Choosing the right wood for the core is critical, as some species can hinder growth. Hardwood trees break down slowly, maintaining the mound longer. Softwoods also work as well, but they decompose faster. Beneficial tree varieties include alder, apple, cottonwood, poplar, and birch. Some trees to avoid are black locust (doesn't decompose well), black walnut (toxic to other plants), and redwood (issues with decomposition and seed germination).
Next, gather the compostable materials, which can include grass clippings, straw, cardboard, manure, and aged compost, to name a few. Place the compostable materials in the empty spaces first, filling in the spaces evenly around the mound. Some people choose to lay entire sections of sod across the mound with the grass-side down against the wood core. Since this will be making up the majority of the hügelkultur, it is essential to keep the angles of the sides at 45 degrees to reduce soil compaction and increase oxygenation.
Finish the mound with a layer of topsoil several centimeters thick. You can begin to plant on the hill immediately after construction, but it is recommended to let it settle over the winter months.
You can make a Hügelkultur any time of the year. The best results occur after the mound has had time to set up. Why? The carbon in the wood needs time to absorb both the nitrogen-rich nutrients and the water. Once the logs become saturated to capacity, they will begin to release the nutrients back into the mound and allow plant roots to wick the water from the hügelkultur's core. Because of this, it is recommended to construct in the fall, allowing the mound time over winter to mature before planting crops the following spring.
Note that the mound will shrink over time. The 2.1-meter mound will turn into 1.8 meters in the first year, with the rate of shrinkage slowing after the second year. If you want to keep a specific height, you can add fresh layers of green manures or composts to the bed each year. Where can you find excess compostable materials? Try reaching out to your local municipality. They clear roadsides of brush, cut back trees from powerlines and pick up green wastes, all of which go to a compost pile and might be available for your use.
The main challenges associated with hugelkultur are nutrients and pests.
Nutrients
In the first years, the wooden core will be absorbing nutrients (i.e., nitrogen). If you notice that your plants are showing signs of nutrient deficiency, you can feed nutrients to the hügelkultur or plant beneficial crops, like nitrogen fixers. When feeding the hügelkultur, too many nutrients can also become a problem, leading to leaching from the mound into the surrounding soil. Too alleviate the risk of leaching, add massive feeding plants—like the brassicas: broccoli, Brussels sprouts, cauliflower, and kohlrabi—at the base of the hügelkultur. Some examples of useful medium-demand plants for the mid-slope include cucumbers, potatoes, squash, and tomatoes. Leave the top for low-demand vegetables like herbs, collard greens, peas, and beans.
Pests
If you live in areas with termites, hügelkultur beds can turn into a nuisance. Termites are recognized for their talent in breaking down the cellulose in wood, meaning your mound could become a great B&B. And while your hügelkultur may handle some termites, your home shouldn't have to. Be sure to research your area before building your hügelkultur if termites or carpenter ants are a concern.
Finally, don't plant trees atop your mounds. As the hügelkultur settles, it can potentially damage or weaken the roots that provide the support structure, causing the tree to tip over in heavy winds, losing the tree and damaging the hügelkultur. Trees can be planted in front or behind the mounds, at ground level, so that as the tree grows, its roots creep out into the areas where it can benefit from collected water and nutrients.
Hügelkultur is an old technique made new again and can be used to take your garden to the next level. So as you plan your garden for next year, you might want to start building a hügelkultur this fall!
