Should you be in Serbia at Christmas, you may well see men running around their houses chanting (in Serbian) the equivalent of Do not listen, God, to Jack, who is full of cack (cack being excrement).

Christmas in Serbia is at a much more sensible time when the crops are gathered, the pantry is full and the weather more suitable for cosy socialising, and it is when they crack the first walnut of the harvest. The chant is done to thwart the bad omen of discovering a shrivelled kernel apparently an ominous sign that the forthcoming year will not be prosperous. Such is the importance of walnuts in Europe and what was once Persia, which is where our common, so-called English, walnut originated from.

As far back as the fourth century, Alexander the Great recognised the attributes of the nut of the god Jupiter (which is where the Latin name for the walnut family, Juglans, originates). Since then, hybridisation and selection evolved the trees that still dominate the forests of Kyrgyzstan into the trees with the larger fruit that we know today. They were traded on the Silk Road, the Romans distributed them throughout Europe, and the Reverend Paul Crath imported tonnes of seeds into America in the 1930s. English merchants took walnuts aboard ships and to the rest of the world, but they were never grown in England on any extensive scale.

In the 1980s, the New Zealand Tree Crops Association (NZTCA) set about to select varieties suitable for our climate. Members collected more than 800 walnuts from trees throughout the country, and selected nuts for quality (taste, colour), suitability for commercial production (size, shape, seal and storage) and crack-out (how full the shell is). The best were then grown at Lincoln University to determine yield, biennial bearing, form and flowering times.

SALLY TAGG/NZ GARDENER

Walnuts are known to benefit cholesterol levels, heart function, gut health and brain power.

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A walnut seedling.

This research has initiated a walnut industry based in Canterbury which produced more than 200 tonnes last year and expects to triple that when trees already planted mature.

The top performer of the Lincoln trial was cultivar number 152, henceforth called Rex, and it constitutes about three-quarters of the New Zealands commercial crop.

Rex is a lateral bearer yielding heavy crops from a young age, but its small nut requires a factory to process.

Growers who sell in-shell nuts directly to customers prefer the larger nuts of the terminal bearing varieties from which it is easier to extract whole or half kernels. (Some walnut varieties bear fruit only on a terminal shoot at the end of one-year-old wood. These are called terminal bearing varieties. Other varieties bear fruit on shoots sprouting from the full length of the one-year-old wood. These lateral bearing varieties lead to higher yields when the tree is young.)

North Island growers in particular favour the long nuts of Meyric, or the heavy crop of Shannon. Trials and research in different locations prove that seasonal influences are the biggest variable affecting nut production, and variety selection to suit regional climates is important.

MELANIE WAKEMAN

Walnuts at various stages of dryness.

Propagation & pollination

Because walnuts can cross-pollinate, a seedling-grown nut can be an unknown mix of both parents.

Seedlings are reputed to take a little longer to start bearing nuts, but several experienced growers refute this, saying tree care and variety selection have a bigger influence on bearing age. In general, seedlings tend to be more resistant to pests and disease, so they grow to become a healthier and more robust tree.

Since grafted cultivars can be expensive ($30$114) and of limited supply, it is certainly worthwhile finding a good walnut tree in your area and planting some nuts from it.

Plant the nut on its side, so when the embryo splits the shell, it does not fill and hold water.

Walnuts are self-fertile, but the chances of selffertilisation are not high. The overlap of pollen shedding and receptivity of the female flower is very short. However, walnut pollen can travel for many kilometres on the wind and even seemingly isolated trees usually get pollinated.

Climate too plays a part with warm weather inducing early pollen release and extending its viability. For maximum production, commercial growers plant two different varieties to cross pollinate one another.

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The first leaves of Juglans regia.

Care & maintenance

Walnuts take seven to 15 years to get into production, and a tree needs about 10 square metres. If you have the space, a walnut tree provides a beautiful shade tree with very little maintenance. They dont usually have single leader and naturally form a lovely rounded shape. They dont usually need to be pruned either.

