For as long as I’ve been potting up plants, one bit of gardening wisdom has been as unshakable as the smell of potting mix on my hands:

“Always put a layer of gravel at the bottom of the pot. Helps drainage. Keeps the roots healthy. Stops rot.”

It’s the sort of advice you hear from parents, neighbours, TV gardeners, and well-meaning plant shop staff. It feels right. It sounds right. It must be right… right?

But for decades, soil scientists have been side-eyeing this claim. In theory, they say, that layer of coarse material at the bottom can actually raise the perched water table — meaning water might sit higher in the pot and linger around roots longer. No one could seem to agree.

And frustratingly, there wasn’t a single scientific study to settle the debate. That is, until February 25, 2025, when Avery Rowe published the first-ever experimental test of this idea in the open-access journal PLOS ONE. This wasn’t hearsay or a potting shed chat — it was a controlled lab experiment measuring exactly what happens when you add that drainage layer.

So, should you still be tipping gravel into the bottom of your pots?

The Long-Standing Debate

If you’ve been gardening in Australia for any length of time, you’ve probably heard the “crushed terracotta trick.” The logic is simple:

1. Water moves faster through coarse materials than fine ones.
2. If you put coarse stuff at the bottom, water should drain away from the finer potting mix more quickly.
3. Roots will stay drier, reducing rot risk.

It’s an attractive idea — and in a country where root rot is enemy number one for indoor plants, it makes intuitive sense.

But here’s the catch: physics doesn’t always play along with our intuition. Soil scientists have long explained that water doesn’t automatically “fall” from fine soil into coarse material until the fine layer is fully saturated. This creates what’s called a perched water table — a zone of saturated soil that “perches” above the coarse layer.

That means your extra gravel could, in theory, be pushing that saturation higher, not lower. And in a shallow pot, that can mean roots stay wetter, not drier.

The trouble is, most of these explanations were based on theory and modelled behaviour — not actual experiments in real pots with real mixes. Gardeners, understandably, kept ignoring the science. “Show us proof,” they said. But still no one did the study.

Now we have it.

About the Study

Title: Effect of drainage layers on water retention of potting media in containers
Author: Avery Rowe
Published: February 25, 2025 in PLOS ONE

This study set out to answer a surprisingly un-studied question: Does adding a drainage layer actually reduce water retention in a container?

To do that, Rowe tested:

• Two layer depths: 30 mm and 60 mm
• Four drainage materials: coarse sand, fine gravel, coarse gravel, and scoria (a lightweight volcanic rock)
• Three potting media types:

1. Loamless organic (think coco coir, peat, bark)
2. Loam-based (soil + organic matter)
3. A sandier, horticultural loam blend

Each setup was saturated with water, allowed to drain freely, then weighed to measure how much water remained.

Two different modelling methods were used to double-check the results:

• Total water retention in the whole container
• Water retention in the potting mix alone (excluding the drainage layer volume)

This gave a much fuller picture than simply saying “more or less water.”

Key Findings

1. Loamless Organic Media

For pots filled with a loamless organic mix — the kind often sold as premium indoor potting mix — the results were surprisingly clear:

• Every drainage layer type reduced water retention in the container compared to having no layer at all.
• Both modelling methods agreed on this outcome.
• The thicker the layer (60 mm), the more effective it was.

This is important because many indoor plant lovers here use peat- or coco-based mixes that behave a lot like the loamless media tested.

2. Loam-Based Media

Things got a little murkier here.

• In total container measurements, most drainage layers made no significant difference.
• But when just the potting mix was measured, smaller-particle layers (like coarse sand) did sometimes reduce water retention.
• The two models didn’t always agree, meaning results aren’t as solid as for loamless media.

So for a standard loam-based mix, the benefit of a drainage layer is possible but far less certain.

