I have been using this technique the last few years with GREAT success. Beds constructed two years prior were VERY productive this past season, but so was my new one!
This link has a general overview of the technique and lots of good photos and illustrations.
In my own words:
The term hugelkultur is German and essentially just mean "hill cultivation". The basic premise is to take some large pieces of wood (fresh, dried or in most often rotten) and bury them underneath a raised garden bed. As the logs break down and turn into "compost" they will absorb excess water and nutrients before these can run through the soil and out. Ever checked out the light, spongy texture in a dry rotting log? It’s very similar to a sponge. Water will actually wick up into the soil above as it dries out. Plus any perennial plants that have reached the logs with roots will be able to access this store of water and nutrients any time. Water use for irrigation can be DRASTICALLY reduced or even eliminated entirely! It's a bit like thermal storage in solar heating, except with water and nutrients for plant growth. Also the decomposing matter will create many air pockets and microclimates that roots and microbes love. Beds built in this way are known to be "self-tilling" as any good garden plot can and should be.
The first year or two the decomposing matter in the hugelkultur bed will also generate some extra heat and generally have temperatures a few degrees higher than raised beds of just soil. Any raised bed tends to warm up a little faster in the springtime and maintain a few extra degrees during the season vs. flat beds.
A few primary considerations to keep in mind.
WOOD SPECIES is important. Most wood is suitable but certain species are known to resist rot for decades and are thus not good for this technique. These woods contain large amounts of highly effective anti-fungal/microbial compounds that can take decades to dissipate/breakdown. Cedar, black locust and redwood are good examples. These properties make said woods highly desirable for building purposes anyway and are best used as such. In most of N. America it is incredibly easy to find "waste" wood that is well suited. Pile of brush you’ve been meaning to haul off, chip or burn? Bottom row of your wood pile getting punky and rotten? Tree services are ALWAYS looking to get rid of wood chips and generally its easy to get a whole tree worth of logs when one is being removed. Source your woody material as locally as possible, you shouldn’t need to go far. Untreated construction scraps(2x4's etc.) are fine, just stay away from anything with glue or chemicals!! NO plywood or OSB!!
Hard woods like Oak and Maple will take longer to begin cycling nutrients /water for you but the trade-off is they will continue to do so for many, many years. Softer woods like pine or cottonwood will breakdown faster thus you see benefits sooner, but their function declines sooner. If possible I suggest large diameter hard wood. Fresh cut wood is actually quite good though some recommend waiting a few weeks to allow any natural anti-microbial/fungal compounds to dissipate a bit before they are entombed. Most trees produce some of these compounds but the aforementioned species are especially effective and very slow to dissipate. “Old” or rotten wood that doesn’t seem to have another use will work just great, in fact it will be broken down more quickly and begin cycling much faster than fresh wood. Again the trade-off is in the long term performance. Old, rotten, and very soft woods won’t perform quite as well as hard, sound wood after a decade, but hard, sound wood often can be put to other valuable purposes while hugelkultur is an excellent way to clean up an ugly old brush pile or get some value out of wood with otherwise limited use.
WOOD SIZE will also affect the performance of the bed along the same general trend. Larger pieces, such as big logs will take a couple years to really get breaking down and storing water etc. however they will give better results in the long haul and are generally prefereable to smaller pieces like wood chips, brush, twigs, or slash. These latter materials can still be used, and they will be well on their way to compost by the end of 1 season. the trade-off is that they will be all broken down into fine compost sooner and won't give as good of results 5, 10 or 25 yrs down the road compared to large diameters. Its similar to the hard/softwood duality from above. A great way to get the best of both is to put down a layer of big diameter logs, and then on top of them some small stuff like chips, twigs or brush . The small stuff on top will break down quickly and give you quick performance, while the larger wood will persist for the long term benefits. Plus adding more layers helps you build up a nice tall bed. Raising the beds up several feet not only packs more plants into your square footage it is also much easier to tend to, especially for elderly or physically limited gardeners. If small, soft wood is what you have available don’t fret it works just fine. You’ll see better short term results anyway and as your thumb gets greener you can continue to improve your beds each year. You may not even notice any “lowered” long term performance once things have totally disintegrated into “compost”. A thick pile of compost has lots of storage capacity in its own right. So just find the best you can and go for it!
