Black Walnut Mitigation - Ask Extension
We have 2 large mature black walnut trees adjacent to our small yard (1/4ac). To avoid any issues, I built raised beds, and used soil from the store ...
Knowledgebase
Black Walnut Mitigation #783516
Asked March 08, 2022, 12:14 PM EST
We have 2 large mature black walnut trees adjacent to our small yard (1/4ac). To avoid any issues, I built raised beds, and used soil from the store to start them off. That worked well for a number of years, but for the past 2 years, I'm having problems with tomatoes wilting, and it was very pronounced this past season. I'm pretty sure this is from the black walnut.
Is there anything I can do to mitigate the effect, and repair the soil, or do I just need to replace it (it's several beds, probably 60 linear feet by 3 feet). Alternatively, are there good crops that aren't affected by the local black walnut?
Thank you!
Baltimore County Maryland
Expert Response
We're guessing leaching juglone from the walnut is not seeping up into the raised beds, but if it were, there would unfortunately be little recourse other than to move the beds further away. Soil replacement would either only be a temporary fix or would otherwise not be practical. How close is the nearest tree?
You can carefully dig down into the native soil layer beneath that bed to see if any walnut roots are present. Generally, the further from the trunk they are, the finer they will be. Roots might be growing up into the bed if they're being suffocated by the added soil, as tree roots typically don't respond well to a drastic grade change (soil depth over the roots), but we suspect this is not the case here or you would have noticed roots while planting or cultivating that bed in recent seasons.
One way to experiment regarding suspected juglone is to grow a couple of test tomatoes, one closer to and one further from the tree(s) to see where a zone of juglone may be. This won't be a foolproof test, as the plants may contract other problems that cause them to struggle, but it might suggest juglone is a problem if, at the very least, the plant closest to the tree succumbs first. The main complicating factor nearest a tree, though, would be its shade, since the lack of full sun would weaken and stunt a sun-loving plant like tomato.
During those past two years, were you been growing wilt-resistant tomato varieties? (Wilt diseases include verticillum and fusarium. Tomato variety tags or catalog listings will mention if they have resistance.) Have any other plants in those beds struggled during the same season the tomatoes failed?
We can share plant lists of juglone tolerance and susceptibility, but this is a horticulture topic with little conclusive evidence either way regarding the real-world risk of juglone to plant health (compared to lab experiments). You can learn more about one main counter-argument to juglone toxicity in this linked Washington State University publication. Juglone-resistant plant lists may differ somewhat or be incomplete for this reason -- observed vulnerabilities may not be consistent because of interference from soil microbes, underlying plant health prior to exposure, and other environmental conditions. Not all juglone lists include vegetables, but here are a few lists that do:
Miri
You can carefully dig down into the native soil layer beneath that bed to see if any walnut roots are present. Generally, the further from the trunk they are, the finer they will be. Roots might be growing up into the bed if they're being suffocated by the added soil, as tree roots typically don't respond well to a drastic grade change (soil depth over the roots), but we suspect this is not the case here or you would have noticed roots while planting or cultivating that bed in recent seasons.
One way to experiment regarding suspected juglone is to grow a couple of test tomatoes, one closer to and one further from the tree(s) to see where a zone of juglone may be. This won't be a foolproof test, as the plants may contract other problems that cause them to struggle, but it might suggest juglone is a problem if, at the very least, the plant closest to the tree succumbs first. The main complicating factor nearest a tree, though, would be its shade, since the lack of full sun would weaken and stunt a sun-loving plant like tomato.
During those past two years, were you been growing wilt-resistant tomato varieties? (Wilt diseases include verticillum and fusarium. Tomato variety tags or catalog listings will mention if they have resistance.) Have any other plants in those beds struggled during the same season the tomatoes failed?
We can share plant lists of juglone tolerance and susceptibility, but this is a horticulture topic with little conclusive evidence either way regarding the real-world risk of juglone to plant health (compared to lab experiments). You can learn more about one main counter-argument to juglone toxicity in this linked Washington State University publication. Juglone-resistant plant lists may differ somewhat or be incomplete for this reason -- observed vulnerabilities may not be consistent because of interference from soil microbes, underlying plant health prior to exposure, and other environmental conditions. Not all juglone lists include vegetables, but here are a few lists that do:
- Landscaping...Around Walnuts -- Penn State
- Black Walnut Toxicity -- Wisconsin Extension
- ...Black Walnut Trees -- Cornell blog
Miri
Thank you Miri... I will definitely check out the links regarding potential resistant vegetables and that's all helpful information.
