Tomato takedown

Well, Dr. Rohwer was right – he thought you’d get this more easily than I did!  Ray and Jon were spot on – this is classic juglone toxicity from those walnut trees (Juglans spp.) you see in the background.  Many of these leaves ended up on the roof, leaching into the rain barrels (good sleuthing Ray and KennyG!), the water in which was used in irrigating the vegetable garden.  In fact, Dr. Rohwer divulged that the rain barrel water was quite brown from the walnut leaves.  Thus, the tomatoes met an unhappy end, because tomatoes definitely do not love walnuts.

Juglone toxicity, an example of allelopathy, is an interesting phenomenon – it does not affect all plants equally.  In fact, this clue was given in the puzzle, when Dr. Rohwer mentioned the "garlic and carrots, amongst young beets, onions, kohlrabi, and bolting radishes and spinach" also in the garden.  Many of these vegetables are known to tolerate juglone, as are various landscape plants.  (There is an excellent publication from Purdue on juglone toxicity with lists of tolerant and sensitive species.)

Juglone is most prevalent in the nuts, roots and leaves of walnut trees.  This makes sense:  germinating walnut seeds and expanding root systems are both able to kill their competitors, retaining more resources for themselves.

Juglone tends to be less of a problem for established trees and shrubs, as their root systems are more extensive; seedlings would be the most sensitive stage.  And walnut wood used as part of a wood chip mulch has not, as far as I know, been shown to cause juglone toxicity in landscape plants. 

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Linda Chalker-Scott

Dr. Linda Chalker-Scott has a Ph.D. in Horticulture from Oregon State University and is an ISA certified arborist and an ASCA consulting arborist. She is WSU’s Extension Urban Horticulturist and an Associate Professor in the Department of Horticulture, and holds two affiliate associate professor positions at University of Washington. She conducts research in applied plant and soil sciences, publishing the results in scientific articles and university Extension fact sheets. Linda also is the award-winning author of five books: the horticultural myth-busting The Informed Gardener (2008) and The Informed Gardener Blooms Again (2010) from the University of Washington Press and Sustainable Landscapes and Gardens: Good Science – Practical Application (2009) from GFG Publishing, Inc., and How Plants Work: The Science Behind the Amazing Things Plants Do from Timber Press (2015). Her latest effort is an update of Art Kruckeberg’s Gardening with Native Plants of the Pacific Northwest from UW Press (2019). In 2018 Linda was featured in a video series – The Science of Gardening – produced by The Great Courses. She also is one of the Garden Professors – a group of academic colleagues who educate and entertain through their blog and Facebook pages. Linda’s contribution to gardeners was recognized in 2017 by the Association for Garden Communicators as the first recipient of their Cynthia Westcott Scientific Writing Award. "The Garden Professors" Facebook page - www.facebook.com/TheGardenProfessors "The Garden Professors" Facebook group - www.facebook.com/groups/GardenProfessors Books: http://www.sustainablelandscapesandgardens.com

11 thoughts on “Tomato takedown”

  1. i am fairly sure the mulch does carry toxins and planting a few tomato seedlings is a great way to test mulch for toxicity. over time the mulch will breakdown and the toxins will leach out of the mulch.

  2. Jon, I agree that in a vegetable problem there might be a problem. In fact woody mulches in general tend to be allelopathic and/or otherwise inhibitory of any seedlings, which is why they control weeds so well. I wouldn’t recommend them in vegetable gardens (though some people swear by them.) But in landscapes, there’s been no evidence that walnut wood as part of a mulch is a problem for ornamentals, especially established specimens.

  3. I recently lost a beautiful tree after muching it. I have a feeling there was something wrong with the muclc. Toxicty may explain it.

  4. Rebecca, it’s highly unlikely that mulch would kill a tree (the exceptions would be deep layers of compost or sawdust, which create anaerobic conditions) or lansdscape fabric). I’d hate for you not to use mulch, as it’s the single best way to protect soil and root health.

  5. This is interesting, because I recently ran across information about monkshood (Aconitum napellus and carmichaelii) stating that these plants inhibit the growth of plants around them, particularly legumes. According to the writer, delphiniums have the same effect (if true, it might explain why my native lupines, planted near native delphiniums, did not reappear this spring).

    This is the kind of info I never thought of looking for, and now I’m wondering what else I might have “killed” by planting in combinations. Sigh.

  6. Thanks, Daniel! Not having a veggie garden myself, I am curious what success others have had with different organic mulches in terms of weed control and moisture retention. Though I’ve been cautious about recommending wood chips for some of the reasons mentioned earlier, I have heard from some gardeners that they swear by wood chips mulches. Let’s see what we can come up with in this thread.

  7. I use pine needles for mulch in the vegetable garden for two main reasons:
    1) There are a lot of pine trees in the neighborhood and I rake up the pine needles from the gutter.
    2) They rake away easily when I’m ready to plant the next set of crops. Wood chips separate when you try to rake them away. Straw/hay also works well for a vegetable mulch, but I’d have to pay for it.
    3) And oh yes, they work well at keeping out the weeds.

    Here in Florida, we can grow vegetables year-round.

  8. I use straw and grass clippings on top of newspaper or cardboard. Plus any yard waste or weeding debris goes right on top of the straw. Rethinking the newspaper and cardboard, based on the sheet mulching info provided here, although I do till in the Fall and plant a cover crop of winter rye, if I have my act together, and I use soaker hoses under the newspaper/cardboard layer.

  9. Peeling very thin flakes of old, old bales of straw and laying them out like carpet sqares on top of my soaker hoses has made problem weeds nonexistent in my veggie garden. Straw is easily pushed aside for plants or rows of seed.

  10. We had a load of manure -brought in to amend our community garden’s soil, and were delighted at how few weeds germinated before it was time to plant. We had a big planting day — seeds and seedlings of chard, peas, beets, basil, carrots, tomatoes, beans, cukes, zucchini, broccoli, peppers — and were excited about the bumper crop we envisioned. But woops — the manure mix turned out to have been mostly sawdust and wood chips, and while it’s impossible to tell if that’s the sole reason we have such a measly garden, I suspect that it’s the main reason that most of the new seedlings bit the dust, and the why the plants that did make it had to struggle to do so. It may be that the wood material was too fresh. Night-eating insects attacked most of the seedlings (perhaps sensing stress), and what’s growing out there now is mainly tomatoes (slow starters, now rallying and blooming), some big pea transplants, and a few bush beans. I’m still trying to find out if we can make amendments to neutralize the effect of the wood chips and sawdust, or if we just have to wait out this season. We had gotten UMass Extension soil tests, and are doing the garden organically. Now we know to be more careful about what kind and age the ‘manure’ is. At least the place still has almost no weeds…..

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