The author’s raised beds for his newly planted shade cottage garden
Raised garden design concepts fall into several different categories.
Tall Raised Garden Design
The first is in gardening for the disabled where the beds have to be constructed tall enough to be wheelchair accessible or tall enough to eliminate stooping over (this is a great idea even for those of us with good backs) 🙂
The second is for reasons of soil depth. In gardens where adequate soil is not available, then raised bed gardening allows plant roots to thrive. This is why we’re building a great many beds like this, our soil is 2-inches of clay over shale rock. Kinda tough to garden on…
The third raised garden design concept would be for esthetic reasons; to fit into the concept of design that allows for differing heights. Think different heights of plants in the same way as we have different heights of furniture in a room. Smaller plants can be shown to greater advantage in taller beds; in fact, we often see specialist alpine flowers in raised bed gardening or taller containers so their diminutive stature doesn’t take away from their beauty (and frankly so the regular garden weeds don’t eat them for lunch).
Note that there are common horticultural concerns in all three design reasons. See below.
Why Raised Garden Beds
In thinking of raised garden design then, the question has to be for what purpose you’re raising the beds.
For The Physically Challenged
The beds have to be far enough apart to allow easy access for wheelchairs or wide garden carts.
The pathways between the beds have to be firmed and not muddy. Getting bogged down or having to work hard to move a device along a muddy path is not good raised garden design.
The beds have to be constructed in such a way to allow the proper height and the proper reach to the beds. There’s little point in having a bed too low or too wide if the gardener can not reach all parts of the raised bed to work it.
For Soil Depth
If the raised garden design is for increasing soil depths, then beds can be closer together but the crop being grown has to be considered.
Annuals do well in 8 inches of soil so one tier of railroad ties (or similar depth) is adequate for a great crop of annual flowers or vegetables. The width of the bed is up to the gardener and what they can comfortably reach.
Perennials or woody plants will require a different thinking. It is not the soil depth that will kill off perennials or shrubby plants. Most of these plants will be fine with 12 inches of soil.
Winter Damage Is Worse For Perennial Flowers
Depending on the winter temperatures, it is more likely perennial plants and shrubs will be damaged or killed in raised beds than in the ground. Plant roots of commonly grown shrubs and perennials tend to die off when the soil temperatures reach -5F. (there are exceptions)
In smaller raised beds or shallow beds over rock, the soil freezes from the top and the bottom and comes closer to reaching air temperatures than does deep soil. If your air temperature goes below the -5F and so does your soil, then you’re going to start losing plants.
The larger the raised bed, the more it acts like a regular ground-level garden
Deep soil has a flywheel effect where the deeper soils that remain unfrozen act as thermal buffers to cold penetrating deeply or for the actual soil temperature to drop too far below freezing.
How Big Do We Have To Build?
Our need then is to create wide beds and deep enough beds so we create our own thermal flywheel and only grow plants such as Iris and Hemerocallis that are tough enough to handle extremes of soil temperatures. More tender plants such as Lavender will not survive these temperatures.
Raised bed design for deep survival calls for beds to be at least 18 inches deep and likely 6 feet wide. These sizes would be variable depending on which zone (colder = deeper and wider) and which plants the gardener wishes to grow. The tougher plants can go to the edges while the more tender plants should be planted in the center.
But it all depends on the lowest winter temperature you get. Colder equals bigger beds needed.
Note that taller raised garden design means that we’ll have watering problems as gravity will pull available water downwards. You will have to water more frequently and deeply to overwinter plants in such raised beds.
Thaws during the winter will also cause the soil to lose moisture and this can cause as much damage as summer droughts to some plants.
This concrete block raised bed is a temporary one I had to build when our foundation was excavated. It was a combined veggie garden and shrub/perennial holding bed for one garden season. As of fall 2014, I’m starting to take it apart and move the soil to other beds. It was – and is – a ton of work to have to do this but it’s just what gardeners wind up doing sometimes. And before you ask, it’s 46 cubic yards of soil and yes, I have a small loader on a small tractor to do the heavy lifting. And yes it was big enough to overwinter perennials and shrubs in the coldest winter in years.:-)
Raised garden design for esthetic reasons is the easiest of the reasons for creating these garden beds. This is simply a matter of constructing the beds to the appropriate depth and growing in them. Note that watering will still be an issue.
Another horticultural challenge is the actual soil to be used. Raised beds share the properties of container gardening and soil structure. Because you have to water more frequently, there is a serious danger of compacting the soil in raised beds that does not exist to the same degree in garden soil. (Water weighs approximately 10 pounds to the gallon so if you thump several gallons on directly with a hose, you’re mashing down the soil with weight.)
Raised beds will require the addition of extra amounts of organic matter to loosen up the soils and prevent compaction. This is easily done with shallow beds for vegetables and annual flowers. Merely dig in peat moss and compost at the ratio of 1 shovel of peat, 1 shovel of compost/manure for every 3 shovels of soil that is turned over in the annual spring or fall digging.
This is more of a problem for perennial beds and tall permanent beds that can not be dug every spring. In these I’d recommend you invest in drip irrigation so now hand watering need be done. I’d also recommend you add extra compost/organic matter every year so the earthworms stay happy and do your digging/aeration for you.
So the last aspect of raised garden design then falls into the category of shapes and structure. And here you’re on your own to create your own garden fantasy. Remember that wood timbers lend themselves to square classic shapes while bricks and pavers can be used in squares or round edge beds.
But no matter the shape, the growing considerations above have to be your initial concerns. You can get more tips on raised beds and container gardening ideas right here
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Raised garden design options and available to fit many garden systems