I switched to VR tomatoes a couple years ago, when I first noticed the wilting. Since the front bed was a solid 50 feet from the tree and just outside the canopy, I assumed it was 'the vert' to blame at the time, because of the symptoms. Things only got worse the next season though (and the one after that). These same beds produced very well when they were first setup around 8-10 years ago, supporting multiple varieties and styles of tomato, but it's gone steadily downhill (I have assumed everything from weather, to watering, to wilt was to blame at first, I rarely see pests). As for proximity, the bed closest to the trees definitely has more issues. I put some blame on the squirrels, since these are mature trees that produce a ton of walnuts, and the squirrels bury them in my beds (I'm always pulling out the shells). Sunlight is 6-8 hours with more in the evenings... the yard faces SW and there's a gap between the trees, which mostly shade the garage area and my neighbor's yard during the day. They are tall enough not to really interfere with the yard except around high noon, which kinda gives my plants a break from the heat in summer (even then, the 'shade' is barely shade, these very tall old trees have a light canopy in general)
So, for other stuff... I planted (red) potatoes in the same area, and those did fine, or didn't die at least. Yields felt small but I was just experimenting with those and I don't have a good measure of those results. Likewise, I have numerous types of peppers that I grow next to the tomato rows, and those do very well depending on the weather of course. The habanero last year produced magnificently in the same soil. My zucchini haven't been doing well for years, but friends blame cutworms and I'm not sure why mine failed, but the butternut grew like a weed, and I even had a volunteer from my compost pile, which was located 3 feet from the trunk of the huge walnut tree. It wrapped up to the trunk and did great actually. My lettuces also do great, my baby spinach not as much, and I gladly lose about half of that to the baby bunnies, which is fine because we have more than enough to deliver salads to the table :)
Oh, and I had strawberries and black raspberry there in the back bed closest to the tree for a bit as well, but I pulled the raspberry two years back (they get out of hand and didn't produce enough to make it worth it). A few volunteers are in the yard, and I just let the birds have at those. The strawberries did well, but are also a lot of tending and the critters get them before I do, so I let them run out and tried the tomatoes in that area. The only thing that does well in the back bed is peppers, and the front bed has done okay with green beans, potatoes, a dozen different peppers, and zucchini in the past. I just can't seem to grow tomatoes, which are my favorite backyard item.
I'm wondering if I might be able to dig out the beds and 'repair' that soil somehow... maybe pile it elsewhere and cover it for a few months (I read that might work). As you mentioned, the advice and information out there on dealing with black walnut is inconsistent and sparse. I'm probably just going to try a few things, maybe do containers for the toms this year, and see how it goes. I appreciate the advice... and by the way, I heard about you via the garderner zoom thing on Tuesday. That was a really great call (topic was cole crops, which I've only played with a bit, and I learned some things!). https://go.umd.edu/get_growing
Thank you so much!
Dave
I switched to VR tomatoes a couple years ago, when I first noticed the wilting. Since the front bed was a solid 50 feet from the tree and just outside the canopy, I assumed it was 'the vert' to blame at the time, because of the symptoms. Things only got worse the next season though (and the one after that). These same beds produced very well when they were first setup around 8-10 years ago, supporting multiple varieties and styles of tomato, but it's gone steadily downhill (I have assumed everything from weather, to watering, to wilt was to blame at first, I rarely see pests). As for proximity, the bed closest to the trees definitely has more issues. I put some blame on the squirrels, since these are mature trees that produce a ton of walnuts, and the squirrels bury them in my beds (I'm always pulling out the shells). Sunlight is 6-8 hours with more in the evenings... the yard faces SW and there's a gap between the trees, which mostly shade the garage area and my neighbor's yard during the day. They are tall enough not to really interfere with the yard except around high noon, which kinda gives my plants a break from the heat in summer (even then, the 'shade' is barely shade, these very tall old trees have a light canopy in general)
So, for other stuff... I planted (red) potatoes in the same area, and those did fine, or didn't die at least. Yields felt small but I was just experimenting with those and I don't have a good measure of those results. Likewise, I have numerous types of peppers that I grow next to the tomato rows, and those do very well depending on the weather of course. The habanero last year produced magnificently in the same soil. My zucchini haven't been doing well for years, but friends blame cutworms and I'm not sure why mine failed, but the butternut grew like a weed, and I even had a volunteer from my compost pile, which was located 3 feet from the trunk of the huge walnut tree. It wrapped up to the trunk and did great actually. My lettuces also do great, my baby spinach not as much, and I gladly lose about half of that to the baby bunnies, which is fine because we have more than enough to deliver salads to the table :)
Oh, and I had strawberries and black raspberry there in the back bed closest to the tree for a bit as well, but I pulled the raspberry two years back (they get out of hand and didn't produce enough to make it worth it). A few volunteers are in the yard, and I just let the birds have at those. The strawberries did well, but are also a lot of tending and the critters get them before I do, so I let them run out and tried the tomatoes in that area. The only thing that does well in the back bed is peppers, and the front bed has done okay with green beans, potatoes, a dozen different peppers, and zucchini in the past. I just can't seem to grow tomatoes, which are my favorite backyard item.
