Cultivation of the eastern American black walnut and hybrid walnut

The black walnut is native to eastern North America, where it has a large distribution area with considerable climatic differences, and where it has developed various adapted ecological varieties. It is common in mixed forests up to about 1200 m above sea level, mainly in association with the white ash Fraxinus americana and the tulip tree Liriodendron tulipera. It grows best on the floodplains of the River Ohio and in the Mississippi lowlands. In these areas it can reach heights of up to 45 metres. One author suggests an average annual temperature of 13 °C, at least 170 frost-free days, and an average annual precipitation of at least 890 mm as the optimum conditions for the black walnut. In its home range, the black walnut is valued not only for its high-quality and valuable wood, but also for its edible seeds (nuts).

The first mention of the black walnut in Europe dates from 1629 in England. In Germany it was first referred to in Leipzig in 1686. The first forestry trials with black walnut in Europe were carried out in the riparian forests along the rivers Rhine and Danube, and particularly extensively in the Prussian state forests from 1881 onwards. In these coordinated cultivation trials with exotic tree species, which aimed to ensure a supply of valuable timber species, the black walnut was seen as a particularly promising tree species. In the Prussian state forests alone, black walnut was sown and planted on experimental areas covering a total of 64 ha between 1881 and 1884. In 1939 it was reported that the trees on only around 10% of these experimental areas had reached the age of 30. The reasons for this high rate of failure are still quite unclear. From 1882, a forester called Rebmann began to establish extensive plantations of walnut on a total of around 60 ha in the Rhine lowlands between Strasbourg and Colmar in Alsace. He became known, sometimes derisively, as the “nut man” as a result of his strong commitment to walnut plantations. However, about 85 % of these walnut plantations in Alsace failed. Today, a few black walnut stands of more than a hundred years old are to be found on the best riparian forest sites along the Rhine (Fig. 1), and otherwise in Europe mainly on the floodplains of the Danube in Hungary and Romania. The provenance of the black walnut seeds used at the time is unclear, but a large proportion of the seeds were obtained through the BOOTH nursery. At the time, around 250 individual trees in parks and botanical gardens were available to Rebmann for the harvesting of seeds for his Juglans nigra and Juglans cinerea cultivations in Alsace.

Another very impressive walnut hybrid stands in the Steiner Wald forest, Lampertheim Forestry Office (Fig. 4). This approximately 115-year-old Juglans intermedia has a diameter at breast height of 120 cm and a calculated trunk volume of 12 cubic metres (standing gross volume).

The advantages of walnut hybrids for forestry cultivation were first recognised in France, and thanks to intensive cooperation between forestry researchers, nurseries and forestry practitioners, various walnut hybrids have been bred, tested and cultivated since the early 1970s. In Baden-Württemberg the first walnut hybrids in the forest were described in an unpublished thesis by a trainee forester in 1985. The author found these incidental crosses in several black walnut stands in the area managed by Breisach Forestry Office. By that time, seed orchards had already been established in France for the production of hybrid walnut seeds.

Two walnut hybrids recommended for cultivation in southern Germany and for which planting material is available commercially are presented below:

Intermedia hybrids (Juglans nigra x Juglans regia); the commercial varieties, available from France, are NG 23 and NG 38. NG stands for Juglans nigra. Black walnut trees were selected for their quality, growth performance and especially their flowering time, and numbered. Seed orchards were established from these individual trees by taking scion cuttings and grafting. The trees are usually pollinated by the FRANQUETTE walnut (Juglans regia) cultivar. The German commercial variety RENI is a protected variety produced by the Schott specialist walnut nursery in the Kaiserstuhl region in south-western Germany. RENI stands for Juglans regia and Juglans nigra. The selection of the trees and establishment of the seed orchard was broadly similar to the procedure in France. The mother tree of these intermedia hybrids is Juglans nigra, and the father tree is Juglans regia. All plants available to date are seedlings from seed orchards. The company is working on the micropropagation of the commercial variety RENI, and the first plants were planted out in 2018. In southern Germany, the Intermedia hybrids are the preferred choice for planting, and recommended. The Intermedia hybrid has become known in southern Germany as “Intermedia”, and this word is now generally used in the German language as a synonym for hybrid walnut. The parent species do not hybridise too well with each other, so the progeny are available only in small quantities. In rare cases, incidental crosses of the Intermedia hybrids may occur in black walnut stands (Fig. 4).

Garavel hybrids (Juglans major x Juglans regia); the commercial variety, available from France, is MJ 209. This hybrid is widespread in France and the hybrid walnut most often cultivated there, as the parents hybridise easily and the plants are available in sufficient quantities. In vitro propagation is possible (Fig. 5). The Garavel hybrid develops leaves somewhat later in spring in Germany than in France and is therefore less vulnerable to late frost.

Both black walnut and hybrid walnut require deep, well-aerated soils rich in nutrients and with pH values between 5 and 8. Free excess lime is not a limiting factor, but black walnut trees are less tolerant of free excess lime than the walnut Juglans regia. The black walnut is the most demanding in terms of its water supply requirements. The hybrid walnut can be cultivated in all areas down to the “moderately dry” category, albeit with a marked decrease in growth performance. Compacted and extreme alternately wet and alternately dry sites are just as unsuitable as sites with clay or gravel layers in the subsoil that are impenetrable for the walnut roots, as well as sites with very high groundwater levels. The average annual mean temperature should be at least 8 °C.

