Black Walnut: Is it True You Can't Grow Underneath Them?

In this episode, Lou Meyer from Davey's mid-Atlantic region shares information about black walnut trees, including if black walnuts are edible and what specific plants you can grow under these trees.
 
In this episode we cover:  

• What are black walnut trees? (1:18)
• What is black walnut dye? (2:45)
• Do people eat black walnuts? (4:09) (6:10)
• How big do black walnut trees get? (7:25)
• Do black walnut trees have pests? (8:04)
• Are there any treatments for thousand cankers disease? (9:25)
• Can you plant something under a black walnut tree? (9:51)
• What can you grow under a black walnut tree? (14:33)
• Have educational institutions and extension agencies accepted this idea? (15:24)

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To hear more of Lou's expertise on Talking Trees, listen to his other podcast appearances, Managing Nature - Storms, Fluctuating Temperatures and Pests and How You Can Successfully Grow Trees in Containers.

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Connect with Doug Oster at www.dougoster.com. 

Have topics you'd like us to cover on the podcast? Email us at podcasts@davey.com. We want to hear from you!

Transcript

Doug Oster: Welcome to the Davey Tree Expert Company's podcast Talking Trees. I'm your host, Doug Oster. Each week our expert arborists share advice on seasonal tree care, how to make your trees thrive, arborists' favorite trees, and much, much more. Tune in every Thursday to learn more because here at the Talking Trees podcast, we know trees are the answer.

Well, he's back. He's been on the show more than any other guest. His name is Lou Meyer. He is part of the Davey business development team in the Mid-Atlantic region based in Baltimore. Hey, Lou. Welcome back.

Lou Meyer: Doug, it's so good to be here once again.

Doug: Well, Lou, I'm very excited about today's topic because I've been taught my whole life that you can't grow much of anything under a black walnut tree because it has some kind of property that stops things from growing. You and I had talked a couple months ago, and I just mentioned black walnuts, and you told me I didn't know what I was talking about. [laughs]

Lou: Well, you're not wrong for thinking that, but we will discuss some interesting stuff that's coming out the past few years regarding those. Before we get to there, let's dive into what black walnuts are and why they're important and where they're growing. How's that sound?

Doug: That sounds good to me. Tell me everything you know about black walnuts.

Lou: Yes. They're part of the Juglandaceae family, so we'll get technical here. The black walnuts, the Juglans nigra, part of the Juglandaceae family. Juglandaceae, it's a family with about 50 different species. It's around the world, but North America, Southeast Asia, and Eurasia are the big areas for that. You would recognize walnuts, pecans, and hickories as part of that group, and all three of those are important trees in North America for sure.

Juglans nigra itself, the black walnut, grows from southern Ontario, west out to the Dakotas. They've got them in Colorado down to Georgia, Florida, Texas, not a whole lot in North Carolina, but here we are in the Mid-Atlantic, myself and you. That's ground zero for black walnuts. They're very populated here. Now the first thing people think about when you hear black walnuts, they go, "Oh, I love eating walnuts." Yes, you're probably eating Persian walnuts, Juglans regia, which is the royal walnut. It's a little different, but we do eat black walnuts too. I'll talk a little about that later.

Do you have any on your property?

Doug: I don't have any black walnuts, but I have a story.

Lou: Okay.

Doug: I had someone send me a message saying that right before school started, their kids got into some black walnuts out in the woods, and she wanted to know how to get that black dye off their hands and I had no answer for her.

Lou: Right. Yes, no, once that sets in, it's there. That's where the name comes from is from the dye. It is a dye. That color, it's found in the droop. A droop is a fancy word for that round ball that falls off the tree. The husk is the outer shell and the kernel is the fruit inside, and together those are a droop and yes, they got the dye on them. Actually, historically, Native Americans used the dye for all sorts of things. Through the mid-1900s, I think, we used it as hair dye. For folks who were graying, you would use black walnut dye in your hair. It's brownish to black, but yes, once it's on, it's set in your hands.

Doug: I think it just has to wear itself out. [chuckles]

Lou: It does. It does. The great news is there's no health effects from it and it's all natural. It's not just some weird chemical dye that you have on you. It's all natural.

Doug: You say they're edible. Do people eat black walnuts regularly?

Lou: Yes, they sure do. You could eat them in all sorts of ways. You can crush them up, the nut itself, use them as toppings on ice cream, used in baked goods a lot, so fudge brownies with walnuts on them. Again, if you're buying them commercially, you're probably going to get Persian walnuts and here's why. The droop that we were talking about, the fruit of it. Persian walnuts are a lot softer and less brittle, so they're much easier to break open and that makes for a much easier processing of that nut in a commercial way. The flavor is slightly different, but for most of us, we wouldn't be able to tell the difference. You get more bang for your buck with the Persian walnuts.

