autumn through winter (May-August). Stake your trees to support them in the early years and shelter is advisable in windy areas – for commercial plantings this means shelterbelts planted several years in advance, or a windbreak structure for small numbers or individual trees in the home orchard.
Culture and care
If you live in a region which experiences less than 750 mm of rainfall per annum, your walnut trees will require additional irrigation for satisfactory growth. Nut development progresses swiftly in the period immediately after flowering and a lack of available moisture during this period will have a detrimental effect on the size, quantity and therefore quality of your nut crop. Getting the irrigation balance just right is key – cold, high rainfall conditions during this growth phase can result in ideal conditions for bacterial blight to set in.
Walnuts are not heavy feeders in terms of fertiliser; a balanced general NPK fertiliser that also includes micronutrients (particularly boron), applied in moderation in early spring, will be beneficial. Try Tui Novatec® Premium at 60 g/m2 applied around the dripline and watered in well after application.
A walnut tree in the home orchard probably won’t require a lot of pruning, as they form an open, spreading branch and canopy structure without much human intervention. The time any pruning cuts to remove the usual suspects (dead, damaged or diseased wood) for late summer, when sap flow is low, as walnut trees lose a lot of sap if cut in spring. Walnuts can actually be tapped for their sap, just like maple trees. Commercially, walnuts are trained to a modified central leader system, with the first scaffold of four to five branches beginning 1.5 m above the ground to allow for mechanical shaking at harvest.
Pests, diseases and what to do about them
The main diseases affecting walnuts in New Zealand are a walnut blight, caused by the bacterium Xanthomonas arboricola pv. juglandis; crown rot, caused by the fungus Phytophthora cactorum; and Armillaria root rot, from the oak root fungus Armillaria mellea. Poorly-drained soils and unsuitable climates provide ideal conditions for walnut blight and Phytophthora to take hold - selection of resistant rootstocks (e.g. J. nigra) can help mitigate the detrimental effects of both Phytophthora and Armillaria. Bordeaux mixture (a combination of copper sulphate and quicklime) has been applied to walnut trees pre-flowering to help with blight management. In terms of insect pests, codling moth (Cydia pomonella), leafroller caterpillars and walnut blister mites (Aceria erinea) may make an appearance. Like the craggy centuries-old roadside specimens, well-situated small or individual plantings of walnuts in regions with dry climates should be fairly easy-care to and require minimal intervention.
Varieties: My top picks
A grafted walnut tree is a significant investment, costing between $80-110+ in the current market.
The New Zealand Tree Crops Association conducted extensive research trials looking for suitable cultivars for the New Zealand industry in the 1980s. The following are two local selections that are widely available:
Rex – a compact tree, this cultivar makes up a large proportion of the current commercial crop in New Zealand. Produces relatively small but nutritious nuts but begins to crop early on and has a degree of blight resistance. Expect trees to reach about four to five metres in height by five years of age and approximately 12 at maturity.
Meyric – found in Hawke’s Bay by NZTCA member Chris Ryan. A larger, more vigorous tree than Rex. Heavy cropper producing large, sweet, easy-to-crack nuts.
Serr – a heat-tolerant American cultivar that is also resistant to sunburn. Large nuts in thin but well-filled shells. Early to mid-season cultivar.
Lara – a French cultivar, imported for industry trials in 2011. A compact, early-bearing tree with delicious creamy-tasting nuts that are easy to crack. (I’m working my way through a box of these at present and rate them highly).
Franquette is a late-leafing, slow-growing traditional French cultivar that may be of interest to those in colder climates, as it should come through late frosts in better shape than earlier-leafing cultivars such as Serr. Some resistance to blight and codling moth.
What to do with your crop
The walnut harvest commences around the end of March and continues until early May. There is usually an early drop of nuts prior to this, but these will be duds – diseased and therefore empty shells. Mow the grass under your trees in preparation – it will make the collection a lot easier. If you have large quantities to pick up, many handy gadgets are available online. Wet weather at harvest time can result in shell and kernel stain, so get your crop off the ground and onto drying racks in a warm, sheltered location ASAP. Wash or brush off any hulls still adhering to the shells. If possible, dry the nuts in the sun, rotating daily. Curing will take approximately six weeks until the kernel inside firms up to the point it will snap easily.
Store walnuts in-shell in net bags in a dry, rat-free location, or invest in some wire-mesh ‘nut safes’ if critters threaten the longevity of your supply! Shell nuts as you intend to use them for maximum freshness, or shell ahead and store in an airtight bag or container in the freezer. Walnuts go rancid at the drop of a hat and life is too short to consume nasty nuts. Walnuts are a nutritional powerhouse, with a small handful daily providing a potent hit of omega-3s, vitamins and minerals (including abundant calcium).
Young green walnuts can be brined and then pickled in spiced vinegar (they must be harvested before the shell forms when a darning needle can be pushed right through the immature fruit). The Italian liqueur nocino is also made from immature nuts.
Walnuts are the baker’s best friend – and now you know the virtues of fresh New Zealand walnuts vs. the supermarket horrors, make your next carrot cake, brownie or batch of chocolate-iced, walnut-topped biscuits sing. My two favourite walnut bakes are New Zealand culinary stalwart Robyn Martin’s Italian Fig Cake with its airy sponge and marmalade-spiked, walnut-heavy fig filling, and the Australian Women’s Weekly test kitchen-approved Armenian Nutmeg Cake – recipe can be found here: https://www.womensweeklyfood.com.au/recipes/armenian-nutmeg-cake-16518
I’m partial to handfuls of fresh walnuts as a snack and as a hearty addition to a Vegemite sandwich. They are a boon in vegetarian recipes, adding texture and richness and making a mean meatless ‘sausage’ roll. They work well in stuffings, pesto and dukkah (even better when the latter is served with walnut oil for dipping) and take coleslaw to the next level. Elizabeth David suggests a simple sauce for tagliatelle consisting of butter, mascarpone, parmesan and roughly chopped walnuts – yes, please!
Whatever your preference is, sweet, savoury or somewhere in between, please make sure to choose fresh New Zealand-grown walnuts, be they begged, borrowed or foraged! For more information, see https://walnutsplease.nz/ and the New Zealand Walnut Industry Group’s website https://walnuts.org.nz/
Anna-Marie Barnes is an active member of the New Zealand Tree Crops Association who endeavours to grow and preserve as much of her own fresh produce as possible. When the weather’s no good for gardening, she can usually be found inside working on a batch of homemade cheese or soap.
The New Zealand Tree Crops Association is a voluntary organisation promoting interest in useful trees, such as those producing fruit, nuts, timber, fuel, wood, stock fodder, bee forage and other productive crops. Find out more about the NZTCA here: https://treecrops.org.nz/
Cracked walnuts – marijana1 via Pixabay.com
Green walnuts – Konevi via Pixabay.com
Mature walnut tree - By Thesupermat - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2822939