Many gardeners use raised garden beds to grow a portion or all of their vegetables during the growing season. While these beds are great, they have a few issues that must be addressed.
One of these issues is soil temperature and the common understanding that the soil gets warm. We set out to find out if this information is accurate and, if so, how hot these beds actually get.
To get the short answer to the question, does the soil become hot in a metal-sided raised garden bed? You bet it does. It can get warm enough to potentially harm plants and soil biology. We have definitive proof of it, and even how much by, as you can find below.
We ran a small test to measure the temperature in a raised bed, and the test was conducted at 2:30 PM. The location is in the wet tropics of Northeast Australia, and the ambient temperature was 34C or 93F ish.
The soil temperature was taken at a depth of 7 inches (175mm) from the mulch surface, and the mulch is about 2 inches (50mm) thick. The thermometer (as shown in the above image) is a long stalk thermometer used when making compost. The color zones on the dial relate to the optimal temperature range for a hot compost pile.
We found that the temperatures varied quite a bit, as you will discover as you read this post. We were shocked by the results and are pleased we now have good data to work with.
We have 10 metal raised garden beds in our back yard where we grow a lot of our food. With the rising summer temperatures, we wanted to know the soil temperature variation in these beds as the sun angle changes during the day.
We have long suspected that the soil becomes warmer at the edge of the metal beds. Still, we never knew how large a difference there is as you move further into the metal raised garden bed from the edge.
In passing, we even mentioned the temperature issue in an article titled “Why use corrugated steel raised garden beds?” and that post got us thinking about this one.
This post that you are reading details the temperature differential in just one of our beds planted with sweet potatoes, which is covered with a light-colored mulch that reflects some of the heat away from the soil surface.
Taking the soil temperatures.
This section is about the process we used to see how hot the soil got at different distances from the garden bed edge. We staged the locations for taking the readings, which can be seen in the images below. The tape measure is a bit blurry, but it is still readable.
As noted earlier, the time was 2:30PM, and the sun had only been striking the end of this garden bed for an hour with any real intensity. Earlier sun angles would be more directly overhead, so they would have far less impact on soil temps via the side walls of the raised garden.
As the images show, there is a substantial difference in soil temperatures as you move towards the edges of these garden beds.
The most significant concern for us as organic food growers who are trying our hardest to be self-sufficient in food production is that the soil life in these garden beds near the edges will not be able to adjust to the sometimes rapid temperature changes that are shown to actually happen.
This then calls into question the viability of growing plants in the areas nearest to these hot spots. One must remember that these locations are not static as the sun is always moving.
We know that diverse soil life equals good plant growth, so keeping the soil cooler seems desirable. Timber garden beds can help here, but this material has a massive weakness. “Does timber garden edging last?” explains this weakness.
When does a metal raised garden bed get hot?
Any piece of metal left in the sun has the potential for the metal to attract and hold heat as the sun strikes it.
The temperatures start to rise when the sun can strike the metal at an angle, and this is what has been measured in the test we ran on our metal raised garden bed. When we took our readings, the sun’s angle was about 30 degrees past the sun’s apex on the day.
The beds were exposed to a good deal of sunlight, which continued for another hour at least before the day’s heat faded. This will be different for all of us and is dictated by your location and the sun’s seasonal intensity where you live.
The sun at midday here is close to being directly overhead, so there is little effect on the raised garden bed walls during the midday time zone. This is why we use the pale mulch to help reflect some of the heat.
To understand the concept of sun mapping and to design the garden beds to best suit the sunlight in your location, “Sun mapping your yard” can get you on the right track.
Sun mapping is just one skill that can help with resilience. Others listed in “23 skills for self-sufficient living” can also benefit.
How hot can soil get in a raised garden bed?
There are a few factors to consider when looking at the soil temperature in a raised garden bed. The first is the seasonal temperature range where you live.
If the sun can strike your garden bed at an angle near 45 degrees, but the ambient temperature is cool, the extra heat that the garden bed can hold may not be as much as we measured in our bed.
Our seasonal temperatures are either warm to hot, then hot to hotter. We don’t get a cold winter, so the soil temperatures in these beds are something we need to sort out.
With our soil temp test, the temperature difference that we measured was 9-10 C or 12F from 1 inch from the edge to 16 inches to the middle of the garden bed.
Does Metal Raised Garden Bed soil get hot?
