If you've just bought a large garden planter, one of the first things you'll notice is how much compost it can hold. Large planters can require hundreds of litres of compost, which soon adds up.
The good news is that you don't need to fill the entire planter with expensive compost. By layering the planter correctly, you can save money, improve drainage and still create an excellent environment for healthy plants.
Why You Don't Need 100% Compost
Plant roots don't usually grow through every inch of a deep planter. Most flowers, shrubs, herbs and many vegetables only require around 20–30cm of quality growing medium at the top of the planter.
Everything below that can be filled with suitable natural materials that gradually break down over time.
Not only does this reduce the amount of compost required, but it also helps improve drainage and keeps the planter lighter than filling it entirely with soil.
Step 1 – Fill the Bottom with Large Natural Materials
Start by filling around one-third of the planter with bulky organic materials.
Suitable materials include:
- Untreated logs or thick branches
- Untreated timber offcuts
- Large pieces of bark
- Pine cones
- Plain cardboard (with all tape and labels removed)
These materials take up space while allowing air pockets for drainage and will slowly decompose over several years.
Step 2 – Add a Drainage Layer
Next, add a layer of coarse material such as:
- Wood chips
- Bark chippings
- Coarse compost
- Well-rotted leaf mould
This layer helps separate the larger materials below from the compost above while allowing water to drain freely.
Step 3 – Add Quality Compost
Finish with approximately 20–30cm of quality compost or a compost and topsoil mix.
This is where the majority of your plants' roots will develop, so it's worth using a good-quality growing medium here.
If you're growing vegetables with deep tap roots, such as carrots or parsnips, you may wish to increase the depth of the compost layer.
What Should You Avoid?
Although you'll sometimes see people recommending plastic bottles or polystyrene, we don't advise using them.
Natural materials gradually break down and improve the soil over time, whereas plastic simply remains inside the planter for years and can make future repotting more difficult.
We also recommend avoiding fresh grass cuttings or large amounts of fresh garden waste, as these can become slimy, attract pests and encourage weed growth.
How Much Compost Could You Save?
Depending on the size of your planter, using this layering method can reduce the amount of compost needed by 30–50%.
For larger planters, that could save you the cost of several bags of compost while still giving your plants everything they need to thrive.
The Highland Wood Recommendation
Our handcrafted timber planters are designed to last for many years, so filling them correctly from the beginning is well worth the effort.
A simple layered approach provides excellent drainage, healthy root development and can save you a significant amount of money without compromising on plant growth.
Whether you're planting colourful flowers, herbs, shrubs or vegetables, your planter will perform best when it's built from the bottom up with the right materials.
Happy planting!
Frequently Asked Questions
Do I need stones or gravel in the bottom of a planter?
Not usually. Modern research shows that a gravel layer doesn't necessarily improve drainage and can sometimes create a perched water table. Natural woody materials are generally a better option for large timber planters.
Will logs rot inside the planter?
Yes, but very slowly. As they decompose, they help retain moisture and add organic matter to the soil, similar to the Hügelkultur gardening method.
Can I use topsoil instead of compost?
A mix of quality compost and screened topsoil often works well, especially for larger shrubs and long-term planting. Pure compost may settle more over time.
How often will I need to top up the compost?
As the natural materials slowly decompose, you may notice the compost level dropping slightly over the first couple of years. Simply top it up with fresh compost when needed.