RAISED GARDEN BED GUIDE BY A SOIL SCIENTIST

RAISED GARDEN BED GUIDE BY A SOIL SCIENTIST
Completely guide to raised beds.

Raised bed gardening is gaining popularity for good reason. There are many different kinds of raised bed gardening ranging from ground contact to table top styles. The material used to make a raised bed garden include the use of rocks, wood and even galvanized tubs. There are huge benefits to a properly designed raised bed. Which includes avoiding flooding, early starts in the spring, & turning otherwise infertile soil into a garden oasis.

My name is Ashley and I am a soil scientist with a passion for gardening. Let me walk you through exactly what it takes to make a raised bed work. We will be looking at recommended heights, positioning, & soil types.

How do you design a raised garden bed?

You can use material that comes in a variety of shapes and sizes. Feel free to get creative with DIY designs. But always take into consideration the structure of the raised bed. I personally underestimated the power of soil and roots in my first raised beds. This means less than three years later and I am redoing all of mine with structure in mind.

To make sure you have a good structure in a DIY wood bed there are few things to take in consideration. The first thing you want to provide is supports and bracing to help keep the sides pulled in. You will want to place these support structures every 2 to 3 feet. The higher the bed the more support you will need. In single board height beds, a proper corner support will be enough. This is because we are not contending with the weight of the soil.

If you’re choosing to go with a longer bed. For example something over 6 feet in length you may want to consider several middle supports to prevent bowing. Once you have your design you will want to take into consideration the wood type. Things such as treated vs. untreated woods is a great example.

Treated vs. untreated wood

If you’re turning your raised bed into a vegetable garden to produce edible foods I suggest skipping on treated wood. If you’re using it mainly for flowers, treated wood will be just fine. Treated wood in a vegetable bed can cause issues due to the chemicals leached into the soil from the wood. While these chemicals are unlikely to harm the plant or the yield they can be stored in the plant tissues. This process of accumulating heavy metals is called phytoremediation. meaning debris in fall from a treated wood bed should not be composted. Phytoremediation is a natural process a plant is designed to perform. The purpose is to lessen things such as competition and ward off predators.

If you’re choosing to do a rock, brick or cement version of a raised bed. Consider using a bonding agent for the brick. We don’t commonly give plant roots enough credit. These plant appendages are well adapted to moving things. This includes brick and rocks that are not secured.

Regardless of which one you choose you will always want to consider using fabric liner. To prevent areas where soil may sneak through. While the raised garden bed initially will appear to hold soil, overtime the roots and water will move the soil profile around. Some soil will work its way out without the addition of fabric.

Where should you place your raised bed?

Raised bed placement is arguably the make it or break it in many cases. You generally want to place your raised bed in an area that gets adequate amounts of sunlight. For Canadians in a cooler climate the ideal range is full sun. If you are in a warmer climate and run the risk of scorching or high rates of evaporation then aim for partial sun. The more sun the raised bed gets the sooner you can get into the garden in the spring.

The second factor to take into consideration when choosing where a raised bed should go is what exists in the area currently. For example, the existing ground cover an old flower bed, a lawn or cement. All three of these options are great choices to go with. All three will be treated a little bit differently.

If you were attaching a raised bed to an existing garden then you want to consider rototilling the area before placing the raised bed. This will help eliminate issues such as a perched water table. A perched water table is a phenomenon that we see in the soil physics and can actually lead to root rot if the plant roots are in that zone.. 

If you’re planning on placing a raised bed on an existing lawn you may want to consider not only rototilling on removing the sod layer but also laying down some cardboard. The cardboard will act as a weed barrier that will eventually decompose. This will help alleviate both the perched water table and the potential of grass peeking its way through. Do not underestimate the power of a lawn working its way back up to the surface.

Raised beds on cement

If you’re placing the raised bed on a cement or brick pad then you will want to take in consideration the crops you want to grow. On average a good height is around 2 to 3 feet. When the base is not soil you will need to provide the extra height to ensure both proper nutrients and water capture. Keep in mind with this method you will not be able to plant shrubs or trees. 