Yes, no, you could use it on baked goods, you can crust chicken or fish. I'm a big grill guy. I like grilling stuff. You can crust your chicken or fish before you grill them or fry them.

Doug: If we had black walnuts on the property, is that dye always on there? I'm just thinking if you're going to crack open black walnuts, how do you do it without getting that dye on yours, or times where the dye isn't as bad?

Lou: No, you're going to get dyed, so use gloves or just accept the fact that you're going to have some dye. Now if you have a black walnut near you and you're looking to capitalize on that, you could either put a bunch of them on your driveway and run them over with your car. Another option is to take a two-by-four, or any piece of lumber, I guess, put it over a five-gallon bucket and drill a one-and-a-half-inch hole, use one of the drills that have the wider heads on them. You place the black walnut with the husk on that and just take a hammer and smack it and the shell falls apart, the kernel goes into your bucket, and then you dry it out and you're good to go.

One other fun way-- and I always love talking about edible possibilities with trees. One fun tradition is actually an Italian tradition. There's a liqueur called Nocello, and you can buy it commercially if you want. My brother-in-law Dan back in Cincinnati, shout out to Dan, makes a really good one, where he collects the walnuts as soon as they fall off the trees, when they're green, shoves them in a bottle or a big jar and puts, I think, vodka in with it and with herbs and spices. Traditionally, you let it sit until just after Thanksgiving, early December, and at that time, you pour it out into a filter and you have this spectacular walnut liqueur that's a lot of fun.

Doug: All right, so next time I'm in Baltimore, I need a little bit of that. Then what is that pie that you love, the maple--

Lou: Oh, the maple cream pie, yes.

Doug: Maple cream pie. Okay, so put that on your list when I come to Baltimore in January. Lou and I always see each other at a trade show there called the Mid-Atlantic Nursery Trade Show, and always tape an episode there. Those are two things to put on the list, okay?

Lou: You better believe it. I like to take care of my buddy Doug.

Doug: How big do those black walnut trees get?

Lou: They can get pretty big. In ideal settings, they can get up to 100-feet tall, and they've got a wide spread of 65 feet or so. They're a very wide-spreading tree. Really pretty. They've got pinnately compound leaves, so they got-- a good way to put it is each leaf is actually made up of multiple leaflets. You've got a long stem down the center with little tiny leaflets coming off the side, which all look like individual leaves, but that one thing is a leaf. They're great shade trees. They create a lot of shade.

Doug: Any pests or diseases?

Lou: Unfortunately, there is. Before 2010, there's been a disease called thousand cankers that's been an issue in the Western United States. Now if you remember from a few minutes ago, black walnuts extend out to about Colorado, so this hasn't been a huge issue for the black walnuts. However, in 2010 down in Knoxville, there was a discovery of thousand cankers disease. Since then, they've found it in Virginia, in your home state of Pennsylvania and a few others. I know here in Maryland, we're on the lookout for it.

It is a disease that is spread by a beetle, a twig beetle, actually, a walnut twig beetle, and it carries a fungus with it. When the twig beetle bores into the tree, the fungus follows it. Then that fungus travels through the vascular system of the tree, the cambium, and the cankers that follow plug up the vascular system, so the nutrient water transportation within the tree is shut down and the tree becomes brittle and dies. Now what I'm reading is it can take up to 20 years for that fungus to really get set and moving, other times it can go faster, but it's not a good sign for the walnuts.

Doug: Any treatment?

Lou: Not exactly, no. There's prevention, but not treatment. Even the prevention is more along the lines of keeping the trees as healthy as possible, so proper fertilization, watering and proper mulching as well, getting that soil ready, so using an air spade to break up the soil so that you have less compaction. That's the answer.

Doug: Let's get to whether I can plant something underneath my black walnut or not. What is it in a black walnut that I've always been told stops many plants from surviving?

Lou: It's a phenomenon called allelopathy, A-L-L-E-L-O-P-A-T-H-Y. It's a tough word for me to say, allelopathy. Allelopathy is a broad term that basically applies to all plants that produce chemicals or other things that prohibit other plants from growing around it. For the walnuts, they're part of the Juglandaceae family, and they produce a compound called juglone or juglones. That is found in those droops, the fruit, and also in the roots of the tree.

Ever since I joined the industry back in 2001, when I started paying attention to gardening landscaping, yes, black walnuts were the tree that you don't want anywhere near your garden because nothing grows underneath them because of the juglones. They kill anything around them. Nature is a wild place, and there's all sorts of defenses that trees and shrubs put up against other competition, against pests and diseases. That can make for a fun episode, the ways they fight each other off, but particularly the black walnut, these juglones.

