Many people often ask, what do I put on the bottom of a raised garden bed. There are many benefits to having your own raised garden bed, not the least of which is saving your back. But do you know what you should put in the bottom of a raised garden bed?
What do I put on the bottom of a raised garden bed? The bottom of a raised garden bed should be a layer of grass clippings, leaves, wood chips, straw, and other organic material. The cardboard should be placed on top of that layer. The organic material will turn into compost, while the cardboard will prevent weeds.
That’s the short and simple answer, but understanding why those materials are best to place at the bottom of a raised garden bed will help you be a better gardener and have a better understanding of your plants.
What’s the Point of Using Cardboard or Newspaper?
You should use cardboard or newspaper in the bottom of your raised garden bed for one very good reason: keeping the weeds out.
For the best results, you should place a single layer of cardboard or five or six layers of newspaper at the bottom of your bed. Be sure to cover the entire base so that no weeds can creep in from the outside. Another thing that will help with that is placing a heavy layer of mulch on top.
The great thing about using cardboard or newspaper in your raised garden bed is that the cellulose in the paper is an organic material. That means that it makes your soil much more fertile and easier to work with.
When you use cardboard or newspaper in the bottom of your raised garden bed, it gets broken down. However, it will last long enough to keep those pesky weeds from getting in.
One of the problems that people often run into with raised garden beds is retaining moisture. You can solve this problem by mulching with straw. Not only will it keep your soil moist, but it will also help to keep the weeds out.
You will need three or four inches of straw or mulch. Then in the spring, you should spade it in. Just be careful that you don’t go too deep because you don’t want to risk bringing up any weed seeds from the underlying soil.
Can You Use Carpet or Some Other Barrier Instead of Cardboard or Newspaper?
Some gardeners have advised laying carpet or another similar blanket barrier in the bottom of
your raised garden bed to serve as a protection from roots. However, this is a mistake.
The problem with laying carpet on the bottom of your bed is that it slows down the drainage. It also limits the root growth for some vegetables.
You will run into the same sort of problems if you use plastic bags.
So What Kind of Soil Should You Use in the Bottom of a Raised Garden Bed?
You may want to consider buying your soil in bulk, or by the cubic foot or cubic yard, if you’re planning on filling more than one raised bed. You can use this Soil Calculator to find the exact amount of soil that you’ll need.
Generally speaking, you should use soil with these proportions:
- 60 percent topsoil
- 30 percent compost
- 10 percent potting soil (you want a soilless growing mix that contains perlite, peat moss, and/or vermiculite)
Please note that these proportions are approximate. The reason for this is that soil volume can vary from source to source.
If the Soil Calculator tells you to buy 0.44 cubic yards of soil for your raised garden bed, for example, you should round up and go with a half yard.
Some areas of the country may not have access to quality topsoil. If that’s your case, then an acceptable alternative would be a 50/50 blend of compost and a soilless growing medium, otherwise known as “potting soil.”
You can also add peat moss to your raised beds. However, it shouldn’t be any more than 20 percent of your total mix.
It’s worth noting that peat moss is actually naturally acidic. That means that it’s not the best medium for growing vegetables.
The Number One Rule for Raised Garden Beds
Perhaps the best thing about raised garden beds is the rich, gorgeous soil. That leads us to the number one rule of raised gardening: you don’t step on the soil!
The fluffy, light, well-drained soil that you can develop is a huge advantage of using a raised garden bed. And that amazing soil means that you will have vigorous plant growth.
If you step on the soil, you will compact it. Doing that will reduce aeration, and it will slow the activity of valuable microorganisms under the soil’s surface.
Should There Be a Bottom on a Raised Garden Bed?
No bottom is required if your raised garden bed is sitting on the ground. You would only need a bottom if you were trying to protect the surface below the raised bed, like a flat roof or a wooden deck.
Even though you don’t need to put a hard bottom on your raised garden beds in most cases, you can still use weed fabric to prevent weed seeds from germinating in your soil.
Weed fabric works in much the same way that cardboard works. It will block weeds while still allowing for plenty of drainages.
A Quick Word of Caution About Using Treated Wood in Your Raised Bed
It’s great that you’re thinking about what should go in the bottom of your raised garden beds. But you should also be cautious about the wood that you use to build your raised bed.
The problem with most treated lumber is that it contains toxic chemicals. And those chemicals can leech into the soil. That means that there’s a chance that your plants can absorb the chemicals. You don’t want that!
This is not really a problem for flower beds, but it could be a real concern if you are growing edibles.
Should I Worry About Green Manure Cover Crops?
In most cases, green manure cover crops are planted between crop rotations. You can also plant them over the winter.
The point is to add organic materials back into the soil. If you do this, it will create a nitrogen-rich fertilizer.
The green manure cover crops need to be turned under before they go to seed. Do these steps several weeks before you replant the bed.
Don’t Forget to Top off the Soil
One thing that you should note about raised garden beds is that the soil is always settling. You will likely find that your soil level is several inches lower in early spring than it was last fall.
Try looking at a spadeful of soil. Is it moist, light, and crumbly? Or has it been compacted? If it has become compacted, you may need some peat to fluff it back up.
You can use the no-till method of gardening to apply amendments by top-dressing. You can use amendments like peat, lime, rock phosphate, and compost to spread onto the soil. Then you should cover it with a thin layer of mulch.
As soon as your plants are up, you can apply more mulch to top off the bed. It’s also best to apply manure about two weeks before you seed in the spring
The Bottom Line
Now that you know how to prepare and what to put in the bottom of your raised garden bed, you are well on your way to a new spectacular garden. Just make sure that you use compost, cardboard, and soil in the bottom of your raised garden bed, and you are sure to be rewarded with a handsome bounty!