We've been enjoying all the wonderful benefits of cover crops in our backyard garden plots for years. But we often get asked about using cover crops in raised beds. Is this something worth doing? Or should we just leave the cover crops for the in-ground garden plots?
Let's start by discussing some of the benefits of cover crops in general, then we'll determine which of these are not going to be applicable for raised beds.
Cover Crop Benefits
1) Erosion Control
When planted densely, cover crops form a thick network of roots in the soil that hold the soil in place. This prevents runoff during heavy periods of rainfall, ensuring your soil stays where it is. This may not be a huge issue if your garden plots are on flat land. But if you garden on a hill, erosion can be a big problem.
2) Nitrogen Fixation
Legume cover crops will utilize atmospheric nitrogen and add it to your garden soil. Many of our favorite cool-season cover crops are nitrogen-fixers, which include clover, vetch, and winter peas. There are also warm-season nitrogen-fixing cover crops which include cowpeas and sunn hemp.
3) Nutrient Scavenging
Many cover crops have long taproots that penetrate the soil much deeper than the vegetables we grow. These taproots scavenge nutrients deep in the soil profile as they grow. When the cover crop is terminated and incorporated into the soil, these "scavenged" nutrients are then available to the next round of vegetables you plant.
4) Weed Suppression
A dense mat of cover crop vegetation will prevent any weeds from thriving. This is one of the main reasons we always plant approximately twice the recommend seeding rate for any cover crop we grow. When we don't let weeds thrive and go to seed, it makes gardening much easier!
5) Adding Organic Matter
Organic matter helps fuel soil biology and also helps your soil maintain nutrient levels. Our sandy soils don't hold organic matter very well, so it's important for us to continually add organic matter via compost and cover crops. This helps us maintain long-term fertility in our plots and reduce our fertilizer inputs.
6) Maintaining Living Soil
Bare soil is not very biologically-active. But if we plant a cover crop on that bare patch of garden, the biology becomes active again. This allows us to maintain the soil ecosystem under the surface and keep our soil "living."
When brassica cover crops are incorporated into the soil, they release chemical compounds that provide a natural way to manage and mitigate soil-borne pests. Mustard, rapeseed, and other "spicy" greens are great for this. This technique is proven and works well to minimize nematodes, soil-borne fungi, and even weeds.
If you have animals on your farm or homestead, cover crops are a great way to reduce feed costs. We can plant a 10 lb bag of cover crop seed and it will feed our chickens for several months. This is much cheaper than buying chicken food, and we get fertilizer on our garden plots as a bonus!
9) Soil Aeration
Cover crops like daikon radish are great for hard, clay soils that are difficult to work. The long roots from the radish penetrate the soil and create pockets to improve airflow and drainage. This makes your soil softer and easier to cultivate or weed.
Do These Benefits Apply to Raised Beds?
Most of these cover crop benefits will apply to raised beds, but some of them won't. Erosion is usually not an issue with raised beds because the soil is contained by the barrier of the raised bed. Even when you get a hard rain, the soil doesn't go anywhere.
Weed suppression is also usually not a big issue in raised beds. Even in one of our larger raised beds, we can pull all the weeds in just a few minutes. It's not like an in-ground garden plot that can quickly become overwhelming when covered with weeds.
Grazing is not a benefit for raised beds because we obviously can't graze a raised structure. I can't put my chickens in a raised bed and convince them to stay there until they eat all the cover crop. I guess you could design a way to keep them in there, but it wouldn't be very practical.
Lastly, soil aeration is not a huge concern in raised beds. I've never heard of anyone filling their raised beds with clay. Most people fill them with potting soil or other well-draining soil media. As a result, drainage is usually not an issue.
Although only five of the nine cover crop benefits listed above will apply to raised beds, I think that's enough to justify doing it. Nitrogen fixation, nutrient scavenging, adding organic matter, maintaining living soil, and biofumigation are all great reasons to grow cover crops in raised beds.
But Is It Worth It?
I think it depends on your situation. If you're growing food in your raised beds year round, you probably don't want to sacrifice that food production for cover crops. But if you have periods of time where your beds are not planted and your soil is bare, you should definitely utilize cover crops in your raised beds.
You can plant a cover crop in a raised bed in a matter of minutes. Scratch the seed lightly into the soil with a hand tool, water daily for a few days after planting, and you'll soon have living soil again. Cover crops are also easy to terminate in raised beds using a fork or a shovel. Simply cut into the dense mat of roots, flip the material in the bed, cover with a layer of compost, give it a couple weeks to decompose, and you're ready to plant again!