In its original homeland, the black walnut has a wide distribution area with extreme climatic differences. It grows best on lower slopes and in river valleys, in moist and fertile soils with an almost neutral pH. The soils are usually sandy loams or loam with alluvial sand. The roots must be able to penetrate to a depth of at least 0.8 m. In Germany, the black walnut shows the best growth on floodplains and in valleys with access to the groundwater, but it also thrives on deep, well-drained loam soils. It tolerates short-term flooding. The many unsuccessful attempts to cultivate the black walnut around 1900 can presumably also be attributed to the selection of unsuitable sites.

The hybrid walnut is suitable for all sites that provide the conditions described above as necessary for walnut trees. The hybrid walnut thrives on a significantly larger range of sites than the black walnut. It is not as demanding as the black walnut when it comes to its water supply, but does of course grow more slowly in difficult conditions. In the district of Ludwigsburg, great success has been reported with hybrid walnuts up to 20 years old on sites that are very well supplied with nutrients, but less suitable for black walnut because of the water supply. Some members of IG NUSS, a [German] interest group for the promotion of the cultivation of walnut trees, are agreed that “the (still young) walnut trees were the ones that survived the drought year of 2018 best” in their forest.

As the risk to the walnut species from browsing is low, there is no need for protective fences. Protection may however be necessary in individual cases to prevent damage caused by antler rubbing. Experiences with tree tubes vary considerably, probably because of the lack of ventilation in some cases. In practice, tree tubes are often used, because they make the plants easy to find, and offer a degree of protection when the area around each tree is mown. Only well ventilated tree tubes should be used (Fig. 8).

Especially in afforestation areas, competition from grass can lead to a severe reduction in tree growth. A circle around the tree with a diameter of a good 1 m should thus be kept free of dense grass. The adding of a few shade-tolerant tree species such as hornbeam or lime can also help to inhibit grass growth by providing shade and early closure of the tree canopy. It is also recommended that forked trees are pruned in the first few years. If forking or bushes of shoots develop (often the case after late frost), the strongest shoot should be selected and others removed with secateurs. This intervention should take place as early as possible in the vegetation period and as long as the wood is green, to ensure that the remaining leading shoot is promoted. The best time to do it is in June.

The question of when is the right time for pruning has been a subject of heated debate within the IG Nuss interest group, particularly in the case of walnut trees, with some even of the opinion that walnuts should not be pruned at all, as they can bleed to death from the sap they lose after pruning. One important hint was added to this discussion by an American colleague who, when asked when walnut trees were pruned in the USA, answered dryly: “when you have time”. In a trial carried out in 1996 and 1997 to study live pruning of the walnut at different times of the year, the walnut trees were pruned in February, June, August and November. Once the wounds had completely overgrown again, the growth performance values and branch samples of the pruned walnut trees were compared with walnut trees that had not been pruned. The results showed that live pruning of walnuts in February, June and August does not lead to problems.

After pruning in November, a wood-destroying fungus was found on a branch sample, so this time of year is not recommended for pruning. In practice, late winter dates for pruning - from February until budding starts - have proven their worth. During this period, especially in the floodplains, the areas are easy to see and walk on. It is possible that sap will “bleed” during this period. This is a natural reaction of the tree that prevents fungi and bacteria from penetrating into it. The flow of sap dries up again after a few days. None of the pruned walnut trees have bled to death; many have developed into wonderful trees (See Fig. 11). The other period in which pruning has been shown to work well is the summer period from June to August. Here it is a good idea to combine tending and pruning. The tending measures make the stand more accessible and give a clearer overview, and the various physically demanding tasks to be completed can be alternated over the course of the day.

Another contentious subject of debate was the thickness of the branches to be removed. In a figurative sense, the debate over the right time to prune the branches continued so long that the branches to be removed ended up being significantly thicker than the 3 cm diameter recommended in the specialist literature at the time. This crucial question is not only important for dealing with the walnut trees. It is also important for the follow-up treatment of stands planted following storm damage, which often have unsatisfactory qualities of oak and other broadleaved trees. For this reason, an extensive trial was set up in 2003 to study the live pruning of deciduous trees. The aim was to investigate the formation of water sprouts, discolouration, decay, growth reaction and the time taken to grow over wounds in oak, beech, ash and sycamore maple as a function of branch thickness and the date of pruning. It was specified that all pruning be carried out exactly according to the findings of SHIGO (1989). The result was impressive: As long as the correct pruning technique is used, the thickness of the removed branches is of secondary importance. The recommended thickness of the branches for removal was from 4 cm to a maximum of 6 cm. This does not mean that thicker branches cannot also be cut, but that the removal of thicker branches leads to a longer period before the tree heals over the wound, an increased risk of wood discolouration, and a thinner sheath of knot-free wood. In addition, the physical effort required for pruning increases enormously and the quality of the cut suffers as a result.

Traditionally, the branches are removed section by section, “from the bottom upwards”. Especially in the case of hybrid walnut trees, the branches in the upper part of the crown very quickly become very thick. This requires a different approach. This is why dynamic pruning, a combination of anticipatory pruning and section by section pruning, is often used in agroforestry (Fig. 12).