ADD NITROGEN to speed up the compost and balance the very high C(arbon):N(itrogen) ratio of the wood. In a nutshell compost needs a balance of these two elements for efficient breakdown by microorganisms. They need more N than the wood has and if you don't add any the logs will actually suck N out of the surrounding soil for a while. Eventually things will reach a balance and the plants will be able to get nitrogen in slow release from the wood, but you'll see sub-par growth for a couple years. Typical Nitrogen sources include, animal manure or green plant matter like fresh grass clippings or weeds. a rule of thumb is if material is brown and dry it will be mostly Carbon, if it is still green it will have significant Nitrogen. Nitrogen is lost easily back to the air as plant matter dries, so use fresh stuff or find some free for the taking manure from a barn or pasture nearby.
Sufficient nitrogen will get things composting faster and generate more heat in the first year. Plus plants need plenty of Nitrogen to support vigorous green leafy growth.
IF your plants are very dark dark green and you experience delayed or retarded flowering/fruiting that is an indication of too much Nitrogen. Bitter greens even when young is another sign of higher than desired nitrogen. don't worry it should be balanced out by next season.
If you lack a good nitrogen source and can't bring in a truckload of someone else's "waste" nitrogen then I suggest planting legumes like beans, peas, and clover in your first season as these plants "fix" nitrogen directly from the atmosphere (~70% Nitrogen btw) with the help of bacterial symbiosis in their roots. Always a good idea to integrate some perennial nitrogen fixers throughout the garden too. There are actually many trees and shrubs that are "legumes", best to find out which are suited to your area, preferably natives. Astragalus and licorice are small bushy herbs highly prized in chinese medicine that also fix N. I have some seeds but have not gotten to sprouting them and getting them established with all the projects I'm juggling. I hear the gophers love'em too and there are lots of "pocket gophers" in my area. Some gardeners tell horror stories about the gophers, but they never bother me much. Last few years they have stayed out of the garden almost entirely until the end of the season and then they showed up and dug burrows but only munched a few plants, pretty much a non-issue so far.
OTHER ORGANIC MATTER (non-woody) like leaves, hay, straw, etc. can be used in layers (search “sheet mulching”) to build up height and create a nice thick layer of compost quickly. I use several layers of whatever is available on top of the woody base layers to build up a nice layer of rich, fertile, compost/soil between the woody base layers and the top layer of topsoil. Again the C:N ratio is the main consideration here. Note the materials your using and estimate if they have a high or low C:N ratio. By alternating layers a few inches thick between high(like dry leaves or straw) and low(fresh clippings or manure) C:N materials you get balance overall yet it is easy to assemble without mixing. The plants themselves, and hopefully a happy population of worms, will mix the layers together as time passes. Within just 2-3 years you can dig into the mound and often won’t be able to distinguish any layers at all, just rich loose, earthy smelling compost/soil.
PH is important in soil and here are a couple ideas to be aware of. Certain plants have special PH needs but the majority of common vegetable crops will thrive in a neutral to slightly acidic PH in the 6.5-7 range. Woody, high Carbon material will tend to break down with a fairly neutral/high PH (7+). Higher Nitrogen materails, especially manures, will tend to contribute acidity(lower PH) as they break down. The neutral buffer of the woody material will balance the PH swing you might expect from adding manure or some of the other ingredients. After a few months or a year the mound will have found a balance and it will be well buffered within the desired range. Resisting any major PH swings from then on. Char(see below), typically has a basic(higher) PH though just how high can vary widely depending on its source material and manufacture. Rock dusts (see below) or other mineral sources will have a strong buffering effect on the PH but some minerals will push the PH towards a certain high or low value depending on the minerals present. Generally a little rock dust goes a long way for mineralization and the buffer effect of rock is not problematic, actually its helpful, once things balance out. Testing the soil PH on site is usually one of the first steps recommended in preparing a garden, but in the case of hugelkultur much of the material in the mound is just composted organic matter and only a thin layer of “soil” to top it off. If you see plants growing around the area your working in you should be fine, but PH tests can be done very cheap if you want to know what your dealing with or you run into problems. I have never actually tested mine at my current location but I have had no trouble either. I inherited an already established, though recently neglected, garden. I think in general if the compost breaks down well, the results will be fine since “compost” tends to buffer extremely high or low PH towards a more suitable neutral range.
