Deaths walnuts really their neighbors? - Of the 1

Many are reluctant to plant walnuts in their (forest)gardens because they have heard that walnuts can poison or even kill their neighbors with the help of a substance called juglone. I think I have contributed to the walnut terror by claiming that apples are very sensitive to juglone and by posting a list of plants that will be resistant to juglone in a post that actually belongs to one of the years most read this blog. As so often, it turns out, upon closer scrutiny, however, that the issue is a bit more complicated than one might think.

Walnut tree in Kalmar, 1799
The grass does not seem to be harmed by this magnificent walnut tree any secretions.

First and foremost, walnuts namely far from alone in wanting to influence their environment by using chemicals and it is the part 1 of this post will highlight. It seems even that most plants engaged in chemical warfare. The phenomenon called allelopathy and takes many forms. It may involve the production of airborne substances (Hosts that coins aromatic fragrances), rotextrudat (as walnuts juglone), substances leaking from the fallen plant parts or substances that are released when the leaves, seeds, flowers, twigs and so on are broken down. These affect other plants negatively and allelopathic plants can make use of one or even several ways to claim their place in the world [6]. The chemicals they produce can, among other things interfere with other plant respiration, photosynthesis, water uptake and transport within the plant, cell division and development, protein production and enzymaktivititet. In many respects, they work the same way chemically produced pesticides, but have far fewer side effects [3].

Mint allelopathic plant
Mint is one of many allelopathic plants that affect their environment by producing aromatic fragrances.

An example of a clever strategy that some plants have chosen is called pollenallelopati. Pollen from one species can put occupy the flower of another species and disturb or even render impossible seeds- and fruit development where. Often it is wind-pollinated species that exhibit this phenomenon [9] and there is some concern among scientists that could arise weeds that hamper pollination by wind-pollinated crops such as corn and wheat [7]. The point of the strategy is that a species can inhibit another species breeding and then take its place.

Other plants use a more indirect, but the cleverer way to disturb their neighbors. They secrete chemical compounds called phenols that completely alters the chemical structure of the Earth. The poison simply some microbes which benefits others may take their place. In this way alters the nitrogen balance in the soil and it is often the plants that survive on very little nitrogen that use this method to gain a competitive advantage, such crowberries (Empetraceae black) and Balsam poplar (Populus balsamifera) [1, 2].

Allelopathy may also be the explanation for the phenomenon known as soil fatigue that is talked about often in connection with the planting of apple trees and roses. When replanting of these are advised not to put them in a place where there has already been an apple tree or a rose bush, because the earth is "tired". The strange thing here is that quite a lot of plants are good to kill themselves. They secrete constantly allelopathic substances that build up in the soil and that makes them very poisoned, and that their offspring can not establish themselves in the same place. The whole thing is called auto-toxicity and is believed to be one of the driving forces for successionen, where some species suddenly and for no apparent reason may disappear and make room for new players [8]. Auto toxicity is very common and has been shown except the apples and roses also with tomato, cucumber, asparagus, peach, grape, cherry and strawberry [5].

Apples on tree
The apple belongs to the plants that are good to kill himself by so-called auto-toxicity.

For timber gardening grower is probably autotoxiciteten that is the biggest concern of all game species of allelopathy. In the wild, they break down toxic substances when burning, which it did with great regularity before humans began to intervene more and more in ecosystems. Therefore recommends the American forest horticulturists burning as a management method in the mature forest garden [4].

So much that is common allelopathy in plant kingdom - but how deadly is the walnut trees now? More on that in second part of this post.


[1] Castells, E., Indirect Effects of Phenolics on Plant Performance by Altering Nitrogen Cycling: Another Mechanism of Plant–Plant Negative Interactions, in Allelopathy in Sustainable Agriculture and Forestry, R.S. Zeng, A.U. Mallik, and S.M. Round, Editors. Springer New York: New York, NEW. p. 137-156, 2008.
[2] González, V.T., O. Junttila, B. Lindgård, R. Reiersen, K. consolation, and K.A. Braathen, Batatasin‐III and the allelopathic capacity of Empetrum nigrum. Nordic journal of botany, 33(2): p. 225-231, 2015.
[3] Haig, T., Allelochemicals in Plants, in Allelopathy in Sustainable Agriculture and Forestry, R.S. Zeng, A.U. Mallik, and S.M. Round, Editors. Springer New York: New York, NEW. p. 63-104, 2008.
[4] Jacket, D. and E. Toensmeier, Edible forest gardens. White River Junction, Vt.: Chelsea Green Pub. Co., 2005.
[5] Liu, Y.H., R.S. Zeng, M. An, A.U. Mallik, and S.M. Round, Autotoxicity in Agriculture and Forestry, in Allelopathy in Sustainable Agriculture and Forestry, R.S. Zeng, A.U. Mallik, and S.M. Round, Editors. Springer New York: New York, NEW. p. 283-301, 2008.
[6] Mallik, A.U., Introduction: Allelopathy research and application in sustainable agriculture and forestry. 1-7, 2008.
[7] Murphy, S.D., The role of pollen allelopathy in weed ecology. Weed Technology, 15(4): p. 867-872, 2001.
[8] Pickett, S.T., M.L. Cadenasso, and S. Meiners, Vegetation dynamics. Vegetation ecology: p. 107-140, 2013.
[9] Roshchina, V., A. Yashina, In. Yashin, and N. Prizova, Models to study pollen allelopathy. Allelopathy J, 23: p. 3-23, 2009.

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4 thoughts on “Deaths walnuts really their neighbors? - Of the 1”

  • When burning as a maintenance measure,, For example, one should burn down, for example, an apple tree and a few meters around it is about larger areas?

    • No clue. However, I suppose you need to come up to a certain temperature in order for the allelopathic substances to break down, so small burns may not be enough. Then it is important to remember that there is no reason to burn down the forest garden as long as there are no problems with autotoxicity.

  • We have planted an Apple tree (Like that) close to two walnut trees, without seeing that the walnut tree affects growth and well-being. From before planting the Walnut tree stood an old apple ( Filippa) which has begun to shrink, probably shallow age and fungal attack ( Raspskjellsopp Pholiota squarrosa). and two years ago, the oldest walnut tree died approx 25 years gml of clear fungal attack on root / tribe. Tommos stands just fine. Otherwise, the literature says that it is Black Walnut- Juglans nigra that secretes most Juglon..

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