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how deep does a raised garden bed need to be

There are a lot of good reasons why you should add raised garden beds to landscapes or fruit and vegetable gardens. Raised beds are great solutions for poor soil conditions like compacted, clay, chalky, or rocky soil.  

They also address limited space in gardens and add texture and height to flat yards. They even help deter pests such as rabbits. Gardeners who have physical limitations or handicaps also find it easier to access raised beds.  

If you want to have your edible garden, raised beds are also the easiest and most effective solutions. This is why it is not a big surprise why more and more gardeners are falling in love with them.  

But, one important question now is how deep does a raised garden bed needs to be?  

soil depth for raised garden beds  

A raised bed can be unframed or framed. Unframed ones are called berms. These are garden beds composed of mounded up the soil. Berms are often made for ornamental landscape beds instead of vegetable or fruit gardens.  

The soil depth of unframed raised beds depends on the plants to be grown, the conditions of the soil below the berm, and the desired aesthetic effects.  

Perennials, ornamental grasses, shrubs, and trees can have root depths of 6 inches to 15 feet or more. You can loosen up the soil under the raised bed by tilling it to allow the plant roots to reach the required depths for proper water and nutrient update. In areas with poor quality, soil that cannot be loosened or tilted, berms or raised beds should be made higher that will result in more soil that must be brought in.  

how deep should you fill your raised garden bed? 

A framed raised bed is often used for growing vegetables. The common depth for raised garden beds is 11 inches since it is the usual height of 2×6 boards used for framed raised garden beds. Compost and soil are filled in the raised bed to a depth several inches below the rim.  

One downside of this is that most vegetable plants require a depth of 12 to 24 inches for good development of roots. Rabbits may also get into a bed with a height of less than 2 feet while gardens with a height of 11 inches might require the gardener to squat, bend, and kneel more.  

To know how deep should a raised bed vegetable garden be, you have to consider the length of the roots of the most commonly planted vegetables. If the soil under the raised bed is not ideal for plant roots, you need to create the bed high enough to have room for the plants.  

The plants that can have roots of 12 to 18 inches include: 

  • Broccoli 
  • Asparagus 
  • Cabbage 
  • Brussels sprouts 
  • Cauliflower 
  • Corn 
  • Chives 
  • Celery 
  • Kohlrabi 
  • Garlic 
  • Onions 
  • Lettuce 
  • Strawberries 
  • Spinach 
  • Radishes  

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Plants with a root depth of 18 to 24 inches include: 

  • Beets 
  • Beans 
  • Carrots 
  • Cantaloupe 
  • Eggplant 
  • Cucumber 
  • Peas 
  • Kale 
  • Squash 
  • Potatoes 
  • Turnips 
  • Peppers

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Plants with deeper root systems ranging from 24 to 36 inches include: 

  • Asparagus 
  • Artichoke 
  • Parsnips 
  • Okra 
  • Rhubarb 
  • Tomatoes 
  • Watermelon 
  • Sweet potatoes  
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Determine the specific type of rich soil that you want for your raised garden bed. You can typically buy bulk soil by the yard.  

To check the number of yards required to fill your raised bed, you can measure the depth, width, and length of the bed in feet then multiply them. Divide the number by 27, as this is the number of cubic feet in one yard of soil. The answer you get will be the number of yards of soil that you will need.  

Take note that you may want to add some compost or some other types of organic matter with the regular topsoil. Aside from that, don’t forget to fill your raised garden bed to several inches under the rim to leave enough room for straw or mulch.  

what is the best design for raised garden beds? 

A raised bed is a freestanding garden bed made on top of the natural terrain. Gardeners have discovered that raised beds can solve most of the common gardening concerns. Sometimes, soil can contain too much clay or sand or has high alkalinity that prevents plants from growing well.  

Poorly aerated soil as a result of poor drainage or compaction can be an issue as well. Soil quality concerns are worse in suburban and urban settings where vegetation and topsoil were removed or construction changed the grade.  