SOIL goes on top of everything else for planting into. It also ensures the organic matter below stays moist enough to break down. Depending on your circumstances you will want to top it off with at least a few inches or up to a foot of the best topsoil you can manage. If the topsoil on site is decent just scoop it to the side before you start building up the wood etc, and then once you've reached more or less your desired height simply pile the topsoil back on top. If the soil is poor don't use too much or it will inhibit your roots from reaching down to the lush compost underneath. In extreme cases of “poor” or very heavy soil it might even be worth mixing some sort of amendments with it aiming for something like a standard "potting mix" to top off and plant in. That would only be practical in extreme cases.
PLANTING! Some people like to wait a few weeks/months to let things settle and get breaking down before planting, but you don't have to by any means. I often plant mine ASAP, even the same day! The heat generated from a freshly constructed and watered Hugelkultur mound/bed will greatly speed up germination in many cases. It takes just the right C:N balance to get a pile cookin hot enough to damage seedlings. Even such a pile is only that hot down into the middle a ways and by the time it reaches the seeds it’s a nice gentle warmth. Especially if you can manage a nice layer of topsoil. By the time roots can get down there the composting will have already produced it’s peak heat and things have started cooling off. If it is still too hot then the roots will just grow sideways until it does. Some like to build new beds in the fall so they will have plenty of time to settle and reach nutrient balance over winter and spring. I’m usually too busy in fall so I end up just building in early spring and plant right away. Only problems so far have been a couple times I’ve added too much nitrogen and retarded the fruit/flower production of crops like tomatos, squash, peppers first season. More on that later. I don’t mind much though cause it accelerates the maturation of the mound and by second year its balanced out and just super productive. Its easier to get good results the first year if you build in the fall and let them sit overwinter. But once you have the feel for your C:N balance you will get lightning fast germination and squeeze in a couple more weeks growing season on new beds by building and planting both in spring.
MULCH is a must for any dry or extreme hot/cold climate. Actually I strongly recommend mulching in any climate. In hot sun mulch keeps the soil surface cooler and reduces evaporation drastically. During cold nights it works like a blanket, keeping the roots in the soil below protected from frosty nights and slows the advancing freeze of winter. PLUS there is a lot of evidence that an actively decomposing mulch layer promotes healthy vigorous microbial activity in the soil. Nearly all healthy terrestrial ecosystems exhibit this layer. Harsh Deserts are the notable exception with exposed mineral soil and very low Organic Matter at all. I have been applying up to a couple inches fresh mulch every year or two and after 4 yrs there is an INCREDIBLE amount of life in this layer if you get down on your hands and knees for a close inspection. Common mulch materials are straw, leaves, wood chips, or dry grass. “weeds” in your garden should be pulled out before they have a chance to flower or seed and they make a highly beneficial mulch layer. If they have started seeding though its best to compost them in a hot pile or perhaps bury them deeply in a new Hugelkultur.
SETTLING: While you don't need to wait before planting, do keep in mind the height of the mound will settle a lot the first couple years as matter works its way into the nooks and crannies, plus the composting process tends to result in shrinkage anyway. Brushy or branchy material maintains lots of air space even if carefully buried and is especially prone to settling. If using brush or slash as your woody layers or if you have lots of “airy” layers like leaves and straw you should build your mound 25-50% taller than you want it to end up! If you want to limit settling try to compact things like brush or sticks and also you can work in some of your Nitrogenous material into the open air spaces as you build up. Fluffy loose soil is great for plant roots though so don't go overboard tamping things down, just limit the big air spaces. Better to anticipate the settling and plan for it than try to fight it. When you dig into it after a year or two you should find fluffy, loose, compost in place of non-woody layers. Big Logs won’t really settle, I bet they actually "expand" a little after some years as they break apart. Everything you pile on top will settle though and probably more than make up for it.
