Deaths walnuts really their neighbors? - Of the 2

I the first part of this post I noted that most plants use chemicals to influence their environment to their own advantage. Best known in this context, as I said walnut plants producing the substance juglone. Juglone is a basically harmless substance found in all plants in the family juglandaceae, Juglandaceae, including the genus Carya (hickorysläktet) and Pterocarya (pterocarya) addition genus Juglans (Juglans). Only when the juglone in contact with oxygen it oxidizes and becomes a herbicide. The only well-studied species in this context is the black walnut, which are shown to continuously secrete juglone through their roots. For the other species in the family we do not know the degree to which they secrete juglone through the roots, although it is known that the substance found in their tissues [1, 3, 6]. It is not entirely clear what happens in detail when susceptible plants are exposed to juglone, but the effect seems to be to plant uptake of water and nutrients is disturbed, which inhibits their growth and which, in combination with other stressors can lead to plant death [4].

year garden book 2018, finally back in trade!

My interest juglonfrågan increased again when I visited the forest Steiner Wald in Germany where black walnut had been planted in a forestry experiment almost 100 years ago, according to the annual rings I counted on fallen trees and uppkapade. I was greeted not only by the towering black walnut tree with straight, knotless strains, but also of an entire forest ecosystem that is so rich in species that the entire forest has become a 200 hectare reserve. It grew a lot of other plants seemingly unaffected by the black walnut. Maple, oak and ash coexisted nicely in the tree cover and the bushes were both Hazelnut, Currants and elderberry. Örtskiktet had just started to wake up when I visited the forest, but where there were already as wild garlic, jordreva, squill, lesser celandine, wood anemone and many other herbs I did not recognize.

Wald, herbaceous layer
How does it look in the nature reserve Steiner Wald. Quite rich in species to be a forest dominated by black walnut.

At the same time, I read headlines such as "The Curse of the Black Walnut"And"Black Walnut: the Killer Tree"Where the black walnut is described as a real threat. How does that work with my own observations from Steiner Wald and from virtually all other places where I've seen black walnut in the real world where it seemed to behave quite nicely against their neighbors?

Soils choice determines

There are several answers to the question. If we start with the farmers' observations that the black walnut kills everything that man planted in it has in my opinion, probably at least as much to do with improper ståndortsval the fact that black walnuts produce juglone. For example, you read that it is difficult to plant things just below the crown of black walnuts. Where is the black walnut is not alone. In nature, it grows namely in the fertile river valleys of the deep, well-drained soils with good access to the moving ground water [2]. It is in other words used to almost unlimited access to water and if planted on a little too dry habitat is only due to water competition is difficult to grow other near a black walnut trees. The same phenomenon occurs around most of the trees that are dry and all who have tried to plant other next to a birch tree growing on a dry habitat know what I mean. Another problem mentioned in the countless farming guides (for example this) is that black walnut shade the ground very much. The combination of shade and drought is perhaps the most difficult site conditions, and it is no wonder many fail to cultivate demanding ornamental trees for their black walnut trees. It is not possible to blame the walnut reigns of forget-me-or rhododendrons do not want to feel comfortable.

What science says?

The scientific literature also provides some (unfortunately somewhat contradictory) answer the question. In fact, all the walnuts secrete juglone and black walnut seems to be the one with the big margin secretes most of the substance [6]. Excretion is continuously all through the roots, and to a small extent also by plant parts that fall down on the ground where it is completely harmless substance hydrojuglon rapidly converted to the plant toxin juglone [5].

Juglans nigra trunk, Wald
Black walnut (Juglans nigra) is an extremely valuable timber trees. But it is also a dead tree?

Juglonets toxicity has mainly been studied in laboratory experiments. There are grown various crops in hydroponic (earth-free) conditions and then subjected to unnaturally high concentrations of pure juglone added irrigation water. Although seeds are exposed to juglone and a through observation is that the germination rate decreases significantly for certain crops Tomato, cucumber and soybean (but not for corn and wheat) [5]. Black walnut is again the culprit, whereas extracts from hickory nuts of point (Carya illinoiensis) and walnut (directed Juslans) have only little effect on sensitive the seeds germination [6]. I can understand the approach chosen in these lab experiments, because it is difficult to study juglonets effect in natural settings. In lab experiments would simply eliminate all possible influencing factors and control as many parameters as possible. Unfortunately, it has nothing to juglonets power of nature to do at all. In natural ecosystems, there is a lot of life in the soil, arts same attachment above and below ground are different in different places, juglonkoncentrationen vary throughout the year and between individuals, there are different amounts of water, heat, light and so on. The few times they have measured juglonhalten in soils have its effect most likely heavily overestimated, because we do not know whether the plant-available juglone is when it is bound to clay particles and organic matter [5].

