Black walnut: plant, propagate & use Juglans nigra

The black walnut can defy climate change. It impresses with its shade-producing crown, heat tolerance and special nuts.

Black walnut (Juglans nigra) could be described as the somewhat more demanding sister of the walnut (Juglans regia). This is because its requirements – for example, soil or water supply – are somewhat higher. Thanks to its good heat tolerance, it can be counted among the future-proof climate trees. If the black walnut feels comfortable in its location, it grows quickly, making it an excellent choice as an imposing shade tree for large gardens. This article explains everything you need to know about growing, from planting black walnut to care and harvest.

Black walnut: leaves, fruits and origin

The black walnut belongs to the walnut family (Juglandaceae). Originally from North America, it came to us across the Atlantic as early as the 17th century. In recent years, its importance has once again increased dramatically. Namely, it needs a warmer climate, wind resistance and little susceptibility to diseases and pests. Thus, Juglans nigra also lends itself to forestry as a substitute for less heat-tolerant tree species. A nice side effect: their noble, reddish grained wood is decorative and in great demand.
The leaves of the black walnut are pinnate like those of the walnut, but instead of five to nine, the black walnut has up to 23 leaflets. These sit in pairs or unpaired on 30 – 60 cm long petioles.

Black walnuts are monoecious, so male and female flowers occur on one tree. However, self-fertilisation is prevented in most black walnut cultivars by their dichogamy. This means that the female and male flowers of a tree bloom at different times and therefore cannot fertilise each other. Therefore, for a rich harvest, it is necessary to have another black walnut variety with a different flowering time in the near vicinity.

At first glance, the fruiting stalks of the black walnut resemble the familiar walnuts. On the tree, the nuts are covered by a yellow-green outer shell, which can be up to 6 cm thick. Only after the fruit falls from the tree in the autumn, it begins to turn black and rot. This reveals the actual nut, which is thick-shelled, dark brown, furrowed and ribbed.

Planting black walnut trees: where, when and how

The location requirements of the black walnut are relatively high. The soil should be nutrient-rich, deep, loose, slightly acidic to calcareous and well supplied with water. In addition, Juglans nigra prefers to grow in sunny, warm places sheltered from the wind, often found in vineyard areas. Since black walnut trees usually grow over 30 m tall and develop large, spreading crowns, they need a lot of space – both above and below ground for the extensive and deep root system.
Once you have found a suitable location, you can start planting the black walnut.

• Best time for planting: April to November; Attention: young trees are still sensitive to frost.
• Dig planting hole comparatively large: at least 50 x 50 x 50 cm.
• If necessary, improve soil with nutrient-rich potting soil such as our Plantura Organic Enriched Compost. The high proportion of organic matter has a positive effect on water retention, among other things.
• Insert tree, set tree stakes for a connection.
• Fill planting hole with mixture of soil and nutrient-rich potting soil and press down well.
• Drive tree stakes deeper if necessary, tie stable connection.
• Model watering ring from excess soil and mulch tree disc.
• Water generously and water regularly in the following weeks.

You can also grow a black walnut tree from seed, but then it takes at least 10 years before you can harvest the first fruits. The procedure is described in more detail in the section on propagating black walnut.

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Tip: If you want an abundant harvest, you should also make sure that there are other varieties of black walnut in the immediate vicinity. This allows the trees to pollinate each other.

The right care

The black walnut tree only really needs some attention in terms of care during the first years of growth, as it is still in the critical growth phase. Thus, in the early years the tree should be watered regularly during prolonged periods of drought and fertilised annually in the spring. Our Plantura All Purpose Plant Food is ideal here, which releases nutrients into the soil over a longer period of time and thus ensures a continuous supply for your black walnut.

When the tree is older and well grown, it usually does not need fertilising thanks to its large root system.
In the early years, the Juglans nigra can also be pruned but this is not absolutely necessary. You can use walnuts as a guide for pruning. The timing should be in summer, otherwise the black walnut tree will “bleed” heavily, that is, pump water and nutrients out of the cut via the water-bearing xylem. Black walnuts should not be pruned at the time of budding. Otherwise, the cut can weaken them so much that they perish.

One maintenance task that becomes more labour-intensive with age is removing leaves in the autumn. In fact, the leaves contain a glucoside that is converted to juglone by microbial degradation and oxidation. Juglone is a so-called allelopathicum that inhibits seed germination and seedling growth in the eaves of black walnut trees, which can even kill turf.