WATER is necessary to composting and dry conditions slows it to a crawl. Sufficient water greatly speeds up the compost process. These beds can absorb a LOT. If its practical I recommend watering each layer heavily during construction. After the mound is finished a soaker hose on top is a good way to get things thoroughly soaked. slow steady dripping allows the material time to soak it up while dumping/spraying a large amount quickly onto a finished bed won't soak in nearly so well. It may seem like its penetrating right in wonderfully but it doesn’t soak in unless it has contact time. Watering well will help things settle too. Even with just an inch or two of mulch on top a well soaked bed can stay moist below for over a week even in the driest weather. In cooler or moist weather you might not need to water for a month, or perhaps ever! Just make sure to give things a good soak to get started and then monitor the moisture an inch or two below your mulch to decide when to water. It’s best to water deeply and less often than to water often, but only soak the top inch of the soil. After a deep water the mulch will dry out in a day or two, but the soil below can retain a nice moisture for a surprisingly long time even with a jungle of thirsty plants growing.
COMPOST TEA is incredibly good for any garden, and it really gets things going fast in a hugel mound. I usually apply tea a few times as I build the bed up so it gets dispersed through the layers well. I don't want to fill this page up anymore than I have but more in-depth info found here http://www.microbeorganics.com.
BIOCHAR is another great complement to this technique. Char is akin to finished compost the way it holds water and nutrients for slow release to the roots. However char is much more stable, basically the most stable natural carbon form besides diamonds, and can persist for hundreds, even thousands of years in soil without breaking down significantly. Also can contribute to a "lag" while it absorbs nutrients until it reaches a balance and starts giving them back. sorry no link you'll have find some more info on your own.
ROCK DUST or another mineral additive like sea solids(salt) will result in the healthiest plants of highest nutritional value. Many sources claim that the lack of flavor and nutrition in many commercial crops is partly because they lack enough minerals. Conversely well mineralized soil is credited with strong, healthy, pest resistant plants during growth and delicious, nutritious plants at harvest. Be very careful if your thinking about using sea salt, it can provide an amazing diversity of minerals, but overdo it and you'll be in serious trouble! Lots more info on soil mineralization here: http://remineralize.org/
PLANT SELECTION can be a big factor in your short-term success. Generally speaking most plants excepting some desert plants will do great in a mature hugel bed, but certain plants are particularly suited to thriving in a freshly contructed one. Particularly heavy feeding (if you have incorporated lots of N) and heat loving plants seem to perform best the first season or two. Often the two categories go hand in hand. Especially good candidates include cucumbers, squash, tomatos, peppers, eggplants, sunflowers. Potatos are closely related to eggplants (solanacea family) and tomatos so they like rich soil and heat but you must consider and be mindful of the manure factor. You generally don't want to plant root vegetables in a bed with manure added until it has had at least a few months, or preferably years, to break down. If using manure, Wait until the second season to plant things like carrots, beets, turnips etc. If you plant them in too "fresh" a mound you will notice it in their flavor as bitter or sort of spicy/sharp. Most stick to aboveground crops for a full year just because the idea of biting into something you pulled out of a pile of poop is unsavory enough. After the first full season it will have turned to nice compost though and root crops usually thrive in rich and loose soil such as found in a mature Hugelkultur. If your concerned just dig into the bed before you plant and see if you can still distinguish any "manure" as such. If you watered well the first year, and got the C:N good enough you won't be able to. plus its interesting to have a look at your lower woody layer and see the progress firsthand after just one season or crop if you have a mild climate.
Greens tend to grow well for me in a freshly built mound too. I eat tons of greens
Here are a few pics of my own garden.