They will grow in most situations and soils doing better in sheltered fertile soils, of course but will not tolerate wet feet.

On planting, dig the hole at least a metre deep to ensure good drainage.

Boron is notably lacking from most New Zealand soils. Deficiencies can reduce a walnut crop by up to 300 per cent and cause new growth to snake head, growing up and over like the handle of a walking stick.

OrganiBOR is a simple way to add boron to your soil.

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Catkins in spring.

If there is any possibility your soil might have Phytophthora, then it is worth the trouble to get a grafted tree on Juglans nigra rootstock.

The most common issue is walnut blight a fungus that attacks the nuts and new growth, and thrives in warm wet spring weather. Some cultivars are naturally more resistant.

Planting where there is good airflow hampers fungal growth, as does copper spray at bud burst.

Once established, walnuts will bear nuts for some 120 years.

Just to be confusing, a walnut is not a nut. Like a peach, it is a drupe with flesh surrounding a kernel. This flesh, aka the husk, contains a strong brown pigment which can be made into a dye for fabric or hair, or an ink that artists such as Leonardo da Vinci and Rembrandt used.

The shells are great fire starters and are commercially crushed for use as an abrasive (the walnut equivalent of sand blasting) or added to facial scrubs as an exfoliator.

The foot of the trunk is sought after for gunstocks and even the leaves were used in a decoction for treating diabetes in traditional Austrian medicine.

All of the Juglans family produce juglone, a toxic substance that reputedly stunts the growth of neighbouring plants.

However, growers and orchardists in New Zealand have not seen any effect of this perhaps something in our soil counteracts this. Juglones is reputedly strongest in the black walnut (Juglans nigra), which though edible is small and hard to extract and usually only grown for its timber.

Juglone has been used as a herbicide and is also toxic to many insects and at least one farmer I know specifically plants them as shade trees for his cattle, observing that they deter the flies.

SHERYN DEAN/NZ GARDENER

Picked green, before the shell forms, any of the walnut family can be pickled.

Foodies favourite

Walnuts are known to benefit cholesterol levels, heart function, gut health and brain power.

Somehow though, they have fallen out of fashion and less than 5 per cent of people surveyed now eat them regularly.

Part of this could be attributed to the lack of fresh, tasty nuts. Without the space or patience to grow our own, we buy old, shrivelled and imported nuts, store them until they are rancid, then throw them into a carrot cake or the bin.

New Zealand walnut growers are horrified by such practices. For those in the know, a fresh, well-harvested and stored walnut is a gourmet food. Its high oil content delivers plenty of health benefits, but these omega-3 and polyunsaturated fats go rancid with time and warm temperatures.

New Zealand nuts are harvested as soon as they fall, and are dried and sweetened with optimum air-flow and temperature. Our cultivars have been selected to produce a honey-coloured, sweet and tasty nut, which, though it will store in shell for about a year, is kept at premium if cracked and stored in an airtight container in the fridge or freezer until required.

SHERYN DEAN/NZ GARDENER

Wanganuis Nuts 'n May Orchard.

Picked green, before the shell forms, any of the walnut family can be pickled. The best are the small and sweet heart-shaped nuts of Juglans ailantifolia, the Japanese walnut, which is classified as invasive as it has overwhelmed many riparian zones in the North Island. Foraging the nuts will help prevent it spreading.

Another member of the family worth growing is the butternut (Juglans cinerea). It is a large tree producing football-shaped nuts which have a sweet, creamy flavour more like a pine nut than a walnut.

Most walnut orchards in New Zealand are certified organic. But walnuts are so easy to grow, even if not certified, it is unlikely the crop has needed a regimen of nasty chemicals.

SHERYN DEAN/NZ GARDENER

Canterburys walnut industry produced more than 200 tonnes last year and expects to triple that when trees already planted mature.

A number of New Zealand growers sell their walnuts. You can find their contact details online, walnutsplease.nz. The website also has recipes and more information about walnut growing and storage.

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Going nuts about walnuts: How to find the best for your region - Stuff.co.nz