How to Fill the Bottom of a Raised Garden Bed
I get asked what to fill raised beds with a lot, and based on the reaction to a YouTube video I recently posted, people have strong opinions about this.
Filling a raised garden bed completely with soil is expensive—I get it. If you're trying to save money on soil by filling the bottom of your raised bed with something else, the key is just to avoid using something that can damage your plants or even cause harm to you over time.
Let's look at different options for things you can put at the bottom of your raised bed.
My Recommendation: Fill a Raised Garden Bed Completely with Soil
Installing a raised garden bed is your opportunity to start fresh with clean, nutrient-rich soil and organic matter. If you're already going through the effort and expense of adding a raised bed, it just makes sense to me to fill the entire thing up, from the bottom to the top, with great organic matter that will feed the plants that will feed you. You'll also be ensuring your plants have the entire depth of the raised bed to stretch down their roots.
Before you shovel your new soil in, add some weed barrier cloth or burlap to the bottom of your bed to keep weeds out and to prevent your soil from washing out of your bed with the first heavy rain.
Save Money by Adding Leaves to the Bottom of a Raised Garden Bed
Leaves can be super beneficial to a garden. They fill your beds with organic matter that will slowly build great soil, and they're free!
The only problem is they break down pretty quickly. I filled two new raised garden beds about two-thirds full of leaves collected around my yard and covered those leaves with soil and compost. After about two months, the top of the soil had settled to about half of the bed's height as the leaves decomposed.
The best way to save money on soil by filling the bottom of your raised garden beds with leaves is to install the leaves in layers just 6 inches thick at a time. Compress them by walking on them and then wet them with a garden hose. Once that's done, add the next layer. Fill the remaining 8 to 10 inches of your raised bed with topsoil and compost. The leaves will still compress over time, but not nearly as quickly as they would without following this method.
Once you've got your bed filled, I recommend waiting a season to plant or planting only annual plants in that bed. That way, you'll be able to easily add more soil to the top of the bed once those plants have completed their life cycle and the leaves have sunk down.
You could also toss your leaves into your compost bin so that you'll have tons of nutrient-rich soil to use in a couple months.
The Verdict Is Still Out on Using Sticks to Fill the Bottom of Raised Beds, as per the Hugelkultur Method
I've been seeing gardeners swear by the Hugelkultur method for years. This method was designed to be used in mounds or rows in the ground, not in raised beds, but gardeners have adapted it and begun filling the bottoms of their raised garden beds with logs or sticks.
I initially thought this was a terrible idea for raised beds. I've talked to clients and students who've tried this method and ended up disappointed with the pH of their soil and the productivity of their plants. When I posted a video saying as much, I got thousands of replies from gardeners who swear Hugelkultur has worked for them.
I figured I would run an experiment on two raised beds, one filled with leaf mulch at the bottom and then soil and compost, the other filled with about 25 sticks I gathered from the woods at the edge of my property, plus leaf mulch at the bottom, then soil and compost. Members of the Gardenary community advised me to use sticks instead of logs and to choose older wood that's already beginning to decompose over fresh wood.
Both raised beds are 20 inches tall, and I plan to plant them exactly the same with a wide variety of plants so we can test the impact of Hugelkultur on different veggies. I will test the pH levels in both beds weekly, as well as measure their productivity over the coming months. Stay tuned for the results!
Don't Use Plastic or Trash Bags at the Bottom of Your Raised Beds
I've seen suggestions online to add plastic bottles or trash bags as filler to the bottom of raised beds to save money on soil. This not only defeats the purpose of installing a raised bed in order to give your plants' roots all the growing room they need, it also can negatively impact the drainage of your raised beds. Over time, that plastic will slowly break down and contaminate your soil with plastic particles.
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You Can Use a Layer of Cardboard at the Bottom of Raised Beds to Prevent Weeds
If your budget doesn't allow for weed barrier cloth, you can add several layers of cardboard to the bottom of your raised bed before filling it with soil. The cardboard will decompose in about 4 to 6 months.
Make sure to not use cardboard with glossy print and remove all tape first.
Shop Gardenary's Raised Bed Kits
I Don't Recommend Putting Wood Chips in the Bottom of Raised Garden Beds
Fresh wood chips can draw a lot of nitrogen from the soil, which can make the soil unideal for your vegetable plants. Wood chips will also break down pretty quickly (faster than sticks) and will cause the soil level in your raised beds to sink unevenly, all while increasing the temperature of your raised bed. For these reasons, I think it's best to avoid mixing wood chips into the soil you're planting in.
If you do want to go with wood chips, make sure you're not using chips from pressure-treated wood to avoid unwanted chemicals in your garden space.
I Don't See a Purpose to Putting Gravel in the Bottom of a Raised Bed
I see people recommending gravel to improve drainage, but the raised bed structure itself should create good drainage for your plants. Also, a layer of gravel might get in the way of plants with deeper root structures. If you're going to pay for a product to fill your raised beds, again, it should just be really good soil and compost. I only recommend a thin layer of gravel at the bottom of your raised bed and under the edges to help you level the area. Save the rest of your gravel for your garden pathways.
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What Can I Put at the Bottom of a Raised Garden Bed to Keep Animals Out?
If you have pests that like to burrow from the ground (voles, armadillos, etc.) and eat your plants, add an extra barrier by putting hardware cloth across the bottom of your raised beds. Make sure to do this before you fill the beds with soil and follow the layer of hardware cloth with some landscape fabric for weed control. This way, you'll get all the protection these barriers can provide without sacrificing good drainage or the organic nature of your kitchen garden.
What Can I Put at the Bottom of a Raised Garden Bed to Prevent Weeds?
Before you install your raised beds, make sure to clear away all grass and debris and then spray the entire garden area with a concentrated vinegar solution. (You can find 30 percent vinegar sprays in hardware stores or online.)
Then, cover the entire area, both the space under the raised beds and your garden pathways, with a layer of cardboard, carpenter paper, or weed barrier cloth. Cardboard and carpenter paper will both break down over time, which is fine for underneath your raised beds, but I recommend using weed barrier cloth under your pathways. (You can find weed barrier cloth here or here.)
Make sure to leave no open spots—that's basically an invitation to weeds. After covering the garden area, there may still be airborne weeds that settle in the pathways or into your raised beds, but this method guarantees that weeds, for the most part, won’t be a challenge for you.
(Find more product recommendations for raised garden bed installation on Amazon here.)
Fill Your Raised Beds with Good Stuff
I attribute so much of my gardening success to having raised beds filled with healthy soil. To me, this is not an area to skimp on, but I do understand wanting to save money on garden installation. Whatever you decide to put on the bottom of your raised beds, make sure to have several inches of great organic soil and compost at the top.
Thanks for helping to make gardening feel ordinary again!
