Purpose of swales between hugelkultur beds?

We have a 3 acre farm in rural central Minnesota. The property is on the edge of a largly uninhabited savannah where it meets mixed woods. The surrounding soil is sandy and level. There are several marshes nearby, along with many lakes and rivers. We are in a class A water district - the water table is +/- 10' below grade.

When we moved here a previous resident had built a 200 square foot raised bed that was 42" high. The unfinished wood sides had begun to deteriorate. It was built on pallets topped with steel panels that didn't hold water well causing a need for excessive watering and the layout was awkward and inefficient to water, but they were filled with rich, horse manure based soil that grew beautiful plants. Their location was also hard to reach with the hose and inconveniently far from the house.

As a replacement we began building 3 - 4 ×16' raised beds using garage door panels for the sides. The beds are 23" high and on the southern side of the greenhouse. Having experience with hugelkultur beds in the past, I was familiar with their benefits. The garage door panels have a very clean feel. The total cost for each bed was under $25. We have plenty of access to downed trees. Most are pine but we haven't noticed the tannins being overly detrimental. The beds are able to be placed fairly close together, allowing for better use of limited space.

The first bed, built the fall prior to last summer, was filled with logs, then recycled soil from the other bed. We top the soil with layers of news paper and straw mulch to block weeds and cut down on watering. The first year crop was mediocre, but the plants this spring seem to be enjoying things better, as expected.

We are working on filling the second and third bed now that I'm confident the garage door panels will hold up and work as well as I had planned. I'm considering lining the edge with strawbales placed on end. My thought is that it would help avoid the 'sinkholes' that form along the edges and increase decomposition of the wood by encasing it in a moisture holding medium. It would also be a little easier - using less wood as well as making cultivation a bit easier.

I'm now learning about beds being 7' high and digging out the space between them to make them deeper, creating swales between them. While I could potentially do this, it doesn't seem practical in our area.

We have marshland less than 500' away in 3 directions. Our water is so close to the surface, digging these would lessen the depth of sand protecting it, not to mention runoff would intially pool in the shallow area, bringing potential contaminants with it to an area with reduced filtering capability.

We have a well established tree line surrounding our property. With our cold weather, perennials needs to go directly into the ground. Other spaces around the yard house different man made microclimates and their appropriate gardens.

Are there other reasons to dig between beds that I'm missing?

You are definitely in a different climate than I am used to, so take my ideas with a grain of salt, but the trenches can be shaped and optionally refilled many different ways. Slope could be used to aid water retention, diversion, or it could be flat for a swale. It could be filled with woody debris and compostables and topped with wood chips. Along the bottom of the trench underneath all that, one could run a drain pipe with perforations for extra drainage and soil aeration. This could all carry water from somewhere you don’t want it (near structures) to somewhere you do (ie a roof-pond-garden-wetland series)

Slope and variation in the landscape create varied microclimates, the edges of which the greatest biodiversity and biomass production. Also, cool and hot spots next to each other stimulates gentle but constant air circulation that is good for many plants.

I’d look at your local landscape and it’s native and naturalized succession process in the wildest places nearby and then facilitate something that mimics that with tweaks to suit your needs and tastes. Where I am, hugelkulture going right on top of undisturbed ground with dug out paths refilled with woodchips gets me above a high winter water table but holds a lot of water in wood for into the dry summer. That and wood is plentiful and easy to divert out of burn piles around here I would not do the same thing where that was not true. Also, I mostly can’t do true swales in flattish places because of 100”+ of winter rain some years. Definitely consider the geographic context of any technique, but this has worked for me.

Ben Zumeta wrote:You are definitely in a different climate than I am used to, so take my ideas with a grain of salt, but the trenches can be shaped and optionally refilled many different ways. Slope could be used to aid water retention, diversion, or it could be flat for a swale. It could be filled with woody debris and compostables and topped with wood chips. Along the bottom of the trench underneath all that, one could run a drain pipe with perforations for extra drainage and soil aeration. This could all carry water from somewhere you don’t want it (near structures) to somewhere you do (ie a rood-pond-garden-wetland series)

Slope and variation in the landscape create varied microclimates, the edges of which the greatest biodiversity and biomass production. Also, cool and hot spots next to each other stimulates gentle but constant air circulation that is good for many plants.

I’d look at your local landscape and it’s native and naturalized succession process in the wildest places nearby and then facilitate something that mimics that with tweaks to suit your needs and tastes. Where I am, hugelkulture going right on top of undisturbed ground with dug out paths refilled with woodchips gets me above a high winter water table but holds a lot of water in wood for into the dry summer. That and wood is plentiful and easy to divert out of burn piles around here I would not do the same thing where that was not true. Also, I mostly can’t do true swales in flattish places because of 100”+ of winter rain some years. Definitely consider the geographic context of any technique, but this has worked for me.

100+ inches of annual rain? WOW!
We usually get about 1/5th that much.

Have you reclaimed any of the leftover carbon chunks from those burn piles?
Cheap way to get & make Biochar.

Swales aren't all created equal. Swales basically do one thing in a variety of different ways, but it's all about water control.

I would dig out the paths/swales between your beds and backfill with woodchips. It's essentially the moisture battery part of the hugelbeet, but extended to the area around the beds. The constant moisture levels and all that tasty woodchip to eat results in an explosion of soil life, which basically means that much of the soil work is then done for you by worms, fungi, and associate microbiota.

Plus, if you drop that excavated layer of subsoil atop the unfinished compost and manures you want to be piling atop your woody layer, it gives a buffer zone between where decomposition of the hugelbeet occurs and the root zones of your garden plants, which don't really like those conditions, live.

The paths between your raised beds will act as swales anyways, as long as there's the slightest depression. Do you need to dig them out? No. But you might find it of benefit.

-CK