When I first started my garden, I had no other option but to place it in the vicinity of a walnut tree. I’d heard they may be toxic to vegetables, but the tree is far enough not to significantly shade or drop leaf litter on my plants. At the end of the season, I have a copious amount of walnut leave and husks to deal with.
Since I love to compost and recycle everything, walnut leaves should be no exception. So what’s the verdict? Is throwing walnut leaves and husks in the compost bin a big gardening no-no, or are there ways around it?
Walnuts are allelopathic trees (toxic to plants growing under their canopy) but you can compost walnut leaves and husks with the right steps. Their toxins are only released when the plant is damaged so if you currently have a walnut tree overhanging your garden, remove any fallen walnut leaf litter.
If you plan on removing a walnut tree next to your vegetable garden you might need to move your vegetable garden, as their roots release toxins into the soil that can prevent fruits and annual vegetables from growing for two years after the tree is gone.
In this guide, we’ll talk about the risks of keeping a walnut tree, vs the risks of removing a walnut tree, the difference between an English walnut, and the notorious black walnut, and how you can safely compost walnut leaves and husks.
Black walnut vs other types of walnut
Juglans nigra, the black walnut tree, is the most toxic tree in the walnut family, with nearly twice as much juglone as any of its relatives (more on that clever chemical later).
Because of their toxicity to other plants, most people who search for walnut trees online are searching for advice about removing them. But while toxic, black walnuts have the richest, most savory, nuts that can be eaten fresh from the tree when they fall or dried for storage, so if you can keep the tree, you should.
Black walnuts are the only wild tree nut in the United States, with most walnuts grown in cultivation as orchard crops, including the common English walnut, which is found more readily around Europe and has less of a toxic side effect on the rest of your garden.
What is the juglone toxin in walnut trees?
The juglone toxin is present in every part of the walnut tree other than the nuts and can be fatal if ingested in its pure form by humans or animals (but that would require eating an entire bowl of sawdust). Juglone is a pesticide, produced by Juglans and Carya trees to kill competing plants, and deter and kill insects that try to eat their young leaves or nest under their bark.
While the toxin is present in all parts of the plant, and in all strains of the walnut family, it is inert until the tree is damaged. If you have a fresh walnut husk, or a recently fallen leaf or twig, Break it open and watch the living green materials change quickly to brown. This is pre-juglone, turning to toxic juglone.
Juglone is most present in the husks that protect the young seeds until they are ready to germinate. The trees concentrate these toxins to increase the chance of germination and ensure the seeds germinate under the canopy and protection of their parents. Thus, benefitting from shared mycorrhizal systems around the roots, and from the pesticides enhanced by the fallen leaf mulch around the young plants from their parents.
What trees contain juglone?
Juglone is a toxin unique to trees in the family Juglandaceae. The Juglandaceae family has one particular grouping that produces juglone, called Juglandoideae, which includes:
- English Walnut (Juglans)
- Eastern Black Walnut (J. nigra)
- Butternut (J. cinerea)
- Persian Walnut (J. regia)
- All other walnuts
- Hickory (Carya)
- Pecan (C. illinoinensis)
- All other hickory
- Wingnut (Pterocarya)
Other Juglandaceaes have no traceable amounts of juglone – these include trees in the families; Rhoiptelea; Engelhardia; Alfaroa; and Oreomunnuea.
Can you compost walnut leaves?
It’s almost essential that you compost walnut leaves and black walnut leaves rather than mulching with them. The juglone toxin begins to break down after around 6 months, but if used as a layer of mulch on fresh ground, will quickly poison the roots of annual plants and vegetables.
By far the best method of composting is to use a three-year compost system, which allows six months for toxins to fully oxidize any pre-juglone into the toxin, and a further 6 months to leach out the toxin into the compost. The toxin will be fully inert by the second year, and by the third year, you’ll be ready to use your compost just as you would with any other deciduous leaf.
How can you make walnut leaves decompose faster?
