QUESTION: How do I know which type of mulch is the best kind to use in a raised bed garden? I’m growing vegetables in a raised bed this year and am not sure which type of mulch to use. — Lorna R.
ANSWER: There’s no one answer when it comes to the best mulch to use in a raised bed garden. There are a few popular types of mulch, and which one you choose to use depends largely on what you’re looking for. In the list below, we’ve outlined the best options for mulching raised beds so you can determine which one will work best in your garden.
With raised beds, it’s especially important to water them well to counteract the potential for erosion. If you don’t keep the mulch in a raised bed garden well watered, your mulch materials might be washed away with rainfall or blow away with the wind. You’ll want to water the mulch well just after you spread it on your raised bed gardens and then keep the garden well watered, too.
One of the advantages of choosing grass clippings to mulch your raised bed garden is that you have the option of collecting your own grass clippings. If you have your own grass clippings available, this mulch option will cost you nothing.
You can either pick up clippings from mowing your lawn or save the clippings when you cut down the grasses in your garden. Do check into the type of grass you plan to save, as some can drop seeds. You wouldn’t want to mulch your raised bed gardens with something that could cause grasses to sprout like weeds through your garden soil.
If you choose to use grass clippings, spread them about four inches thick around the plants in your raised bed garden. Grass clippings are known for their ability to form a protective barrier—a woven mat that keeps weeds at bay. They also provide lots of nitrogen, making the soil more nutritious for your plants.
Make sure that the grass clippings you use as mulch have had plenty of time to dry out. You don’t want to spread fresh grass clippings or grass clippings that still hold any moisture over your raised beds. If you collect your grass clippings yourself, just spread them out in a single layer on a tarp you’ve placed in the sunshine to allow them to dry out.
You should also check into the possibility that grass clippings have been treated with fertilizers, pesticides, or weed killers before you use them in your garden. After all, you don’t want any substances hitching a ride into your garden on your mulch that you wouldn’t otherwise allow.
Shredded newspaper or cardboard mulches are especially effective when you need a mulch to help you handle invading weeds in your raised bed garden. These mulch materials are porous enough to let needed oxygen and moisture reach the soil, but they form a protective barrier against weeds.
The best way to use newspaper or cardboard as mulch in a raised bed garden is to spread a two-inch layer of newspaper, which is a minimum of eight layers, or a single layer of shredded cardboard. Paper mulches can be a protective layer against weeds and get topped with another type of mulch to keep them from being eroded by rain or wind.
Gardeners who choose to mulch their raised bed gardens with straw or hay should be aware that these materials have the potential to contain weed seeds. These seeds could be transferred to your garden by the mulch and sprout there as weeds. Straw and pine straw are good choices for budget-conscious gardeners because they can sometimes be gathered for free on your property or that of a neighbor.
However, you can test your bales of hay or straw to find out whether they contain weed seeds. Just leave one bale out and let it get rained on for a few weeks. If your straw or hay contains weed seeds, you’ll see the seedlings start to sprout out of it. Look at them and try to identify what plant they are, as straw can contain oat seeds, and oat seedlings do not pose a problem for your garden. But if your straw or hay contains weed seeds, you won’t want to use it as a mulch in your raised bed garden. Feed hay is known for its propensity to contain weed seeds, but golden straw and wheat straw are safer options.
Straw and pine straw are especially noted for their ability to help the soil retain moisture so it’s more consistently available for your plants. A layer of straw or pine straw as mulch also helps protect your plants against the dangerous temperature changes that go along with inclement weather.
Some gardeners swear by using wood chips on a raised bed garden, and others warn against using wood chips as a mulch. The warning is because some gardeners find that wood chips have too much carbon. Materials that are high in carbon can tie up the nitrogen that vegetable plants in your raised beds need to thrive. If you choose to use wood chips as mulch in your garden, make sure that you’re providing more than enough nitrogen for your plants.
Gardeners who don’t recommend using wood chips also say that it takes longer for them to break down in the soil than it would take other types of mulch. But other gardeners find that these issues don’t pose a problem for their plants, and they believe in using hardwood mulch in the garden because of its benefits.
One of the pros of using wood chips as mulch is that they can be quite affordable or even free. If you live in a state where there is a lot of pine tree logging, you may be able to find pine bark nuggets for free or very cheap. If there is no pine tree logging in your area, you can find wood chips to use as mulch at the garden center or nursery.
Wood chips are an especially good choice as mulch wherever the soil needs to become more acidic. That’s because wood chips and bark add a bit of acidity to your garden’s soil as they decompose.
With all the mulch options for your raised bed garden, you’ll need to leave a few inches of empty space between the mulch and the foliage of your plants. If the mulch touches your plants, it can help spread disease through your garden, and no one wants that.
Now you know the basics about the four types of mulch that we recommend for raised bed gardens. Each type of mulch comes with its own benefits and potential drawbacks, and only you know what’s best for your garden. But now you’re armed with the knowledge you need to make the right choice.