Gardeners have been lauding about the benefits of raised garden beds for decades, and in recent years there has been a resurgence in raised beds, especially using different materials like galvanized steel or cement blocks.
But for those of us in colder climates, are raised beds warmer than the ground? Many growers report that the soil in garden beds is workable in spring while the rest of the ground is practically frozen solid.
In winter, you will find that most raised beds are in fact just as cold if not colder than in-ground beds. However, in spring, they will also warm up faster. You can use various season extenders to greatly enhance the thermal properties of your raised garden beds.
Do Raised Beds Hold in Heat?
While at first glance it makes sense that raised garden beds would hold in more heat than the ground, it turns out this is actually the opposite in most cases.
Raised beds will lose heat faster than in-ground garden beds, however, they will also warm up faster, and we can use this to our advantage.
Raised Beds Warm Up Faster in the Spring
In early spring, you will inevitably notice the snow in your raised garden beds melting faster than the snow in the rest of your yard. This is why many people think raised beds are always warmer than in-ground beds.
However, what’s really happening is your raised beds are getting exposed to more sun on the sides of the raised bed, which allows the soil to warm up from the top and the sides. The result is your raised beds will be warmer than most of your yard. The exception would be the ground very close to your house, which warms up from the heat radiated from the walls of your home.
What this means is that raised beds can allow you to plant some vegetables sooner than in-ground beds. The soil will be workable sooner and will be warmer, allowing faster germination.
The taller your raised bed, the greater this warming affect will be.
Raised Beds Get Colder in the Winter
The reason raised beds warm up faster in the spring is the same reason they get colder in the fall and winter.
The difference is that instead of the sun’s heat warming up your raised bed, the cold air will draw out the heat from the sides of your bed. If your raised bed is taller, it will get colder than a short raised bed.
If you live somewhere where it snows, a thick layer of snow will act as an insulator, keeping the ground protected from the cold air.
Raised Bed Materials Have Different Thermal Properties
The material you use to make your raised beds will have a direct impact on how quickly your bed warms up in the spring. Most raised garden beds are made out of wood, but you can also use steel or even stone, brick, or cement. Each of these have different thermal properties. I have already written a more detailed comparison of wood vs. galvanized raised garden beds.
Wooden Raised Beds
Wood has a surprisingly high amount of thermal insulative properties, but for standard 1- to 2-inch thick planks used in raised garden beds, using wood alone won’t make a significant improvement in the heat absorption and dissipation compared to in-ground beds, unless you use wooden raised beds in combination with other season extenders like a cold frame.
The main advantage of using wood is that it’s a very cheap material, relatively durable, and easy to use for constructing raised beds. And since wood can act as a good insulator, if you combine a wooden bed with a cold frame on top, you can substantially increase the temperature of your raised beds.
Galvanized Steel Raised Beds
Steel has the worst thermal properties of all raised garden bed materials. Steel rapidly heats up but also readily releases that heat into the air just as quickly. Furthermore, the corrugated steel used in raised beds is too thin to hold that much heat.
Galvanized steel raised beds, however, are becoming more popular because they are very durable in all environments and will last decades. Galvanized steel beds are often rated for 25 or 30 years, and require very little maintenance.
Stone, Brick, and Cement Block Raised Beds
While stone, bricks, and cement are not as good insulators as wood, they have a great ability to absorb a lot of heat from the sun, which they can release slowly into the surrounding soil.
The soil in raised beds made out of stone, brick, or cement will therefore thaw out much more quickly, because those materials will absorb a lot of heat during the day and release that heat gradually in the evening. This means your raised beds will warm up faster in the spring and cool down slower in the fall.
These are often the heaviest and most expensive materials, but the benefit is that they are truly sturdy and have the potential to last many years. However, cement blocks and some bricks can crack more easily in climates with lots of rain and freezing temperatures in the winter.
Using Wooden Raised Beds to Extend Your Growing Season
One of the many benefits of using raised beds is that you can use them to extend your growing season.
As mentioned above, wood is a good insulator, but under normal circumstances, it won’t make a significant improvement to warm up your raised bed alone.
However, if you were to take a wooden raised bed and fit a cold frame on top of it or a row cover, then you would be able to unlock the full potential of your raised bed for early spring or late fall gardening.
A cold frame or row cover over your wooden raised bed will act as a greenhouse, trapping in thermal energy from the sun and warming up your soil. At night, the soil will gradually cool down to the ambient air temperature. However, in a wooden raised garden bed, the soil will be insulated from the sides, which will slow down this cooling process. By morning, the raised bed will start warming up again from the cold frame or row cover.