Oak leaves are slow to decompose, but does that mean you should ban them from compost piles and mulch?

You often read that you shouldn’t put oak (Quercus spp.) leaves in the compost, because they’ll be toxic to micro-organisms … or is that rather that they’re too acid? (The proponents of this garden myth never seem to be able to agree on the explanation!) Nor, say these same authorities, should they be used as mulch for the same reason(s). But the whole idea is essentially false or at least, highly exaggerated … but it does contain a pinch of truth, as is often the case with garden myths.

The Truth About Oak Leaves

It’s true that oak leaves contain a lot of tannins, phenolic substances that would be toxic to humans if we ate too much of them … but nobody munches on oak leaves. Tannins in too high a concentration are also toxic to certain herbivores (horses, cows, etc.) … and they’ll avoid eating oak leaves if they have any other possibilities, largely because tannins make the leaves very bitter. There are many micro-organisms that will turn toxic tannins into harmless byproducts … but others that won’t touch leaves until the tannins have been broken down.

So, a bit of truth there, however…

The same tannins are present in most other tree leaves too, not only in those of oaks. Tannins are, in fact, very abundant in forests. It’s tannins, for example, that give the rivers that flow through forested areas their brown tea-like coloration. This happens even in northern forests well beyond where oaks grow. Water rich in tannins remains as drinkable as other similar water sources and harbors a wide range of fish species.

Also, humans regularly consume tannins without any harm. The clear brown color of tea comes from tannins, as does the somewhat astringent taste of red wine. And if distillers age whiskey in oak barrels, it’s so the tannins oak wood gives off enrich their taste.

20170515B colin grice, WC
Fresh oak leaves are acidic, but by the time they decompose, most of that acidity is long gone. Photo: colin grice, Wikimedia Commons

As for acidity, true enough, the freshly fallen oak leaves are certainly acid, but they become less and less so as they decompose. At the end of the process, they end up being, depending on the species, slightly acid to even a bit alkaline! And “slightly acid” is actually the acidity most gardeners want for their garden soil.

As a result, the acidity of oak leaves does no harm to plants when they are used as mulch, nor does it make the soil more acidic than it originally was, much to the disappointment of rhododendron enthusiasts, who often mulch their favorite shrub with oak leaves under the mistaken belief that they will acidify their soil. (Rhododendrons and azaleas are, with blueberries, among the few plants that grow best in very acid soils, but mulching is not going to acidify their soil.)

In fact, if you analyze the soil under large oaks where their own leaves have been allowed to decompose for decades, you’ll find it to be … acidic, neutral or alkaline, depending on the pH (acidity level) of the underlying rock. Even after 100 years of superimposed layers of oak leaves, they will have had almost no influence on the acidity of the soil.

The simple truth is that mulch almost never modifies the pH of the soil underneath, no matter what it is made of.

But Oak Leaves Aren’t Perfect

So, when oak leaves are accused of poisoning soil or compost or being too acid, that’s essentially a myth … but that doesn’t mean oak leaves are necessarily a boon to gardeners.

First, oak leaves are very slow to decompose. Not only do they tend to be tough and leathery compared to most other leaves, but the presence of a lot of tannins does seriously slow down decomposition … and usually what you want if you add leaves to compost is fast decomposition.

Oak leaf mulch.

Also, when oak leaves are entire, they make a poor mulch, as they tend to clump together, forming an almost impenetrable layer that perennials and ground covers have trouble breaking through.

That’s why it’s always best to shred oak leaves before using them. Run them under the lawn mower, vacuum them up with a leaf blower (it will chop up the leaves as it picks them up), pour them into a garbage can and shred them with a string trimmer, or whatever. The method is up to you, but when you do break oak leaves into small fragments, tannins will be largely rinsed out the first time it rains, reducing the so-called toxicity to almost nothing, and bacteria will start to decompose the leaves in earnest.

It’s interesting to note that, in several of the world’s greatest gardens, shredded oak leaves are actually the preferred mulch, as they last a bit longer than other leaf mulches!

So, don’t be afraid to use oak leaves in your compost bin or as an ingredient of your mulch: they are essentially harmless and can even be most useful. Just make sure to shred them first!20170515A

23 comments on “Garden Myth: Oak Leaf Toxicity

  1. Ben Jackson

    I have a large bin I made from cattle fencing from Tractor Supply which I use to place my Oak leaves. They are breaking down very well and I plan to use them around all my yard plants and in my raised garden of pepper plants which I grow to make Cowboy Candy.

