I've been asked via email why would I use a box container garden versus a keyhole garden versus a hugelkultur bed. On my homestead, each is used. Each has its benefits and drawbacks. The idea is to choose the one that best works for a particular situation.
The box container gardens that I prefer are made from 4 wood pallets that sides are lined with something impervious, such as plastic sheeting. They are easy and quick to make, cheap, simple to fill and empty. And they work wonderfully within the framework of their design. Issues to keep in mind with these boxes are:
...they drain readily thus will dry out quickly
...they are easy to fill with just about any combination of biomass
...they can be easily disassembled and relocated. Great for temporary use.
...they make root crop harvesting very easy.
...they don't look pretty unless you put extra work and money into them
...the nutrients deplete within 6-9 months, so they need rebuilding.
I use these grow boxes especially where there is little soil, such as at my seed farm. They are also very well suited to areas where I want to add more soil because once I harvest the crop, I can dismantle the box and spread the "manufactured" soil around. The box holds almost a whole cubic yard of soil, and that's a lot for spreading!
Keyhole gardens work. I find that they are a fun alternative to the pallet boxes. I make mine out of cut down pallets, lined with plastic. The central basket which holds the composting nutrients I usually make from old metal fencing. Unlike packet boxes, coarser and denser material is often used. This adds a longer life to the bed.
...they drain readily thus have a danger of drying out
...the lower strata often utilizes wood, keeping the beds moister
...the refillable nutrient basket prolongs the useful life of the bed
...they can be more attractive than the pallet boxes
...will eventually need rebuilding in a few years
These can be used several years before they need to be rebuilt. That's because of the nutrients being constantly added via the central basket. Depending upon their size they can hold 3-4 times more "manufactured" soil than the pallet boxes. So if I were to need a bunch of soil for some project, I could always open the keyhole, rob the soil, then refill it by starting over again.
Hugelkultur beds have a permanent place on my farm. I make most of mine as pit gardens on the main farm, and as pit/mound combinations on the seed farm (not there yet but it's in the plans). Reason- main farms has lots of holes and pits that need filling in. Seed farm doesn't. Simple.
...good use to dispose of waste tree trunks, branches, and punky wood
...good way to fill in holes and make them food producing areas
...good for disposing of coarse, woody, brushy waste
...great way to store moisture in the ground for trees. I find them excellent for bananas.
...use in place of swales for spot capture of water runoff.
...I don't foresee needing complete rebuilding in my lifetime. Top dressing should suffice. Or perhaps harvesting the top two foot for using elsewhere then rebuilding that two foot cap.
The above ground hugelkultur beds that will be built at the seed farm will act as windbreaks. The tradewinds can be fairly brisk at that location. The other big benefit is that the seed farm is pretty dry. Hugelkultur methods retain moisture. At the seed farm I am creating an expansive hugelkultur bed to change the contour of the land. Once completed the area will be flatter and create a moisture retaining area for food plant production. This is a big project that will take months to complete, but I'm in no hurry. Trees planted in conjunction will have access to improved moisture and nutrients.
The other two biggest differences as I see it are:
1- the amount of biomass and time needed to create them. The pallets boxes require the least of both. The keyhole is next. The hugelkultur pits by far require a lot! It often takes me weeks if not months to fill up a hugelkultur pit.
2- the amount of time and power... I can throw together a pallet box in a day and have it planted. Tools needed- hammer, some nails, a staple gun, a scissors, some wire and a wire cutter. The keyhole requires a few days to make and fill, plus a circular saw to shape the pieces. The hugelkultur bed requires tons of biomass, and quite a bit of that is woody. So now I need to have a chainsaw, a come-along and chain, a wedge, tree loppers, hand pruners, something to move logs and big branches, an o-o bar (a heavy steel bar often called a spud or tamper). I cut all my wood up into handleable sized pieces, otherwise one would need a skidsteer, backhoe, or an excavator.