Tip: Like walnut trees, black walnuts can also be infested by the walnut fruit fly (Rhagoletis completa), although the pesky critters usually only destroy the green hull. Nevertheless, you should collect and dispose of infested fruit, otherwise the larvae will pupate in the soil.

Propagating black walnut trees

Propagation of black walnuts can succeed in two ways: firstly, through cuttings and secondly, you can also try to grow trees from the nuts. Another option would be grafting, but this is rarely successful in the case of black walnuts and is carried out only by professional garden centres.
Propagation via cuttings is usually much easier and faster.

Propagating black walnut via cuttings:

• take non-woody, healthy cuttings without fruit set between June and August.
• Suitable cuttings are young shoots on the black walnut tree, which are at least 20 cm long. The tip of the shoot is cut off after 15 cm here. Even slightly woody shoots can still be used well. It is also convenient to select multiple shoots. This way you have a replacement if one of the cuttings fails to grow.
• Put the shoots in planters filled with soil as rich in nutrients as possible. Our Plantura Organic All Purpose Compost is a sustainable choice. If necessary, you can put several cuttings in one planter.
• Press the soil firmly and water the cuttings well.
• The cuttings like it warm and protected, so a place with indirect sun is optimal. They should also be watered regularly, but waterlogging should be avoided at all costs.
• For the first few years, the cuttings should still be overwintered in a cool but frost-free location; they are only winter hardy after about two years of growth.
• If the first, new leaves form on the shoots, you can tell that the cultivation was successful and the cutting has taken root. Now the cuttings can move, each in its own pot. The rule is: the larger the pot, the larger the root system of the black walnut seedling. Also, the stronger and more robust it is when planted in the final location.

Black walnut trees can also be grown from the nuts. For this purpose, you should preferably use fresh fruit because the longer they are stored, the more the germination capacity decreases. You can try to sow the nuts directly in the garden about 5-8 cm deep in the soil. However, in the process, many nuts fall victim to mice, so growing in pots offers greater chances of success. Alternatively, you can store the nuts cold in the refrigerator for 3 – 5 months over the winter and sow only in the spring.

• In the autumn, put a few fresh nuts that you have freed from the fleshy fruit shell in a pot filled with potting soil and cover them with soil. If sustainability and organic quality are important to you in the selection of soil, we recommend our Plantura Organic Herb & Seedling Compost. It is peat-free and even the packaging is certified with the Blue Angel.
• Ideally, place the pot in a bright, sheltered place where it is cool, but frost-free.
• The soil should be kept constantly moist over the coming period, but without waterlogging, otherwise the seeds may form mould.
• The first shoots should appear in spring, which shows the germination of black walnut. Then the time is right to transplant the black walnuts. Each should be given its own pot, which may already be filled with a slightly more nutritious substrate.
• Plants should be watered regularly and still protected from late frosts.
• It is best that these black walnut seedlings also spend the next two years in pots so that in winter when temperatures are very cold, you can put them in a frost-free place for a short time.
• After two years, the plants should then be ready to be planted in the garden.

Harvest, effect and use: is the black walnut edible?

Black walnuts can be harvested in the autumn. However, you should not pick them from the tree but wait until they fall off by themselves. They should first be freed from the green/black skin and then dried.

Black walnuts are non-toxic and even very healthy. It is just not that easy to get your hands on the edible black walnuts. Cracking is hard work and usually cannot be done with regular nutcrackers. But with a little creativity, you can certainly find a solution; for example, you can get to the nuts with a hammer. You should use cracked nuts quickly or store them as cool as possible, otherwise they quickly become rancid. Use other nuts as a guide when using the black walnut. In America, where it is considered a delicacy, it forms the basis for pastries or ice cream.
Some healing properties are also attributed to black walnuts. For example, the nuts can lower blood pressure. The hard nut shells are true health wonders, which can be ground and then processed into tinctures. These can be applied externally, for example, for herpes or warts, but also help with digestive disorders or high blood pressure, among other things.

Did you get a little respect for planting black walnut trees in your garden after all? Ordinary walnut trees offer a good, somewhat less demanding alternative. Everything you need to know about cultivation of walnuts is summarised for you in a separate article.