Black walnut has juglone in it, which can be harmful to other plants. I've read claims that you shouldn't use black walnut for wood ash in your garden because of this, although I'm not sure if that's true. It does seem pretty sure that soil black walnut is grown in is harmful, but it seems like burning the wood might change the chemical. Anyway, I don't know if it does, but that's not my question (it's a preface to my question):

I've heard that black locust (I don't mean black walnut) has allelochemicals of some sort in it that may hamper other plants (probably not juglone). I'd like to use wood ash that contains a small percentage of black locust ash on ground for tomatoes. Maybe one in ten pieces of wood burned was black locust. Is this ash going to be okay for tomatoes and other Solanaceae plants, or will it hurt them significantly?

If you have experience with black locust wood ash on plants like tomatoes, that would be preferable. I guess I could germinate some tomatoes and add some of the ash to the soil to see what happens.

I've heard people use black locust for nitrogen fixing. So, it doesn't seem like it would be that harmful to other plants. But, maybe they just use it for tolerant plants.

Squash are said to be tolerant to juglone. I'm not sure what tolerates whatever allelochemical(s) black locust uses, whether it persists in soil for years like juglone, or what. I'm not sure how potent it is, or if it's even in the black locust wood. I don't know it if affects plants at all (maybe it just affects animals). Black locust does have toxic parts to animals.

  • When you say black locust, do you mean Robinia pseudacacia?
    – Bamboo
    Jan 17, 2017 at 17:28
  • That would be the only plant I know with the common name Black Locust. Not at all Black Walnut. I am confused...@Shule, which one do you have? Robinia or Juglans? You are correct about Juglans and yet you know Robinia is a legume plant.
    – stormy
    Jan 17, 2017 at 20:42
  • @stormy - He says in the question he's not asking about Juglans... I just want to make sure that the common name 'black locust' is not being used for a plant that isn't Robinia... that being the nature of common names, to be used for more than one plant.
    – Bamboo
    Jan 17, 2017 at 21:10
  • Grins, thanks, Bamboo!! I kinda thought so but puppies make one very distracted and exhausted and that is my excuse these days!! I need to go check out ash because as I remember ash ain't that great. Remember when burning field debris was common? Not at all indicated these days...Graham's information makes me hesitate...tiring to keep checking out our information to see if it was real or old wive's tales, grins!
    – stormy
    Jan 17, 2017 at 21:29
  • @Bamboo Yes, I mean Robinia pseudoacacia. :) Jan 17, 2017 at 23:02

5 Answers 5


I've used black locust ash for many years in my garden which thrives! In fact, I had a problem with my broccoli and brussels forming leaves but few heads, and after trying many different soil amendments found that wood ash (incinerated at high heat) solved the problem. I have gorgeous heads now and the largely black locust wood ash was the only variable which changed. It's a great wood for flooring, siding and beams, just saying. It's had a bad rap.

  • Since I asked the question, I tried it, too, and it seemed as good as other wood ash. :) Stormy is right that it can raise the pH, but there's something magical about it. Tomatoes and maybe even peppers really seem to like it, despite the pH, while tomatoes don't seem to like rockdust nearly as much by comparison (maybe it's because it's already a composition of minerals utilized by plants, in forms plants have used, unless the burning changes the salt forms). Feb 8, 2018 at 1:35
  • Tomatoes use a lot of calcium and potassium anyway, though (so if those are the only alkalizing components, I doubt the pH will be much higher for long—unless too much is used—due to the calcium and potassium being used up). I could be wrong, but as someone who possibly suffers from chronic mercury toxicity (from other sources), my symptoms don't worsen when I eat tomato fruits grown in ground with wood ash; if I were growing rice, which is known to absorb it easily, that might be another matter. My symptoms do worsen when I eat some (not all) brands of rice, though. Feb 8, 2018 at 1:52
  • Tomatoes do lessen blood levels of mercury, though; so, that might be why (even if it's in the fruits), but I haven't noticed symptoms worsening with other garden crops, either. Feb 8, 2018 at 1:54

If you're incinerating the wood, then any allopathic chemicals would be oxidised by the fire, and what's left is wood ash.

Much wood ash contains calcium carbonate as its major component, representing 25[6] or even 45 percent.[1] Less than 10 percent is potash, and less than 1 percent phosphate; there are trace elements of iron, manganese, zinc, copper and some heavy metals.[6] However, these numbers vary, as combustion temperature is an important variable in determining wood ash composition.[5] All of these are, primarily, in the form of oxides.[5]



Robinia pseudacacia is thought to be allelopathic to some extent, particularly with regard to fallen leaves, but not much research exists as to which plants it might affect. The bark wood is often fashioned into fence posts and used, untreated, when it lasts some years, with no ill effect on surrounding planting. As for its toxicity, its certainly a toxic plant for horses, but many other animals forage from it with no apparent problems. More information on this plant here, where some reference is made to its possible toxicity and allelopathic effects http://tcpermaculture.com/site/2013/11/18/permaculture-plants-black-locust/

I agree that the ashes from the burned wood are unlikely to contain any toxins which might affect your plants, but use of any wood ash should be limited because of its alkalising effect.


I use ashes for my garden. AFAIK, the problem is on the content: the heavy metals: some plants accumulate much of them in wood, some vegetables absorb them, and it is toxic for us (but it is also needed on small quantities).

So now I use not so much ashes on vegetable garden. I still use it on small quantities, and with much manure. Most of the ashes go to the lawn: it seems that my "mosses" doesn't like it, and I don't like them on my lawn.


Ashes are alkaline, big time and have high salts! For the lawn with lots of rainfall to leach salts they are fine. But to use them in the soil is not a good idea. Salts are awful! And compounding the problem is the use of tap water (chlorine)...I would never use ash on tomatoes! They love acidic soils (6.0 to 6.3). Ash is great if you have heavily acid soils...great for lawns that want alkaline...7.0. Otherwise ash is another 'unknown' added to your soil that will definitely mean a proper soil test, before and after. Working with ingredients that one doesn't know the pH, the percentages of chemicals is just making more unknowns that aren't worth the trouble to add to your soils. They stopped burning sugar cane crops for example...they stopped burning the debris of crops when they found how ash is not a great addition to any soil. The 'good' stuff is volatilized and unusable to any ecosystem. I don't want to be cremated. I want to feed the decomposers, soil organisms, thus plants, the small and larger mammals...otherwise ash is kind of worthless. My opinion...of course.

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