Raised Beds - Soil Depth Requirements
• Preparing the ground for raised beds
• How tall should a raised bed be?
• Soil depth requirements for popular vegetables
• Height of popular vegetables at maturity
• Container gardening planting tips
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|A frequent question we receive in our online store is "How tall a raised bed do I need to grow vegetables?"
The ideal height for a raised bed is a matter of preference for gardeners. Considerations include the cost of raised beds, the condition of the soil beneath the bed, soil depth requirements for the intended crop, and especially important to mature gardeners, how much bending over you want to do.
Container gardening also requires gardeners to understand the rooting requirements of different crops. Since gardening containers and planters have bottoms, the soil depth is limited.
These considerations are discussed in the article below, with charts showing the rooting depths needed for different vegetables and the sizes of plants at maturity, which helps to determine the location and spacing of seeds and transplants.
|~ Preparing the ground for raised garden beds|
• 'double dig' the ground beneath the raised bed
Raised garden beds are open on the bottom which enables plant roots to access soil nutrients below ground level. When first setting up a raised bed, gardeners should ‘double-dig’ the soil which underlies the raised bed. This only needs to be done once, and is much easier if done before the raised bed is assembled in place.
Double-digging refers to two shovel blade lengths, or approximately 24” in depth. This is done to remove rocks and any debris which would obstruct root growth, and it gives the gardener a chance to see if there are any other roots encroaching into the soil space. Nearby trees, for example, may be sending roots as far as 50’ laterally underground in search of available nutrients. Double-digging provides a reservoir of nutrients and water which can be accessed by the plant’s deeper roots.
Double-digging also lets the gardener see the condition of underlying soil so it can be determined which soil amendments to add. For example, if the soil appears clay-like then it will need to be lightened with peat to provide aeration and improve drainage.
• add amendments to improve the subsoil
Once the ground plot has been dug and cleared of rocks, add peat moss as needed to lighten the soil. Since peat is acidic, add lime to balance the soil ph. Rock phosphate can also be sprinkled in at this time. Now the raised bed can be assembled and topped up with soil. Once the bed is filled to within a few inches of the top, add fertilizer and compost. These final amendments should be added within a week or two of planting, since you don't want early spring rains to wash these valuable amendments too deeply into the soil.
To learn more about building rich soil, see 6 Tips for Building Soil in Raised Garden Beds and Planters.
|The soil in these raised beds is being prepared for planting. Compost and fertilizer will be added to top up the beds.
If you look carefully, you can see the aluminum cross-braces on the lower bed. These will be covered with more soil.
Note the beds are built with a taper to the sides. This is because the garden is on sloping ground. Read our article How to build raised beds on sloping ground.
|~ How tall should a raised bed be?|
• consider drainage
Besides the aesthetic appeal of raised beds and container gardens, they also provide good drainage for the soil within the bed. The most popular height for raised beds is 11". (This is the height of two standard "2 x 6" boards, which actually measure 1.5" x 5.5".) This height provides sufficient drainage for most crops. For best results, there should be another 12" or more of good soil below the bed. This gives your plants at least 18 - 20" of soil. (The soil in raised beds is usually a few inches below the rim of the bed. This is because.soil compresses after several waterings. Having the soil level down a few inches is useful because you will likely want to add a few inches of mulch.)
Since raised beds are well-drained and the soil is above ground level, the soil within the bed warms up earlier in the spring. This enables gardeners to set out transplants sooner which helps extend the growing season. A cold frame can be set on top of the bed during early spring to protect young seedlings and transplants from late frosts and strong winds. The cold frame can be lifted off the bed once the crops become established, and moved to another bed to help protect successive plantings.
• how much bending over do you want to do?
If you are young, fit and full of energy, then bending over or kneeling down to tend your garden may not be an issue. But if you are prone to back strain, or if you have any mobility issues, then a taller raised bed will make gardening easier. In our garden, since the ground is sloped, the beds vary in height from 8" - 24". Since we are 'mature' gardeners, we really notice the difference in working the various beds. The 24" beds are much easier on the back when tilling the soil, setting in transplants, weeding and thinning, addding mulch, and harvesting.
Container gardening in elevated planters is another option for gardeners with limited mobility. These gardens can be tended while sitting in a wheelchair. However, the same principles apply in container gardens with respect to soil depth requirements, top-dressing amendments such as compost, fertilizer and mulch. And taller plants grown in container gardens usually require staking or tying to trellis.