Multiple extension agencies have gone back and said that this is a big issue, that you shouldn't plant underneath these, but they all cite two articles. One was an extension article from Cornell University in 1973, and the other one's from Ohio State in 1993. There's a professor out of Washington State University, Dr. Linda Chalker-Scott, who in 2019 started digging into this. She said, look, all these peripheral articles, they're not original research. They all cite these two articles, Cornell and Ohio State.

When she went to look for these two articles, the Cornell one had been pulled because it had been such a long time; and the Ohio State one, she couldn't find, nor could the Ohio State Archives. She started running research on this. In lab settings, yes, when you apply the juglone directly to seeds or seedlings, those seeds will be stunted, they will die sometimes, it really wrecks them, but there's been no field studies that have conclusively shown that there's juglone in the soil that affects the trees and shrubs around them.

Now there is limited growth underneath walnuts sometimes, but no less than other shady trees. You can't grow tomatoes under an oak tree. It's not because of juglones. It's because there's no sunlight and they need a lot of sun. Walnuts are also very fast-growing trees, so they require a tremendous amount of input. They pull a lot of water and a lot of nutrients out of the soil to fuel that growth, so it's really tough for stuff to grow underneath them.

One other thing that I feel worth mentioning because you did an article on mulch recently, a lot of people shy away from walnut mulch, walnut wood chips for the exact same reason. Juglone doesn't make its way up into the canopy of the tree. It's only found in the droop and the roots. Walnut chips are great to use, no problem.

Doug: Well, this is fascinating and so wild that just three- I'm sorry, two scientific papers, which to this point can't be found, has perpetuated to almost everyone in the garden industry that you can't grow anything under a black walnut tree.

Lou: Yes. Actually, it even goes back to the first century. Historian Pliny the Elder from way long ago, he actually has a quote that said, "The shadow of walnut trees is poison to all plants within its compass." I don't think he was doing many lab studies.

Doug: [laughs] Yes, but he knew, just like we do. When I tell people, "What should I grow under an oak tree?" I say, a bench, grow a bench underneath there.

Lou: Yes, a mulch bed is spectacular. [crosstalk] No, I have all sorts of properties that I've taken care of that have beautiful hosta gardens under their walnuts, beautiful coleus, all sorts of shade-loving plants that can grow underneath them.

Doug: Yes, what else could I grow underneath there? Could I grow a tough, shade-loving shrub underneath it possibly, or there's not enough water, not enough fertilizer?

Lou: Well, if it's in your landscape that you have irrigated and you have an arborist coming through and fertilizing it, I don't see why not. Some hydrangeas that can do shade. My dad has an oak tree back in Cincinnati that is huge and it's heavily shade underneath it. He has a bottlebrush buckeye that is just gorgeous that surrounds the bottom of the base of the tree. Yes, if you plant it early enough before the shade sets in, you can get away with it. Again, irrigate and fertilize to make sure that the trees and the shrubs are not fighting over the resources too heavy.

Doug: Now have some of the extension agencies and other, like Ohio State, Penn State, have they realized this or accepted it, or what's going on?

Lou: The only one that I found was the University of Maryland. Oh, Terrapins, my Terrapins have said, hey, we should really start rethinking this. They put a citation on their extension page on their Walnut page saying that this is starting to- we're starting to realize that this may all be wrong, and shifting the thought. What's really fun about this, Doug, is horticulture is something that's been practiced forever and a day. We think we know it all. We think that it's set in stone. It's really important to realize that there's a science to this and there's new research coming out all the time. We're always learning and we always should be learning and open to new things. Nature wins.

Doug: Well, I can't wait to talk about this on the radio show and write something about it. This is pretty exciting. I really appreciate your input on this and waking me up to the power of the black walnut.

Lou: You bet. It's one of my favorite trees. When I started learning that it's not as aggressive as we thought, it just rose higher in the list.

Doug: All right, Lou. Thanks a lot. I'm sure we'll talk to you soon.

Lou: Doug, it's been great. Have a wonderful time.

Doug: I can't wait to share this information with all my horticulture friends. I'll be interested to see what they have to say. I always learn something new from Lou. Great stuff. Now tune in every Thursday to the Talking Trees podcast from the Davey Tree Expert Company. I'm your host, Doug Oster. Do me a big favor, subscribe so you'll never miss a show. If you've got an idea for an episode, maybe a comment, couple of different ways to reach us. Send us an email to podcasts@davey.com. That's P-O-D-C-A-S-T-S@D-A-V-E-Y.com. You can also click the link at the end of our show notes to text us a fan mail message. Your ideas might be in a future podcast and we'd love to hear from you. As always, we'd like to remind you on the Talking Trees podcast, trees are the answer.