Raised garden beds are an excellent way to add green space to your existing garden or to create a garden in a reduced or smaller space. The type of raised garden bed you build depends on the type of surface where they will rest, such as soil, pavement, or a concrete surface. In this article, we will look specifically at a few considerations you need to take before you build a raised garden bed on a concrete pad or deck.
The first question we need to answer is, is it OK to build a raised garden bed on top of a concrete pad or deck?
Concrete is a completely appropriate surface to build a raised garden bed on. Construction of the raised bed and drainage will have to be accounted for but, concrete is acceptable for raised bed gardening.
From drainage and height, to soil quality and stains, it is important to keep a few things in mind before turning your concrete deck into a lush and thriving raised garden bed.
Building a Raised Garden Bed on Concrete
Raised garden beds are boxes built directly on the ground and filled with soil. Raised garden beds have no bottom, which is why it’s important to consider the surface they will be built on.
In terms of size, when building a raised bed on concrete you are able to follow most of the standard raised bed sizes. While the length varies, most raised garden beds are between 2 and 4 feet wide, and a minimum of 2 to 12 inches high.
The most common materials used to build the walls of raised garden beds include wood, brick, concrete block, stone, and recycled plastic. (source). These materials work equally well on raised beds built over soil or over concrete.
Raised garden beds on concrete offer many of the same advantages than those built on soil or other surfaces:
- Control over the quality of the soil. Raised beds sit aboveground, so you are able to control and adapt the type of soil you will use depending on the type of plants or vegetables you will grow.
- Improved drainage. Both the elevation and the fact you do not need to rely on the existing soil means you are able to create the perfect conditions for appropriate drainage.
- Accessibility. Raised beds also make gardening easier with less bending and kneeling required when tending to the plants. The paths created around the raised beds make access easier as well for the gardeners with limited mobility or using a wheelchair.
- Longer planting season. Raised beds warm out quicker under the sun, which means gardeners are able to start planting earlier in the spring.
- Increased access to gardening space. A raised garden bed is a great option for those with limited of no suitable soil for gardening as they can be built on top of different types of surfaces, including concrete. (source).
Ideal Height for Raised Beds on Concrete
Raised garden beds that sit over grass or soil are usually a minimum of 10 to 12 inches high. This allows for ample space for healthy root growth and to maximize the plants’ access to nutrient-rich soil. However, raised beds built over a hard surface such as concrete should be higher, at least 18 inches high.
This is because many plants, including the most common raised bed plants such as tomatoes, carrots, and potatoes, prefer deeper soil to grow into healthy and strong plants.
Insufficient soil depth on a raised bed on concrete may result in roots not having any space to grow.
In addition, building higher raised beds on concrete helps prevent excessive drainage or water runoff as the deeper soil helps to hold onto moisture longer. (source).
Should There Be a Bottom on the Raised Bed?
The short answer is yes. Unlike a raised bed built directly on top of the soil, a raised bed on concrete needs to have a liner or a layer of drainage material between the soil and the concrete to avoid leakages or waterlogging.
The Royal Horticultural Society recommends adding a layer of coarse gravel or stones in the base of the raised bed. This layer should be at least 3 inches high and be covered with a geotextile membrane (drainage fabric).
The drainage fabric prevents clogging. However, the fabric can be omitted if you plan to plant deep-rooted plants such as shrubs and conifers that need a surface to hold on to.
The Importance of Good Drainage
In addition to building a good drainage layer between the concrete and the soil, a good combination of soil and compost will ensure raised beds have appropriate drainage.
However, compost is key for raised beds on concrete, both to provide nutrient-rich soil for the plants and to help retain moisture. It is recommended you avoid potting soil on its own–its light texture leads it to drain and dry off too fast. (source).
Before building the raised bed, you can also pour some water over the surface and see in which direction the water drains. This will help you determine the best place to build your raised bed. Ideally, you should avoid any area that gets waterlogged. (source).
Concerns When Building a Raised Garden Bed on Concrete
Even the most well-built raised garden bed may experience occasional leakages due to heavy rain or excessive watering. The water that comes off the raised bed is full of soil and may stain the concrete around it.
To prevent water runoffs that may cause stains, avoid overwatering, and use a soil mixture that provides good water absorption. Another good idea is to cover the raised bed during winter. If leakages do occur, promptly wash off the runoff water to keep the concrete surface clean. (source).
Soil pH Content
Concrete is alkaline and, over time, may impact the pH of the soil in the raised bed. (source).
Knowing the pH of the soil is important because it will help determine which plants are more appropriate for the soil you add to your raised bed.
Most garden plants prefer a pH between 6.5 to 7.0, which is slightly acidic to neutral. An alkaline or sweet soil has a pH of 7.0 and above.
Gardeners are easily able to test the pH of their soil using a store-bough test kit and adjust the acidity of their soil as needed. Acidic soil can be treated with grounded limestone or wood ash, while alkaline soil can be treated with compost, ground sulfur, or gypsum.
Consult your local garden experts to help determine the best way to adjust the pH of your soil. (source).
Raised garden beds on concrete are an excellent way to increase limited garden space or create a garden in a barren area. A base layer of rocks or gravel covered with a drainage fabric should be placed between the concrete and the soil to avoid leakages or waterlogging.
Raised beds on concrete should be at least 18 inches high to give ample depth for roots to grown healthy and strong. Choosing a good soil mix will not only help prevent leakages due to overwatering but provide you with strong and healthy plants and vegetables that any gardener would be proud of.
Before starting any new gardening project, it’s important to consult different sources. Here are the sources used in this article.