Black walnut on ramson
Ramslök (Allium ursinum) seems in any case do not have any problems to grow in black walnut.

Yet there are some indications that at the black walnut can kill a few species under the right circumstances, even in field trials. For the other species in the family juglandaceae missing my knowledge field trials. The results are coming primarily from the alley cropping and commercial forestry where black walnut planted with other plants. On our pine quite closely related weymouthtallen (Pinus strobus) seems to be extra sensitive to black walnut. What role juglonet play in this context is not clear. While North American species of elm (Ulmus), ask (Fraxinus), björk (Betula) and even our native alder (Alnus glutinosa) grows worse or die if planted along with black walnut. In the natural forests have been discovered is specialized bacteria that can break down the juglone [3]. Perhaps it has somehow emerged such bacteria in the German forest I visited?


What conclusions can we draw from this? I feel in any case not very worried about all the walnut trees we have planted will create problems in our plantations. Our black walnuts grow on a deep, humus sandy soil in a valley crossed by lots of small streams, which almost perfectly match their favorite habitat. Here comes the water competition does not become a problem, and the trees are so widely spaced that there is no greater risk of juglonkoncentrationen the soil becomes extreme.

For the other species in the family juglandaceae there who said no evidence that their juglonproduktion would be a problem at all. In my German homeland planted walnut interspersed among the apple trees of the old fruit orchards and apple trees do not seem to suffer any harm by it, although they are often alleged to be extra sensitive to juglone.

Apple trees and walnut planted together
It has planted a number of apple trees near a slightly older walnut trees.

Do you still worried and want to create planted together around your walnut trees, you can enjoy access to juglonkoncentrationen decreases rapidly with increasing distance from the tree. Experiments have shown that juglonkoncentrationen already at barely 2 m of a black walnut trees decreases to less than half compared to the concentration at 1 m [3].

The important thing is to simply place the walnut trees in the right place from the start and live with it is not possible to create complex planted together everywhere.


[1]         HAJ, M.M. and K. Kamel, Determination of Juglone (5-hydroxy 1, 4-naphthoquinone) in Pterocarya fraxinifolia by RP-HPLC. 2006.
[2]         Honkala, B.H. and R.M. Burns, Silvics of North America. Volume 2: Hardwoods. Forest Service, United States Department Agriculture, 1990.
[3]         Jose, S., Black walnut allelopathy: current state of the science, in Chemical ecology of plants: Allelopathy in aquatic and terrestrial ecosystems. Springer. p. 149-172, 2002.
[4]         Jose, S. and E. Holzmueller, Black walnut allelopathy: implications for intercropping, in Allelopathy in sustainable agriculture and forestry. Springer. p. 303-319, 2008.
[5]         Jose, S. and E. Holzmueller, Black walnut allelopathy: implications for intercropping. Allelopathy in sustainable agriculture and forestry: p. 303-319, 2008.
[6]         Qin, C., M. Nagai, W. Hagins, and R. Hobbs, The allelopathic effects of juglone containing nuts. The Journal of Experimental Secondary Science, 1, 2011.

4 thoughts on “Deaths walnuts really their neighbors? - Of the 2”

  • What does the practice; I live in Malmö and has
    Two walnut tree that grows and bears fruit (self-seeded).
    I have directly during coughing rhododendron hydrangea, pink
    And within radius of 2 m apple trees.
    The drought has reduced the amazing cough but I replant.
    I have not seen the impact als!

  • I have planted a manchurian walnut tree 2009. Beside there were five stately plants knightly spores that after four years completely thinned away. I thought it was some kind of pest insect that was ticking them off. But it can thus be the influence of juglon? Two years ago, I planted five plants “Jack Frost”, caucasian forget-me-not, in a row there instead. It was not easy to dig there in the soil because the walnut had very superficial roots. Now this year, only two plants are ok, three are probably giving up (unless I move them of course) What thrives directly during the walnut is with me, flocknävan, has grown there ever since 2009.

  • A few years ago I planted 3 apple trees in very rocky soil here on Gotland. The following year I planted approx 20 small walnut trees (from Czech walnuts) in pallet collar about 1 / 2m from apple trees. Now one of the apple trees has died and the other two look pathetic much like when they were planted..The walnut trees are fine and should be moved to other places. First thought there was something wrong with the apple trees (bought at Lidl …!) Or the earth, but after reading here about the possible effects of walnut trees on neighboring trees, it seems to be a probable explanation.

    • Tack för att du delar dina erfarenheter! However, I have a bit of a hard time believing that small walnut trees could kill apple trees nearby. There are many other things that can kill apple trees, not least sork, water shortage, soil diseases, etc.. But nice to hear that the walnut trees are doing well! /Philipp

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