Raised garden beds provide a focal point for your plants and make it much easier to maintain a herb garden or keep your flowers in perfect condition without bending and stooping.
While normal flower beds at ground level have a natural soil base, you’ll need to think about the substrates and layers underneath your compost and fertilizer in a raised garden bed to ensure your plants remain healthy.
Most raised garden beds are layered with natural, organic materials, such as:
- Grass cuttings
- Wood chips
To prevent weed growth, you’ll normally place cardboard on top, while the bottom garden bed layer will gradually become compost.
In this guide, we’ll look a little closer at which materials work best on the bottom of a raised garden bed and why!
Building a Raised Garden Bed – the Basics
If you’ve not had a raised bed in the garden before, the main priority is to ensure you have rich, fertile soil that will support a range of herbs, shrubs, plants and ferns.
Because the bed is elevated, you shouldn’t ever need to step on the soil.
Having a raised bed at height is a perfect way to ensure good drainage, so your soil is light and fluffy – helping plants grow faster and stronger.
Putting anything heavy on the soil, or standing on it, compacts the soil and damages the aeration quality, so you should always keep your raised beds protected from kids and dogs!
If you have a raised garden bed on the ground, you don’t need to install a bottom panel, but if the planter is on a surface such as a wooden deck, it’s still necessary to prevent damage to the ground beneath.
Raised beds situated on soil work best with a weed fabric as the lowest layer, stopping weeds from creeping up into the rich soil.
Why Add a Cardboard Layer to a Raised Garden Bed?
The cardboard or newspaper layer is essential to keep unwanted weeds away.
The best option is to:
- Use either a single cardboard layer or around 5-6 newspaper layers at the bottom.
- Cover the entire surface area of the base, right up to the sides.
- Add a heavier mulch layer to keep your cardboard weed-preventer in place.
Paper and cardboard are great for this job because the organic material contains cellulose that contributes to the health of your raised bed soil.
While it will break down in time, the cardboard will last long enough to stop weeds from harming new roots.
If you find that your raised bed isn’t draining well and starts retaining moisture, you can add a straw mulch layer, which also keeps weeds at bay while helping the soil stay moist.
About three or four inches of mulch or straw is fine, and you can spade it in during the spring without digging deep enough to bring weed seeds up to the surface.
What is the Best Type of Soil to Use in a Raised Garden Bed?
If you have a large new raised garden bed or several beds, it’s probably more cost-effective to purchase in bulk according to the total volume of soil you need.
The ideal concentration is usually 60% topsoil, 30% compost, and then 10% potting soil to get your raised bed ready to go. The perfect option is a growing mixture with peat moss or something similar for the latter.
If you don’t tend to get decent quality topsoil in your area, you could opt for a 50/50 combination of a soilless potting mixture, blended with compost.
Peat is fine to use in raised garden beds but should be 20% of the mix as a maximum. That’s because it’s quite acidic and isn’t suitable for growing veggies and some other plants.
Should I Treat the Wood in my Raised Garden Bed?
Treated timber is standard in gardens – think sheds, fencing and trellises. The issue with a raised garden bed is that stains and varnishes contain toxic chemicals.
If you use a treated wood base for your raised bed, you run the risk of having those chemicals seep into the soil, which your plants, in turn, soak up.
While that may not be a serious problem for general flowering plants, if you want to grow anything edible, it could be very dangerous indeed.
What Are the Best Materials to Use in the Bottom of a Raised Garden Bed?
We ran through a list of the common options earlier, but in essence, you’ll want to build your raised bed in layers:
- Weed fabric or wiring at the base of your bed will stop weeds and pests from interfering – you won’t need a solid base slab if you’re putting a raised bed on natural soil but might want to use concrete or a natural wood base to protect timbers or paving.
- Next, you’ll add your natural materials, such as leaves, straw or wood chips.
- Over the top, a layer of cardboard or several newspaper sheets stop weeds from penetrating up from the soil underneath.
- Following that, you’ll add your compost, topsoil and potting soil in the ratios we discussed previously, and be ready to get started!
If you’re looking at alternatives, you might choose landscape fibre instead of weed fabric. It’s more expensive, but a fibre sheet made from recycled materials can be used as a raised bed liner.
There are drawbacks, as a recycled fibre layer will stop worms and beneficial microbes from getting into your soil, so we’d usually recommend going for cardboard.
The key is to avoid anything containing plastic – it might be a great weed barrier but will prevent proper draining.
Plastic is a synthetic substance that is not biodegradable, and therefore not suited to an organic garden.
If you need help maintaining a raised garden bed or would like a gardener to construct a perfect bed to help your flowers thrive beautifully, you can always hire a gardener to advise on the optimal soils and layers for the type of plants you wish to grow.