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How Black Walnut Trees kill the competition

"Could my walnut trees have killed other plants nearby?" This is a question we often get from people who bought trees and shrubs from us that died when planted near Juglans nigra.  When you sell as many plants as we do, this happens sometimes (and we replace them under guarantee so all is not lost, dear customer!)

To the Latin scholar chemists amongst you that should tell you all you need to know.   For the rest of us it might be a mystery unless you are lucky like me and happened to see an information leaflet on the stand of Hadlow College who were strutting their stuff at the Chelsea Flower Show.

I must admit I "sort of knew something" about black walnut's ability to kill other plants but the students of Hadlow made it very clear.  Black Walnut is called Juglans nigra in Latin.  Juglans is the root (no pun intended) for juglone which is an allelopathic drug. That means it stunts or kills.  And black walnut is the biggest natural producer of juglone, which it uses to great effect to kill unrelated plants and trees nearby. If you have a black walnut and trees and shrubs relatively close to it suffer wilting, yellowing foliage and either die or stop growing, now you know why. Incidentally, juglone is probably one of the reasons that black walnut is so resistant to Honey Fungus.

So here is (as the Americans would say) - the take away.  Mother nature likes balance, so there are a number of plants that don't mind juglone and can be planted near your black walnut. They include:

Acer negundo (Box Elder)
Acer palmatum (Japanese Maple)
Acer rubrum (Red Maple)
Acer saccharinum (Silver Maple)
Aesculus (Horsechestnut)
Betula pendula (Silver Birch)
Betula nigra (Black or River Birch)
Catalpa bignonoides (Indian Bean Tree, Foxglove Tree)
Cornus Mas (Cornelian Cherry)
Crataegus (Hawthorn)
Cydonia oblonga (Quince)
Fagus (Beech)
Gleditsia triacanthos (Honey Locust)
Juniperus (Junipers)
Liquidamber styraciflua (Sweetgum)
Liriodendron tulipifera (Tulip tree)
Picea abies (Norway Spruce)
Platanus (Plane)
Prunus serotina (Black Cherry)
Pyrus calleryana (Pear)
Quercus (Oak)
Rhus (Sumach)
Robinia pseudoacacia (Black Locust)
Tilia platyphyllos (Broad leaved Lime)
Tsuga (Hemlock)
Ulmus (Elm)
Viburnums

Of course, we sell a fair selection of the above, if you are interested.....

Relax and enjoy watching your garden grow!

Creative Commons License
Death by Black Walnut by Julian de Bosdari is licensed under a Creative Commons Attribution-Share Alike 2.0 UK: England & Wales License.
Based on a work at blog.ashridgetrees.co.uk.

2 thoughts on “How Black Walnut Trees kill the competition”

  • CHAU, Nguyen-Minh (Ms.)
    CHAU, Nguyen-Minh (Ms.) 30th April 2013 at 5:11 pm

    Thanks for your detailed info.
    I have a more specific question, below:
    There are two wild black walnut growing on my property's eastern edge. Two years ago I planted four quince bushes at the foot of the privacy 8ft fence erected by my neighbor on that side, hoping that the quinces' growth and blooms (with morning sun--four different color shades) would alleviate the sterile look of that new fence. .
    Unfortunately, although the quinces arrived (from a local, reputable nursery) with vigorous blooms, they are either short-lived or almost absent in the current spring.
    Could this be due to the effect of the quinces being planted in a space located between the two black walnuts?
    Thank you in advance for your advice.
    NMC
    Garrett Park, MD

    Reply
    • julian

      Thank you for your email. The short answer is that there are other variables that may be cause and not enough research has been done on black walnuts to really know. However these are my (random??) thoughts:

      1. If the quinces are not at all under the canopy of the black walnuts then they are away from the walnut's poisonous root system and it is probably not the reason. The converse may also be true.

      2. Were the quinces pot grown, rootballed or bare rooted when you bought them. In the latter two cases it is perfectly possible that they are still establishing in their new homes (especially after last year's miserable summer).

      3. Did you prune them? If so, please don't as they flower on the ends of their branches and pruning removes flowering wood

      4. Assuming you are in the UK, the spring is incredibly late - our quinces have not flowered at all yet. Patience may yet pay off.

      I hope this helps

      Reply
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