To speed up the composting of walnut leaves you simply need to damage them as much as possible. If you have a large, even, area of hard ground, lay out the gathered leaves and twigs throughout autumn, and run your lawnmower over them.
Breaking them up like this speeds up the oxidation of the toxins, and means there is more surface area of each scrap of leaf litter to be broken down as efficiently as possible by the heat of the compost.
If you are removing a black walnut tree, the same applies to the trunk, bark, and branches. Shred them with a heavy-duty chipper if you have access to one, and compost the chippings for at least 18 months (If you don’t have access to a chipper, dispose of the logs, or air dry walnut logs under cover for 6-12 months for firewood – the ash can be used to support fruit bushes, carrots, and legumes).
Can you compost walnut husks?
Walnut husks are a touch more complicated as they are the second most toxic part of the plant (after the roots), and when they dry out they take an absolute minimum of 6 months to fully compost.
First, you need to understand the seed, to avoid mistaking the hull for the husk. A nut is a seed with a casing (a hull) so when we eat walnuts, we eat the seed and discard the hull. The husk is the fruit that surrounds the nut.
There is no risk of poisoning from walnut hulls. It’s the green husk you need to worry about, which instantly releases the toxins when it falls from the tree as the bruising damages the husk.
How long do walnut husks take to decompose?
Walnut husks take around 6 months to compost to a reliable state where you can use them as a general purpose mulch for most of the garden, and they will continue to oxidize out any toxins in minimal levels after that. But to be safe, they should ideally be composted for 2 years as part of a three-bin compost system.
What’s really incredible about walnut husks, is that they produce alkaline compost if composted by themselves for 12-18 months (or longer), which is quite hard to achieve in most domestic gardens.
Uses for walnut compost/leaf mould
Compost made from walnut leaves or husks is more alkaline than most garden composts and varies between a pH of 7 and 8.5, which is ideal for growing beetroot, celery, cauliflower, and cut-and-come-again lettuces (like rocket, butterhead, and spinach).
When mixed into an existing compost heap, walnut husks and leaves help your compost pile retain balanced nutrients and minerals in the same way as any other deciduous tree, but there are no particular benefits reported over the use of walnut compared with other trees.
Can you plant where a walnut tree used to be?
In theory, you can plant anything in the site of an old walnut tree, but black walnut trees are slightly different. Because of the high concentrations of juglone, as soon as the tree is felled it oxidizes pre-juglone down to the roots into the ground creating a toxic root system, which won’t re-grow and won’t allow anything else to grow in its place for at least one year.
Acers, willow, apple, cherry, and pear trees will struggle to grow at all, so you should ideally wait at least two years after felling a black walnut to plant any fruit or ornamental trees in its place.
What vegetables will grow under a walnut tree?
There are a few gardeners online claiming that squashes and melons will grow under walnut trees, but they would have to be in a greenhouse for most gardeners in the northern hemisphere, which would be shattered every time a walnut husk dropped from the tree above. So, here’s our brief guide to the plants that will actually grow under a walnut tree.
Carrots, parsnips, and beetroot will happily grow under a walnut tree, as will onions and garlic, but make sure to rake up any fallen leaves after harvesting to reduce the amount of toxins in the soil. You can also grow leek, sweetcorn and cauliflower, but protect any overwintering crops from falling walnut husks in mid-late autumn.
The best choice is a low-maintenance crop like Jerusalem artichoke, which can withstand the annual onslaught of juglone from the walnut tree and can be harvested lazily without risking too much damage to the tree’s roots.
While I’m being overly cautious with the information above, it’s because walnuts can be very difficult plants to manage, and very damaging to vegetable gardens if you don’t maintain them, and the soil beneath them properly. Almost all parts of even the most toxic black walnut can be used as a mulch or compost safely after 6 months of composting with good aeration, but they will still have traces of juglone.
By composting fully in a three-year system you will remove all traces of juglone and will be able to plant even the most sensitive plants into pots of walnut compost, including carrots, apples, and fruit bushes.