  2. Anecdotal evidence, but I have shredded my oak leaves in fall, put them in the plastic bag outdoors for winter, and in June put a heavy thick layer of mulch for my tomato’s raised beds. When I took the shredded leaves out of the bags, the water at the bags bottom was all black in color, obviously I drained it away from my tomatoes. But I have noticed that when I was deep watering my tomatoes, the water that was trickling down from the bottom of the raised bed was still brown in color, like tea, from the the oak leaves mulch. This year my tomatoes do not grow as well as they usually do. I am growing a few tomatoes in bags, and they seem to grow much better. One tomato even had its leaves all curled. I am suspecting the tannins from the thick layer of shredded oak leaves mulch is stunting tomato growth.

  3. My property is mostly moss, which I love and try to encourage. The oaks dump huge amounts of leaves and while I prefer to blow them off and into my wooded lot, this year my partner insists on mowing them and leaving the mulch on top of the moss. Is my moss doomed or should I covertly blow the mulch off the top in secret?

    • The mulch won’t harm the moss at this season: it will dormant or close to being dormant. However, if no light gets through in the spring, yes, that will reduce the moss.

  4. What about using oak leaves, intact, as part of a hugelkultur? I have a huge oak and so much debris after leave drop.

  5. https://csuvth.colostate.edu/poisonous_plants/Plants/Details/5

    Oak trees are poisonous. Farmers advised to take uo acorns which horses and cattle consume if grazing.

    • Be careful of generalizing. Not all oaks are poisonous. Reread your reference and you’ll see. In many countries, acorns (of the right species) are used as fodder.

  6. Michelle

    With the “leave the leaves” movement, what would be a happy medium? I have oak leaves I raked into my new native perennial beds this fall. I don’t want to shred them because of all the beneficial insects who use them. I also don’t want to leave them to smother the new growth. Should I cover in the fall and relocate in the spring, once the insects are “awake”?
    Thank you

  7. Libby Neves

    I have cut down three huge boxwood from underneath a 10 year old red oak. This tree grows in a triangle of soil in the middle of a concrete patio next to my house. It gets morning sun and afternoon shade. The air circulation is good. The triangular bed is about 8x14x12.. My plan is to have no more shrubbery under the tree, just a few Chrysanthemums along the outside of the bed where there is sun during the summer.
    Until now the bed has retained, and I have sometimes added, a thick layer of oak leaf mulch. Today as I raked the bed I found a layer of white matter under the newest leaves and on the surface of the soil.
    I have a challenge with this tree. It harbors a scale infestation, so I think I have to remove all the old mulch to remove next year’s potential infestation. I also plan to paint the tree And treat the soil according to horticultural guidelines.
    My question is, what is this white matter? Is it beneficial? It looks like a layer of rotted newspaper, but I haven’t put newspaper in the bed. It feels spongy, like a fungus.
    Thank you.

    • It’s simply a fungus that digests leaf matter. There are thousands of species of them and they’re harmless. And, in fact, beneficial, as they release minerals to tree roots. They come and go as leaves decompose, so if you don’t dig into the mulch at just the right time, you likely wouldn’t notice them.

  8. Would the same “facts” be true for pecan tree leaves…? Years ago I was cautioned to not add pecan leaves to my garden soil….. reason given was that the tannic acid contained in the leaves acted as a natural herbicide and would not be good for the soil….

    Appreciate your reply…

  9. Thank you for this comprehensive article. I am about to establish a blueberry farm. My farm is surrounded by beech and oak wood forest. I would like to use forest duff and/or shredded branches of beech/oak wood for mulching material. I have been told that forest duff rots too fast and oak is high in calcium.. Given that mulch has a very slight or no effect on the soil Ph, can I use a mixture of shredded beech and oak wood and forest duff as mulch material for highbush blueberries.

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    • You haven’t fully done your research. Intact oak leaves do take a long time to decompose (but not necessarily 2 years: that depends on a a whole host of factors, including the species of oak), longer than most other leaves (you’re right there), but when finely chopped and exposed to the elements, their tannins disappear much, much faster.

  11. What a great article! Thank you for clearing up so many questions on this topic – both succinctly and with ample supporting information. I will be sharing this for sure.

  12. Pingback: Garden Myth: Pine Needles Acidify the Soil – Laidback Gardener

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