• taller beds may need cross-supports added
The taller you build your raised bed, the more volume it will hold. As the soil is watered it becomes heavier and this exerts pressure which may cause your bed to bow outward in mid-span, near the top. If the bed is taller than 12" and longer than 6', it will likely require a cross-support in the center of the span, across the width of the bed, to keep the sides from bowing. These supports are often supplied by manufacturers of raised beds, but if you are building your own beds, then you may want to include this feature. The cross-support can be made using wood, composite plastic or aluminum. We use aluminum 1/2" flat stock and cut it to length with a hacksaw, then drill the ends for screws. It is a simple job, and the aluminum stock is available at most hardware stores.
|~ Soil depth requirements for popular vegetables|
• be sure there is adequate depth for the roots of the crops you plant
In most gardens, the top 6" of soil contains the most nutrients needed for plant growth. This is because most root growth in vegetable gardens occurs in this relatively shallow depth. Nutrients such as compost and fertilizers are added to the bed from above and lightly tilled in. Mulches are also 'top-dressed' throughout the growing season, and gradually decompose into the top layer of soil adding additional nutrients.
Tap roots will travel deeper into the soil if nutrients and water are available, and this also brings more trace minerals to the plant. Larger vegetable plants will send down deeper roots. When plants are able to send their roots deeper, they are less likely to fall over in windy conditions or if the ground becomes too wet. Large-leafed, shallow-rooted plants such as broccoli, brussels sprouts and cauliflower will still require staking to ensure they stay upright as they grow to maturity.
When preparing a garden bed for planting, it is helpful to know the root depth of vegetable crops since this has a bearing on where you may decide to plant certain crops and how deeply the soil is prepared. For example, in our garden we may plant shallow rooted crops like lettuce in beds where the subsoil has more clay and does not drain well. The shallow roots are not affected by the deeper clay, and they benefit from the added moisture. Plants with deeper roots, such as tomatoes, would not do well in this soil depth.
Some raised beds are set on cement patios or on gravel surfaces which prevent roots from going deeper than the height of bed sides. In these cases, it is especially helpful to know the soil depth requirements of different vegetable crops. Gardeners can compensate by building the raised beds higher to allow for more root space. While raised beds are commonly 8" – 12" tall, some raised beds have sides which are 3' or higher. These taller beds enable deeper rooted crops to be planted even if there is no soil beneath the bed, but drainage must be provided by blocking the bed up an inch or so, or drilling drain holes near the bottom of the bed sides.
Root Growth Pattern
For most vegetables, the bulk of the root mass is within the
top six inches of soil. The soil should be light and well-aerated
to enable roots to access available nutrients.
Deeper soil provides additional nutrients and trace minerals
which further facilitate plant growth. Plants will send some
roots deeper if the soil conditions permit. When preparing soil
for raised beds, 'double-digging' the soil will aerate this
deeper soil and clear it of rocks and debris.
|~ Height of popular vegetables at maturity|
• arrange your crops so that shorter plants are not blocked from the sun by taller plants
When laying out a raised bed garden, the beds should be oriented to gain the maximum benefit from sun exposure. Beds should face south, and we suggest they be arranged horizontally facing south. This optimizes sun exposure from side to side across the bed, and keeps adjacent plants from shading each other as the sun moves east to west during the day.
Some gardeners prefer to layout their raised beds vertically to the sun (north to south), reasoning that this arrangement minimizes plants shading each other. This may make sense if you plan to grow a variety of crops in a single bed, and want to locate the taller plants at the rear (north) of the bed to prevent them from shading the shorter plants. However, we find it simpler to grow only one crop per raised bed because, since the crop matures at the same time, the bed can be cleared after harvest and planted in a 'green manure' crop to restore nutrients in preparation for subsequent crops.
Regardless of how your garden beds are arranged, it is important to know how tall each of your planted crops will grow when mature. This is because you don't want some of your plants blocking others from the available sunlight. Shorter plants like lettuce and radishes should be planted on the south side of the beds, with mid-size plants behind, and the tallest plants to the rear (north) of the planted area. Plants such as pole beans and peas which are planted against a trellis will block a lot of light, so locate trellises with this in mind.
Wind is another factor to consider. Taller plants are more vulnerable to wind damage so they may need to be staked, secured to a trellis, or located against a windbreak. Plants get much heavier once they begin to put out fruit, so trellises need to be well secured. The chart below gives the approximate heights at maturity for popular vegetables. This will help determine planting location and distance between plants when planting seed or setting out transplants.
Planting by Height
In this garden the shortest plants, lettuce, are in the front facing south. A row of bush beans, medium-sized plants at maturity, is planted behind the lettuce. The back row has the tallest plants, tomatoes and climbing (pole) beans. The fence is used to help secure the bean trellis.
image: agrotek.net .
• container gardening tips: growing vegetables in planters and elevated containers
~ To buy a raised bed or planter suitable for growing vegetables, visit our Online Store. ~