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. [fusebox_track_player url=https://traffic.libsyn.com/secure/journeywithjill/QA60_-_What_do_I_put_on_the_bottom_of_raised_beds_-_3921_8.28_AM.mp3]
When it comes to creating a thriving raised garden bed, what you put at the bottom matters. The materials under the soil can impact drainage, discourage pests, prevent weeds, and even add nutrients to enrich your soil over time. With so many options to choose from, it can be confusing to know the best approach. This comprehensive guide covers everything you need to know about how to fill the bottom of raised garden beds.
Why Pay Attention to the Base?
You may be wondering if it’s necessary to put anything at the bottom of a raised bed After all, the soil and compost you fill it with are what really matter, right? While that’s true, a proper base provides these key benefits
-
It separates the native soil from the soil mix in the bed This prevents weeds from below from creeping up into your garden soil,
-
Certain materials like wood chips add organic matter as they break down, This enriches the soil for better plant growth
-
Some bases enhance drainage, preventing waterlogging.
-
Insect barriers keep pests from burrowing up from below.
-
Lining materials like landscape fabric smother weeds.
So in short, the base lays the foundation for optimal performance from your raised bed.
9 Best Materials to Use at the Bottom of Raised Beds
Here are the most common and effective options gardeners use to line their raised garden beds:
1. Cardboard
Cardboard is all-natural and free if you reuse shipping boxes. It suppresses weeds well. As it decomposes over several months, earthworms can enter from below. Avoid glossy cardboard from food packaging.
2. Newspaper
Newspaper is an inexpensive weed barrier that eventually decomposes. Use 5-10 sheets and overlap them to prevent gaps. Avoid glossy paper with colored inks.
3. Leaves
Leaves add organic material to soil as they break down. Rake up leaves from your yard or neighbor’s yards for free material. Compress leaves in layers, wetting each layer to compact them.
4. Wood Chips
Wood chips suppress weeds until they decompose into rich organic matter. Cedar and pine chips may inhibit plant growth so use hardwoods like oak. Avoid pressure-treated wood.
5. Straw
As straw decomposes, it increases drainage and adds organic material to soil. Use 5-6 inches of straw and wet it after laying it down to mat it into place. Be sure straw is seed-free.
6. Landscape Fabric
This woven geotextile fabric allows water to pass through while blocking light to stop weeds. Slits are cut for inserting plants. Fabric typically lasts 5-10 years.
7. Hardware Cloth
A mesh of woven wire, hardware cloth keeps burrowing animals like gophers from excavating in beds. It also permits water drainage and has 1/4 inch openings for worms to pass through.
8. Weed Guard
Thick sheets of black plastic weed barrier mats are durable and effective at stopping light and weeds. However, water does not permeate the plastic so drainage holes need to be added.
9. Gravel
A gravel layer improves drainage but does not stop weeds on its own. Use 1-2 inches along with landscape fabric or cardboard. Larger rocks settle unevenly.
Common Mistakes to Avoid
Now that we’ve covered effective options, here are some materials to avoid using at the base of raised garden beds:
-
Pressure-treated lumber leaches dangerous chemicals.
-
Black plastic blocks water drainage.
-
Fresh wood chips rob nitrogen from soil.
-
Non-compostable materials like plastic bottles don’t enrich soil.
-
Too thick of a gravel layer prevents deep rooting.
-
Any glossy, dyed papers may contain toxins.
-
Grass clippings invite pests and weeds unless they are seedless.
Step-by-Step Instructions
Follow these steps for installing an optimal base in a new raised bed:
-
Remove grass/weeds from the bed site. Dig down 4-6 inches removing roots.
-
Optional: Install boards around bed perimeter. Boards contain soil but a bottom is not necessary.
-
Place cardboard or landscape fabric over area, overlapping edges by 6 inches.
-
For hardware cloth, place it on top of other materials if using them.
-
For gravel layers, spread 1-2 inches evenly.
-
Add thin layers of compostable materials like leaves, straw, or wood chips. Wet and pack each layer down.
-
Build up with layers of compost and topsoil or planting mix to fill the bed.
-
Plant! Amending with fertilizer gives plants nutrients for vigorous start.
-
Mulch the bed after planting to retain moisture and prevent weeds. Replenish mulch as needed.
-
Check drainage holes after first heavy rain or overwatering. Repair leaks or add more holes if needed.
With a properly prepared base and rich soil mixture, your raised bed will get plants off to a great start! Amend soil annually with compost to replenish nutrients. And keep the area around the bed free of weeds. Your raised bed will provide a bountiful harvest for years to come.
Frequently Asked Questions
Do you need to put anything at the bottom of a raised garden bed?
It is not an absolute requirement, but lining the bottom offers benefits. At minimum, put down a weed barrier like thick cardboard.
What depth should fill materials be at the bottom?
Aim for 6-12 inches of biomaterials, though more is fine in deeper beds. Compress leaves and straw before adding soil mix.
How long do materials at the base of beds last?
Most decompose fully within 6-12 months. Landscape fabric lasts 5-10 years. Hardware cloth is permanent but may eventually rust.
What can I use along the bottom if I’m on a tight budget?
Cardboard, overlapping sheets of newspaper, straw, and leaf litter are all inexpensive, readily available options.
What if my raised bed will sit on grass?
Remove grass under bed or the grass and roots will creep up. Cover with landscape fabric or thick cardboard. Or elevate bed on blocks.
Can I just put compost or garden soil in the bottom?
Yes, but it is expensive to fill the entire depth that way. Use compost and soil for the top 6-12 inches where roots actively grow.
Do I need drainage materials if my raised bed has a bottom?
The structure itself should provide sufficient drainage, so gravel is not required. Ensure the floor has drainage holes.
Conclusion
A properly prepared base for a raised bed sets your garden up for success. Effective weed control, enriched soil, and good drainage give your plants their best chance at thriving. Follow these tips to reap the benefits for many gardening seasons to come. The small upfront investment of time and materials will pay off with healthier, more bountiful harvests year after year.
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 for rodent prevention
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.
