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The very first layer of a Hugelkultur mound is comprised of wood. The wood layer is covered in additional layers of leaves, compost, manure, soil, and mulch, all of which encourage the wood layer to break down slowly over time. The type of wood you use in this layer can have a huge impact on how well your garden performs. So what are types of Hugelkultur wood to avoid, and which woods should be used instead?
Allelopathic trees that produce substances that kill surrounding plants, such as black locust and tree of heaven, shouldn’t be used in Hugelkultur. Other trees, like cedar, juniper, and Osage orange, can take too long to break down. But oak, apple, and maple woods work well, so choose them instead.
What kinds of Hugelkultur wood should you avoid?
There are many kinds of wood that are suitable for your first Hugelkultur layer. But there are also a few that you should be careful to avoid. So before adding more layers to your Hugelkultur mound, double-check to make sure you’ve excluded the following trees.
Tree of heaven
Also known as ailanthus, most people consider tree of heaven to be a weed. This invasive species has the added disadvantage of being popular with the spotted lanternfly, a planthopper that can destroy vegetable plants and fruit trees almost overnight. Even when their preferred host has been removed, lanternflies will stick around to consume other vegetation.
There are a couple of reasons why you don’t want to use tree of heaven in your Hugelkultur mounds. The first is that the logs can perk back up. Placing pieces of tree of heaven at the bottom of your Hugelkultur could encourage them to sprout — and because these trees are so invasive, that could leave you with a serious problem on your hands.
The second reason why tree of heaven is a Hugelkultur wood to avoid is that this species inhibits plant growth. Ailanthus tree extracts work to destroy their competition leaving more nutrients for them. Trees that secrete substances designed to kill surrounding plants are called allelopathic trees. Using tree of heaven logs in your Hugelkultur can stunt plant growth and reduce crop yield.
Another allelopathic tree, black walnut produces a compound called juglone. This substance can poison plants up to 80 feet away and is so strong that it can prevent many plants from germinating at all. Signs that a plant is suffering from juglone poisoning include brown leaves, withering, and stunted growth.
Because black walnut actively works to kill nearby plants, it’s a poor choice for your Hugelkultur mound. Interestingly, some fruit plants are immune to juglone poisoning. So you can still use the black walnut trees on your property to create wood chips to use as mulch for quince, peach, cherry, and plum trees.
Pecan wood also contains juglone. Because these trees require a ton of nutrients and water to produce, they can’t afford the competition. But, because of this, they’re not an ideal choice for Hugelkultur beds and should be avoided. Pecan wood chips, on the other hand, are highly beneficial to the soil, significantly increasing the organic matter that plants need to grow well.
The good news is that cedar doesn’t release any harmful toxins, so it won’t inhibit growth. Unfortunately, cedar is resin-heavy and extremely rot-resistant. It can take years for cedar logs to start to break down, sometimes decades. So while it’s okay to use a couple of cedar stumps in your Hugelkultur, it shouldn’t make up the bulk of your log layer.
While cedar may not be the best choice for your log layer, it can work very well as a mulch. Because cedar is rot-resistant, cedar mulch can last much longer than mulches made from other woods. Cedar extracts also work to repel harmful insects to keep the plants in your Hugelkultur mound bug-free (homemade insecticides can help keep insects at bay, too!).
Like the black locust tree, Osage orange trees can take years before they start to degrade. This is because the vascular system of the Osage orange becomes clogged with dead cells, so there’s no airspace for water to penetrate. The result is an incredibly rot-resistant timber that won’t provide enough nutrition for the plants in your garden because it simply doesn’t break down fast enough.
Also called false acacia, these trees are covered in thorns, leading many homeowners to chop their black locust trees down. Because these trees are invasive, it’s not uncommon to remove huge groves of black locust trees at one time. If you have a bunch of black locust trees on your property, it can be tempting to use them in your Hugelkultur, but because they take so long to break down, you shouldn’t.
Juniper trees are much like cedar trees — in fact, in certain parts of the United States, juniper trees and cedar trees are considered interchangeable. They’re not, of course. Cedar trees are tall evergreens with fan-like leaves. Juniper trees are also evergreens, but they’re not really trees at all, they’re more like tall shrubs. And they produce hundreds of tiny berries that deer love to nibble on. But because juniper is just as resin-heavy as cedar, it, too, should be excluded from your Hugelkultur beds.
Which hardwoods can be used in Hugelkultur?
Hardwoods are the gold standard of Hugelkultur woods. These woods break down at just the right rate, providing the plants in your garden with ample nutrition for years. A good Hugelkultur mound can last for more than a decade, and using the appropriate hardwoods is the best way to extend the lifespan of your Hugelkultur mound. Here are some of the best hardwoods for Hugelkultur.
Oak wood is widely available throughout the United States, so it’s pretty easy to find. Even if you haven’t got any oak on your property, it’s not too difficult to track down oak firewood — and because firewood is already chopped up, using it for the log layer of your Hugelkultur mound can save you an extra step. If you have plenty of oak at your disposal, you can use all oakwood for your log layer, then turn the extra into wood chips to use as mulch when you’ve finished your Hugelkultur mound.
If you’re lucky enough to live near an apple orchard, try to get your hands on some apple wood, because it’s some of the best wood for Hugelkultur. Applewood starts to break down much more quickly than resin-heavy hardwoods cedar and juniper. But because apple wood is also a hardwood, it can last for years, prolonging the life of your Hugelkultur mound.
Maple trees are among the fastest-growing hardwoods. These trees can grow in zones 3 through 9, so they’re easy to find all across the U.S. Maple wood is an ideal Hugelkultur wood because it starts to break down quickly, but because it’s hardwood, it degrades slowly.
Which softwoods can be used in Hugelkutur?
The log layer of your Hugelkultur mound can be composed entirely of hardwoods. But, if you’re having trouble tracking down enough hardwood, you can pad the layer out with some softwoods, too. Here are some of the best softwoods to use in Hugelkultur.
Some argue that pine wood can lower the pH of the soil, making it more acidic over time. But, while pine needles can be acidifying, pine wood is not. And even though this wood, like cedar and juniper, can be resin-heavy, it’s also much softer by comparison. Pine wood is fairly porous, and when covered over in mulch, compost, and dead leaves where it can be kept warm and moist, it breaks down very nicely.
Spruce is another Hugelkultur wood that’s worth your consideration. Even though it’s somewhat resin-heavy, it also breaks down fairly quickly. For best results, it should be used in tandem with hardwood, like oak, to help stagger the breakdown rate. But, otherwise, spruce can be great for use in Hugelkultur.
This one’s trickier than pine and spruce. As a softwood, willow can be very useful in Hugelkultur. It breaks down nicely. But, unfortunately, willow is incredibly adaptable. These trees never say die. In fact, willow trees secrete regenerative substances that can be used to create your own rooting hormone.
Baby willow trees frequently spring up from old stumps. And new willow shoots can grow from seemingly dead logs. So if you want to use willow in your Hugelkultur layers, make sure they’re old, dehydrated, and have no chance of springing back to life.
Creating the best log layer for your Hugelkultur
When building your Hugelkultur, it’s best to use whatever materials are available locally. Hugelkultur mimics what happens on the forest floor, with logs being covered in dead leaves and other organic matter. The trees native to your region are best suited to restoring minerals that your soil is naturally deficient in.
While the softwood and hardwood trees mentioned here can help you build the best Hugelkultur possible, it’s even more important to avoid allelopathic trees as well as those that break down slowly. Keeping these harmful woods out of your Hugelkultur helps the plants in your garden stay healthy.