A question that new raised bed growers ask me often is “What do I put on the bottom of my raised beds?” Maybe you’re just starting your garden for the first time or maybe you’re adding more to your existing garden (I add one or two every year myself).
Either way, when you’re building a new bed, it’s important to consider whether you should line or fill the bottom of the beds — and if so, with what.
This question actually has two parts to it and we will address them both. The first is, what do you line the bottom of your raised bed with? The second is, what do you fill the bottom of your raised bed with? We covered all of the details of this in a recent Q&A podcast episode. You can listen to that here or continue reading.
What’s the purpose in lining raised beds?
I currently have 13 raised beds in my garden and 11 of those I lined with nothing. I simply built my raised beds and filled it up with soil. (Learn about raised bed soil mix options here.)
That’s not to say, though, that there aren’t benefits to taking that extra step and lining your raised beds. There are certainly several. But first, you have to ask yourself what your purpose is in wanting to line.
Lining Raised Beds to Prevent Weeds and Grass
Many gardeners line the bottom of their raised beds to cut back on weeds and grass. This lining serves as a barrier, preventing weeds and grass from growing up from the bottom. This is more of a benefit if you have a more shallow raised bed. If you’re bed is 6-10″ tall, you may want to consider lining for this purpose. With deeper beds, weeds and grasses aren’t as likely to reach the surface.
Lining Raised Beds to Prevent Burrowing Rodents
A second reason to line your bed is to prevent ground-dwelling rodents like moles, voles, and gophers from getting to your garden from underneath it. If this is the reason you’re wanting to line, then you would use a completely different liner than to cut back on weeds, which we’ll discuss below.
Lining Raised Beds to Prevent Soil Contamination
The third reason for lining your raised beds is that you want to keep a barrier between your raised bed and native soil that contains soil contaminants, such as lead. You’ll likely not know this until you do a soil test that specifically tests for contaminants.
Lining options for weed prevention
If you are lining your bed for the purpose of weed and grass barriers, one option is to lay cardboard in the bottom of your bed. Cardboard is great to smother the weeds and grass trying to pop up and it eventually breaks down right into your soil. Earthworms love it, and you’ll promote the health of the underlying soil while preventing weeds.
If you’re building a bed on bare soil, gravel or concrete, this really isn’t as necessary. If your new bed is built on weedy or grassy areas, it’s definitely a benefit to smother those things.
Because cardboard breaks down over time, some people use landscape fabric to line their beds with. I’m not a fan of this method, as it restricts the growth of deep-rooted plants and doesn’t break down into the soil like cardboard does. There can never be a natural blending of added soil to the native soil, nor does it allow any earthworms into the raised bed soil, which we know are good for it.
But, landscape fabric does have its perks. It creates a solid barrier while allowing for drainage. And because it never breaks down, it might be more effective against particularly invasive grasses (although, I have seen Bermuda grass power through even landscape fabric).
Using landscape fabric to line shallow raised bed will generally limit you to growing shallow-rooted plants, and in most cases I don’t see deep raised beds needing it. That’s why it’s not my go-to choice.
Lining for rodent prevention
Wire mesh and hardware cloth
If you suspect or know you’ll have problems with ground-dwelling rodents, you may want to consider a hardware cloth or wire mesh. This will be stapled to the bottom of your bed after you build it. This is great at keeping the rodents out, but also allowing the beneficial earthworms in.
Lining for contaminated soil
Food grade plastic
You’ve tested your soil and it’s contaminated. You have a couple of options. First, you can build an elevated raised bed so that there’s zero contact with contaminated soil. Second, you can line your raised beds with food-grade plastic. You will most likely have to poke some holes on the bottom of this for drainage.
Because this plastic is heavy and, really, you’re completely closing it off, you’ll want to treat your raised beds as containers and add additional drainage methods to your bed and choose a lighter soil like a potting mix.
What do I fill the bottom of my raised bed with?
Thus far we’ve discussed lining options, and again, I’ll remind you that most of my beds aren’t lined. But filling the bottom of a new raised bed is a different question.
If you don’t have access to soil on hand, as most of us don’t, you probably know that filling those raised beds can be pricey. A good way to do fill your bed on a budget is to put something different on the top than you do on the bottom.
No matter if your bed is super shallow or very tall, your best soil needs to be on the top six inches. This is where plants’ feeder roots tend to be, so save your pricey soil and compost for the top. So, what do you fill the rest of the space with? There are so many options.
I have used sticks, pinecones, logs, woodchips, straw, partially broken down compost, old okra and corn stalks, even weeds! Look at what organic matter you may have on hand, and use what you have. It will all break down in time anyway, contributing to the tilth and fertility of the soil.
If your bed is very deep (24″ and deeper), you can add any kind of bulky, organic material such as leaves, shredded paper, broken down boxes, etc. All of these materials are going to break down eventually.
Will the organic matter settle?
Yes. As these items break down, your soil will settle and the level will drop. Keep in mind that the bulkier items on the bottom will eventually leave you more top space to fill. But, you can top that with compost and plant your annual plants. It will all just build on itself and you’re going to be so impressed with the long-term results.
But, I do have to caution you. If you’re planting perennials (plants that live year after year), I’d caution you against using too many of the bulk items. As the soil settles in time, it’s very hard to top those off when you’ve got plants growing year-round inside it. Having struggled through this dilemma, I recommend you use a different place to plant the perennial plants or use fewer bulk items.
One final caution, that most gardeners understand these days. Avoid using rocks to fill the bottom of your raised bed. Rocks themselves aren’t a problem but if you line your beds with rocks, the bed will not have proper drainage. A water table will form on top of the rocks. With a deep bed, that’s not going to be as much of an issue as with a shallow bed. Still, it’s best to just avoid using rocks at the bottom of a raised bed to add bulk.
Hopefully, you’ve learned a new way to fill or line the bottom of your new raised beds. But if you’re still trying to decide what to fill the raised beds with, grab the free download, Raised Bed Soil Options Guide for Any Budget below.