3. Sandier Loam Media

In the sandier mix, the effects varied even more:

• Some drainage layers made a small difference, some didn’t.
• The results here suggest that in very free-draining mixes, adding a coarse base may not improve things much.

4. Across All Tests

Despite decades of warnings from soil scientists that gravel layers can make pots wetter, this study rarely found increases in water retention. In most cases, water content went down.

The most consistently effective option?
A 60 mm layer of coarse sand under the potting mix.

This outperformed gravel and scoria across all media types.

🌱 Myth vs Fact: The Drainage Layer Debate

🪨 Myth:

“Gravel at the bottom of your pot makes water drain faster and keeps roots drier.”

📊 Fact (2025 study):

In most cases, a coarse sand layer — not gravel — was more effective at reducing water retention.

🪨 Myth:

“A drainage layer always raises the water table and makes roots wetter.”

📊 Fact:

The new study rarely found increased water retention. In many setups, water content decreased with a layer.

🪨 Myth:

“Any size or depth of drainage layer works the same.”

📊 Fact:

A 60 mm coarse sand layer was the most consistently effective across all mixes tested.

💡 Pro Tip:
If you’re using a loamless organic mix, a coarse sand layer could help — especially in humid climates or for plants that hate wet feet.

What This Means for Aussie Gardeners

When a Drainage Layer Might Help

If you’re using a loamless organic mix — very common for indoor plants, tropicals, and seedlings — a coarse sand layer at the bottom could help reduce waterlogging risk.

A 60 mm depth worked best in the study, so aim for that if your pot size allows. This could be particularly helpful in:

Humid climates (Brisbane summers, Sydney’s coastal areas)

Cooler regions where pots dry slowly (Melbourne in winter)

For plants sensitive to “wet feet” like succulents, cacti, and some natives

When It Might Not Matter

If your mix already drains fast — think sandy native mix or gritty cactus blend — adding a layer is unlikely to make much difference.

And for loam-based general mixes, results are mixed enough that you may not see a noticeable benefit.

Other Factors to Consider

Pot Depth & Shape
Shallower pots have less vertical space for perched water, so any change in water table height has a bigger effect.

Plant Choice
Succulents, some orchids, and many natives prefer drier roots — they’re more likely to benefit from improved drainage. Ferns, calatheas, and tropicals often like moisture; they may not need extra drainage.

Climate
In Perth’s dry heat, retaining water might be more valuable than losing it. In Sydney’s muggy summer, drainage becomes more important.

Practical Tips from the Study

If you want to try this in your own pots:

• Skip the gravel — Coarse sand was more effective and didn’t create big air gaps for mix to fall through.
• Go thick — 60 mm layers worked better than 30 mm in nearly every test.
• Test on one or two pots first — See if your plants respond well before overhauling your whole collection.
• Don’t over-layer — This study tested single layers. Combining sand, gravel, charcoal, and pot shards may not give the same results.
• Keep drainage holes clear — No matter what’s in the bottom, blocked holes = bad news.

Why This Study Is a Big Deal

This is the first real data we’ve had to test a claim that’s been repeated for generations. It doesn’t mean the conversation is over — the study author notes there’s still a need for:

1. Field trials in outdoor conditions
2. Testing with more plant types
3. Long-term studies to see if layers affect root growth

But it gives gardeners something we’ve never had before: evidence to back up (or challenge) what we’ve been doing for years.

Bottom Line

For loamless organic mixes, adding a 60 mm layer of coarse sand under your potting mix can reduce water retention and potentially help prevent overwatering issues.

For loam-based mixes, results are mixed and benefits aren’t guaranteed — but you’re unlikely to harm drainage by trying.

And for sandy mixes? You probably don’t need it at all.

As always, let your plants be the ultimate judge. Observe, tweak, and keep growing smarter.

At last, the “crushed terracotta in the bottom” debate has moved beyond pub chat and into peer-reviewed science. Your next potting session might just be a little different — or exactly the same — but at least now you’ll know why.