The table below shows the temperature difference.
garden bed edge
soil depth 7 inches
soil depth 175mm
|1 inch / 25mm
|4 inches /100mm
|8 inches / 200mm
|16 inches / 400mm
As you can see, any plant within 4 inches of the garden bed edge is subjected to higher temperatures than the middle of the bed. This can be seen as a problem, but it can be used to your advantage if you think about it.
Some plants prefer warmer conditions, and the warmer soil will allow us to plant these varieties in the bed alongside the plants that prefer cooler roots.
The soil temperatures can also allow some plants an earlier start to a growing season, i.e., sweet potatoes benefit from warmer soil.
How do you keep the soil cool in a metal raised garden bed?
Several methods can be used, based on shading the raised garden bed sides from direct sunlight during the hottest parts of the day.
We intend to use a curtain of shade cloth that is held in place with clips of some sort so it can be easily removed and cleaned if required. It is a simple method that will work and is probably one of the more cost-effective.
If you have metal garden beds that are not yet filled with soil, you could place a layer of insulating material inside the bed against the wall that will get the sun. This can work well, but it is best done before the bed is filled.
You could scrape back the top 20 inches or so and insulate the bed that way, now that I think about it.
What are the disadvantages of warmer soil in raised beds?
There are several disadvantages to having the soil at the garden edges become hotter during the day as plants grow. One is the smaller area that can effectively grow vegetables if the plants being grown are temperature sensitive.
The amount of space lost to this effect will depend on the aspect or orientation of the garden bed relative to the sun’s path at that general location.
The garden bed in the images in this article is oriented along the sun’s path. The rising sun hits the narrow end first instead of the long side. It is the side-on sun during the middle of the day where a lot of growing areas will be affected by the heat.
Another disadvantage of having warmer soil at the edges is the potential for problematic soil conditions from bacterial blooms. These could be beneficial or harmful, but the potential is still there.
The next disadvantage we would like to mention is the probability of harm being done to the soil life, like worms and similar critters. We strongly recommend that mulch be used to help counter the heating effect, so the soil life has a chance to thrive.
The final disadvantage that we want to raise is the moisture content of the soil will vary across the garden bed from the heat buildup.
One thing to remember is that it will take time for the heat to dissipate as the day cools off, so there is a larger window of time where the moisture will be driven from the soil near the edges.
We have a more extensive article that details several disadvantages of raised beds. It is titled “Organic raised bed gardening: what are the disadvantages?” we recommend you look at it before getting this type of bed. You may not require them at all.
What benefits are there to warm soil in raised garden beds?
One benefit is the opportunity to fast-track some seedlings by planting near the edges where the soil is warmer, which can be helpful in certain situations.
Do all raised garden beds get hot soil?
Not all garden beds will behave in the same manner as the corrugated steel raised garden beds. Other construction materials should be considered as you plan out your garden beds.
Some materials will rot and break down before the garden bed has grown enough produce to pay you back for the initial investment of building or buying it.
Stone walled garden beds can also hold heat, but they can’t transfer the heat into the soil as efficiently as the metal sided beds can. This article, “What is the safest material for organic raised bed gardening?” also gives other alternatives to metal.
It is safe to say that corrugated metal raised garden beds are the worst offenders for heat transfer into the soil, which can harm soil life. We know a bit about building soil full of beneficial life forms, and “How to maintain organic soil in raised garden beds” gives the details.
How do you avoid getting hot soil in a metal raised garden bed?
There are a few ways to help avoid heating issues. The first one, probably the best, is the initial planning of raised garden bed orientation and yard design where the beds should go. What should you consider here?
- Sun movement during the seasons.
- Shading elements that already exist.
- Land contour for suitable locations.
If you consider the hottest season and the sun’s movement for that period, place the garden bed so that the sun rises on one side of the bed and sets on the opposite side. This will have the smallest face of the garden bed towards the sun during the heat of the day.
This depends on your location because there may be a reason to have the garden bed pointing end-on to the rising sun, which is opposite the first example. Your site and the seasonal situation will be unique to where you are.
Sometimes the size of the garden bed ends up being the major heat issue because a wider bed has more soil to act as a heat sink. A narrow garden bed can get hammered, as we discovered with two of our beds. “How wide should a metal raised garden bed be?” gives the details.
We thought the soil in corrugated metal raised garden beds is warmer near the edges where the sun hits it. Still, we had no idea how much the difference was until we measured it ourselves.
We hope the information above has helped with your planning and food growing potential because we know it helped us.
Article by Tim Blanch for TheTropicalHomestead.com. He is a qualified Permaculture designer.