The shorter you make the bed the less soil volume you will have, this means you may need to water these beds more often. Other aspects to keep in mind is the colour of the hard surface. If the surface is asphalt for example increase the height to avoid hot roots in the warm summer months. Hot roots can show up as a nutrient deficiency issue. But in reality it is being caused by a damaged root system.

However, due to the cement layer we will also end up with the perched water table. The only way to avoid the plant roots from sitting in the perch water table area is through raising the soil profile and making it deeper. The depth is essential for ensuring you do not have root rot caused by the junction where the soil and cement pad meet. 

What should be the raised bed height?

The height of the raised bed is important when it comes to cement pads like we mentioned earlier. However, if you were placing the raised bed on any soil surface there are a few things to take into consideration when determining the raised bed height. For example if it is on a heavy clay soil that tends to compact then you may want to consider additional height. If it’s going onto an existing lawn or a soil that is simply sandy then a shorter height will be just fine. A good rule of thumb would be if the existing soil acts similar to cement when watered with a hose, height is your friend. If you notice pooling or “streams” forming then this would be considered a semi impermeable soil.. Meaning it should be treated similar to a cement platform.

What soil should you use in a raised bed?

The choice for a raised bed soil is going to be based on your budget. While you can use the more expensive option of potting soil I suggest saving the cash and going for an actual soil mixture. The reason being is because potting soil overtime will degrade and shrink in volume. You will need to add a lot of potting soil to fill your raised beds and will need to top it off every year.

The exception to this rule would be if you are using a raised bed on legs. This means you do not have any ground contacting the bottom of the raised bed. This is a scenario where you will benefit from a potting soil mixture because soil will be much too heavy and can lead to root rot. The potting soil mixture also will not degrade as quickly because we do not have the same level of microbe & macro fauna activity. Without earthworms and millipedes there is less organic material being eaten and decomposed.

Soil types for raised beds

If you have an open bottom bed that is being placed on cement, existing lawn, or an old garden you can use natural topsoil. You’re going to find out there are a lot of variations that you can purchase. Don’t get too caught up in that and simply pick something called a loam soil or topsoil.

A loam soil or topsoil is ideal for a raised bed set up. It’s an added bonus if compost or manure is added into the mixture. Some mixes will be 25% compost or manure and others maybe 50% compost or manure either of these options will work wonderfully. This in combination with a loam soil and compost/ manure will provide nutrients while also providing soil structure that will benefit the plant roots.

If you choose to go the loam soil route you will be able to use the soil forever. Because it is inorganic meaning physical particulates not old organic material. It will not degrade overtime as the microbes munch away. A soil less medium such as peat moss or coconut coir will degrade as time goes on. Meaning you will have to add or top up your potting soil every year to two years.

Using pure compost or manure

Avoid using a pure compost or manure. While this may seem like a great idea it is a waste of your hard earned cash and potentially harmful to the environment. When we use pure compost or manure we have a lot of water soluble nutrients. While the plant will try to utilize as much as it needs there is a risk of nutrients being leached out of the system. This excess nutrients is easily washed away without the presence of an inorganic compound such as clay or silt. This process of losing nutrients can cause issues such as eutrophication.

Loam soils & topsoils act similar to a battery. When we add in nutrients or fertilizer we are charging the soil battery. Soil has a fancy mechanism called the cation exchange capacity that holds onto nutrients and will not let the water run away with it. This nutrients is held until the plant tells the microbes and soil that it is needed via exudates. 

After the addition of soil you may want to consider using a mulch after planting to help preserve moisture. Raised beds due to their inherent nature of being above ground tend to use moisture a bit quicker than an inground flower bed. It’s most likely due to the heating of the soil and therefore increased rates of evaporation. 

There you have it you’re well on your way to building your first set of raised beds. There are a lot of benefits to using raised beds. These benefits include things such as higher yield and quicker harvest. If you follow the rules outlined in this article you will be able to achieve greatness. Be creative in what you decide to use to make your raised bed and remember to have fun.

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