Raised beds can improve the plants’ growing conditions through lifting the plant roots above low-quality soil. Soil amendments can be done to give plants a better growing medium, including plants that don’t thrive there naturally. Raised bed soil warm-up earlier during spring and is less prone to get invaded by tree roots and some grasses. The height of a raised garden bed makes its maintenance easier.  

Raised beds must blend with their surroundings. The design of the bed can be informal or formal depending on the shape and type of chosen edging. Rectangular beds edged with the low brick wall then filled with boxwood or yaupon pruned into straight topiaries or hedges have a more formal appearance that makes them suitable in front of the house.  

Irregularly shaped perennial borders tucked at the back of a dry stonewall are less formal yet these are more attractive in almost any part of the landscape. A garden for growing vegetables has an informal appearance that can work best in private parts of the area. The bed’s size should be proportionate with the surrounding space. 

If you want to know how deep to make a raised garden bed, it is important to remember that you don’t need it to be very deep so that it will be effective. Most of the time, 8 to 12 inches is already enough. If there is a problem with drainage or your plants prefer drier soil, you can go for a taller bed filled with a porous growing medium. 

The depth of vegetable beds should be 12 to 18 inches. The material you use for edging a raised bed must be attractive, durable, and stable. The edging gives the bed a unique look in the landscape. This also forms the bed’s outline while holding the soil in the proper place.  

Edging could be simple like landscape timbers, railroad ties, or metal strips or more complex like mortared stone or brick. Crested beds are where the soil is mounted from the bed’s edges to the middle and may have an edging or not.  

The metal edging can be 8 to 10 feet long. This can be installed easily and is perfect for edging a curved bed. But, rust can develop and unless plants overflow the bed or a more aesthetic material camouflages the edging, it might not look attractive.  

Timbers and ties could be laid in layers or singly to create a rustic appearance. Creosote-treated railroad tires don’t pose health issues since most of the creosote gets leached away. The use of treated landscape timbers is associated with some controversy but studies revealed that compounds that leached out are within the EPA’s set safe levels in both the harvested produce and growing media.  

If you are worried about the use of treated timbers, you can line the inner part of the bed walls with roofing felt, polyethylene, or similar materials to create a protective barrier. Beds also look more interesting with stonewalls and can be made with openings and cracks for creative plantings.  

You can also use timbers that are rot resistant and can last for years. Among the great options are Garapa wood that is known for its impressive high-quality features, density, and lifespan. 

Stone or concrete blocks, however, may be pricy. Interlocking pavers have become popular with easier installation than mortared stone. No matter what your chosen edging material is, it must be strong enough for holding the growing media and withstanding getting ridden over or bumped into by the riding lawnmower. This must be properly installed while complementing the landscape as a whole.  

how to prepare the ground for your raised garden beds 

The bottom part of raised garden beds is open to give plant roots access to soil nutrients underground level. During the initial set up of your raised bed, you need to double dig the soil under it. It must be done once and is easier done before assembling the raised bed in place.  

Double digging pertains to two lengths of shovel blade or depth of around 24 inches. This is done to eliminate rocks and other debris that can hinder root growth. This also gives gardeners the chance to check if the soil space has encroached with other roots.  

For instance, nearby trees might send roots as far as 50 feet laterally underground searching for available nutrients. The process of double digging creates a reservoir of water and nutrients for the deeper roots of your plants.  

Double digging also allows gardeners to check the condition of the soil underneath to identify the best soil amendments to add. If the soil is more clay-like, for instance, it can be lightened using peat to improve drainage and provide aeration.  

enhance the subsoil with amendments 

After preparing the plot and clearing the rocks away, add peat moss as required to make the soil lighter. Add lime for balancing soil pH because peat is acidic. This is the time to sprinkle rock phosphate as well. You can now assemble the raised bed and top it up with good soil.  

After filling the bed with several inches of the top, add compost and fertilizer. The final amendments must be added in one or two weeks of planting because you wouldn’t want the rains of early spring to wash the important amendments too deeply in the soil.  

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