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The Curse of the Black Walnut

Black walnut trees have a well-deserved reputation for making life difficult for gardeners because they release a toxic compound called juglone, which can adversely affect many kinds of plants.

Contributors: Rick Schoellhorn

For many gardeners across the United States and Canada, the words “black walnut tree” can signal severe depression. For anyone who worries about this tree affecting their gardening, there is even a Black Walnut Society to help you learn more about gardening around the dreaded black walnut! Black walnut trees have a well-deserved reputation for making life difficult for gardeners because they release a toxic compound called juglone, which can adversely affect many kinds of plants. That’s the whole point from the black walnut’s point of view; by releasing its own natural herbicide, it has fewer plants to compete with.

Black walnut trees are native to most of the Central and Eastern United States from Florida north to Quebec and west to Utah, so you’ll find them growing in a lot of gardens in North America. But all is not lost! You can grow plants near and even under a black walnut tree if you select carefully.

Key Points to Remember:

  1. The drip line of a black walnut (everything under the canopy from trunk to farthest branch tips) is the hardest place to grow plants because the tree concentrates the chemical juglone under its own canopy.
  2. Remove all fallen black walnut leaves, stems and walnuts since they also contain the chemical. The roots are also toxic and juglone can persist in dead wood for years even after the tree is removed.
  3. Learn which plants can tolerate being near black walnut trees and which cannot. See our plant list below.
  4. Black walnut trees can create heavy shade. Limb branches up high to allow enough light in to grow plants below.
  5. The bad news is that tomatoes will not grow near black walnuts, so your dreams of a salsa garden might be compost for now…

Many plants can tolerate being near a black walnut tree, including the list of Proven Winners® plants shown below. Remember, every garden situation is different so start slowly and add plants as you see success. Remember the black walnut is a native tree that can provide shade and atmosphere while being drought tolerant and relatively pest-free. Use care in making your decisions about gardening beneath a black walnut, as this native tree may benefit your local ecosystem more than the garden you imagine replacing it with.

     

Herbaceous Plants That Tolerate
Black Walnut Trees

Shadowland Hosta (Hosta spp)

Amazing Daisies Shasta Daisy (Leucanthemum superbum)

Pardon My Bee Balm (Monarda spp)

'Opening Act Blush' (Phlox hybrid)

Lemon Drop® Evening Primrose (Oenothera spp)

Rock ‘n Grow Stonecrop (Sedum spp)

Rainbow Rhythm Daylily (Hemerocallis spp)

Dolce® Coral Bells (Heuchera spp)

Sweet Caroline and Illusion® Sweet Potato Vine (Ipomoea spp)

Laguna and Lucia® Lobelia (Lobelia spp)

Surefire® Begonia (Fibrous and Tuberous Begonia spp)

Anytime® Pansiola (Viola spp)

 

Shrubs That Tolerate
Black Walnut Trees

Sunjoy® Barberry (Berberis spp)

Deutzias (Deutzia spp)

Show Off® Forsythia (Forsythia spp)

Lil’ Kim, Chiffon and Satin® Rose of Sharon (Hibiscus syriacus)

Incrediball® and Invincibelle® Spirit Smooth Hydrangea (Hydrangea arborescens)

St. Johnswort (Hypericum spp)

Blue Holly and Winterberry Holly (Ilex spp)

Elderberry (Sambucus spp)

Viburnum (Viburnum spp)










Oh the curse of the black walnut tree! See its mighty trunk in the left rear of this photo? The juglone it is sending out through its roots, fallen leaves and fruits are slowly poisoning the magnolia tree in the foreground. By poisoning other trees around it, the black walnut is effectively eliminating its competition for water and sunlight. It’s a very smart tree indeed, but a sad situation for the poor magnolia!
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Anonymous's picture
Anonymous Tue, 11/13/2018 - 5:11am

I've seen variable information that puts lilacs on both sides of the juglone sensitivity chart. Do you have any definite information for the Bloomerang Lilac? I have a new hedge that was planted last season, so I could still move the lilac if I needed to. I want to put a walnut tree close to the hedge because all the other shrubs in it are resistant. Please help.

Stacey Hirvela's picture
Stacey Hirvela Thu, 11/15/2018 - 8:23am

Unfortunately, we don't have any definitive information on this - it is hard to get any 100% accurate data about juglone tolerance in anything because it is so dependent on other factors. For example, the biggest challenge is from fallen fruits, not the roots as most people think. So if you have a walnut that drops a lot of fruits that stick around the lilac, that would be a problem. Ditto with leaves. If the issue is just roots, ample irrigation or rainfall should mitigate any risk to most plants. In short, if you can manage the leaf and fruit litter and amply irrigate, it should be okay. And if it is not, then you should still have time to move it before it dies.

Anonymous's picture
Anonymous Tue, 08/14/2018 - 8:11am

Hi, when I saw your title about back walnut trees causing depression, I thought that I finally found the answer to my question, NOT!!

I'm becoming increasingly frustrated with all the young 0 to 2 yr. old saplings we have that I want gone!

I'm physically not able to dig down to the roots very well! Is there an environmentally safe products to spray or soak into fresh cuts I'll make?

Thanks in advance for any support and assistance that you can give me! Susan

Rick Schoellhorn's picture
Rick Schoellhorn Wed, 11/14/2018 - 2:41pm

Hi there Susan -
The only consistent answer for this seems to be glyphosate from what I have found so far. I know a lot of people refuse to use it and that is their choice, however it is the simplest method I have found in numerous on line conversations. You would apply the concentrate to the fresh cut seedlings do not dilute, and try to keep the chemical only on the stump you are working on).

I do know that applying salt as some people recommend is extremely dangerous for the surrounding plants, so I would definitely avoid that.

Sorry I can't be more help! Best of Luck!

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