I'm wondering if I might be able to dig out the beds and 'repair' that soil somehow... maybe pile it elsewhere and cover it for a few months (I read that might work). As you mentioned, the advice and information out there on dealing with black walnut is inconsistent and sparse. I'm probably just going to try a few things, maybe do containers for the toms this year, and see how it goes. I appreciate the advice... and by the way, I heard about you via the garderner zoom thing on Tuesday. That was a really great call (topic was cole crops, which I've only played with a bit, and I learned some things!). https://go.umd.edu/get_growing
Thank you so much!
Dave
Hello Dave,
It sounds like juglone may be a suspect, then, though this may take further experimentation if any other vegetable crops known/suspected to be juglone-susceptible were doing well at the same time. Verticillium-resistant selections is great, but perhaps Fusarium is still a culprit if the juglone theory doesn't pan out. (Since information is limited regarding juglone impacts in real-world conditions, perhaps there are soil interactions that make juglone-exposed plants more vulnerable to certain pathogens, and it's the pathogen which ended-up actually killing the tomato. We can't say for certain, except to suggest you try Fusarium-resistant cultivars too, if possible.)
The sunlight levels sound good. Worst-case, as you suggested, maybe try a tomato or two in large containers (plant them solo) this time around to see how they fare.
"Repairing" the soil by letting it rest, so to speak, is probably not practical. Similarly, we don't know of any way to remediate it with, say, a cover crop to speed-up juglone removal. If juglone proves to the core issue, an equal or lesser amount of effort, time, and expense to fix the problem at that point would be more useful if spent just moving the beds further from the trees. Or raising the existing bed level so the depth of soil is more above-grade. Regarding the idea to remove the soil, yes, juglone does biodegrade eventually, but how fast this occurs depends on conditions like microbe community, weather, and soil type. We couldn't predict how long it would take, and there is no test available for juglone content so you'd know when it's ready to use again.
Good luck!
Miri
It sounds like juglone may be a suspect, then, though this may take further experimentation if any other vegetable crops known/suspected to be juglone-susceptible were doing well at the same time. Verticillium-resistant selections is great, but perhaps Fusarium is still a culprit if the juglone theory doesn't pan out. (Since information is limited regarding juglone impacts in real-world conditions, perhaps there are soil interactions that make juglone-exposed plants more vulnerable to certain pathogens, and it's the pathogen which ended-up actually killing the tomato. We can't say for certain, except to suggest you try Fusarium-resistant cultivars too, if possible.)
The sunlight levels sound good. Worst-case, as you suggested, maybe try a tomato or two in large containers (plant them solo) this time around to see how they fare.
"Repairing" the soil by letting it rest, so to speak, is probably not practical. Similarly, we don't know of any way to remediate it with, say, a cover crop to speed-up juglone removal. If juglone proves to the core issue, an equal or lesser amount of effort, time, and expense to fix the problem at that point would be more useful if spent just moving the beds further from the trees. Or raising the existing bed level so the depth of soil is more above-grade. Regarding the idea to remove the soil, yes, juglone does biodegrade eventually, but how fast this occurs depends on conditions like microbe community, weather, and soil type. We couldn't predict how long it would take, and there is no test available for juglone content so you'd know when it's ready to use again.
Good luck!
Miri
