Does adding rocks in containers to improve the drainage of your pot plant sound familiar?
Well, if you’ve been gardening for a while it should.
After all, this is what most green thumbs do. Spanning from the days of our great grandparents to the current generation, it’s a trusted procedure.
But what if I told you that placing rocks or gravel in pots to aid in drainage is dumb?
Would you believe me?
If you’re like most gardeners I know, you wouldn’t.
After all, why doubt something that has "worked" for your plants for many years. You’ve enjoyed happy, healthy, and thriving plants in pots with holes and gravel without major problems.
You believe with a good potting soil, the rocks give the water a place to go so that the soil isn’t sitting in water.
To be fair, I agree with you, but let me explain why doing this ACTUALLY works against you.
Do this Simple Experiment
Requirement: A small to medium potted plant.
Step 1: Use a watering can to irrigate your plant until the water comes out of the drainage holes. That way you know the soil is getting moisture all the way to the bottom
Step 2: After 5 – 10 minutes carefully remove the plant from the pot.
To do this, place one hand around the base of the plant, on top of the potted soil. With your other hand, tip over the pot so that the plant and soil slide out together.
You’ll likely need to tap the pot to loosen the soil from the edges. You generally don’t want to pull the plant out, especially from a larger pot, as it may rip out part of the root system.
Step 3: Carefully observe how moisture is distributed across the potting soil/mix and take note.
If you do this correctly, you’ll observe excessive sogginess at the base of the potting mix.
This experiment demonstrates one thing, and that is irrigation water will often sit at the base of the container.
There’s actually a name for this phenomenon - it’s called a perched water table normally abbreviated as PWT.
I’ll explain where I’m headed with this. Therefore, read on:
The Concept of perched water table in pots
In gardening, especially container gardening, you will often run into a phenomenon called a 'Perched Water Table' (PWT).
This is a common scientific phenomenon known to scientist. But it is a foreign subject to most gardeners despite the fact that it affects them every day.
When watering your container plants, you will notice excess moisture at the bottom of the pot as we’ve seen in the experiment above.
The water doesn’t drain as intended, but why?
How and why PWT form
I’ll get a bit scientific at this point but will try to explain as simply as I can.
First, let’s start with the basics:
Your favorite potting mix or growing media holds water by means of two natural forces.
The first force is through adhesion and the second one, cohesion.
These two forces are generally referred to as Matric Potential (MP). Simply put;
Adhesion + Cohesion = Matric Potential (MP)
Matric potential is responsible for retaining water within the pot so that the moisture is available to the roots. This force varies depending on the planting media.
For instance, porous media has a lower matric potential than non-porous media.
It’s worth noting that for each media, whether soil or soilless potting mix, the MP is always uniform within the entire container.
However, when you water your plants, there’s another opposing force that ensures, water penetrates into the media, moves through and across, and finally drains from the bottom of the pot.
We call this force – gravity.
It’s also the reason why the planet holds everything together – otherwise we would all be floating in the galaxy – I don’t know if that’s even possible.
Scientifically, it has been proven that gravitational potential (PG) is higher at the top of the container and lower at the bottom of the container.
Now, here’s the catch:
From the illustration above, part A, which is also the upper zone, gravitational potential (GP) is higher than Matric Potential.
This means that irrigation water will flow seamlessly down the container without much resistance. Hence, the upper zone (A) is always the most porous and well drained.
Point B is the area of equilibrium, where GP is equivalent to MP. At this point the water slows the movement down the pot considerably.
The last part (C) is the saturation zone. There’s also another name for this zone. And yes, you guessed right.
It’s called a perched water table (PWT).
Perched water table is therefore, formed as a result of a higher matric potential than the gravitational potential.
I hope I’ve not lost you.
Now, do you understand why there was a saturation zone at the bottom of the pot in our earlier experiment?
If you think about it, PWT isn’t necessarily a bad thing:
As long as the upper zone has the required air spaces to allow for healthy root growth, you could simply consider PWT or the saturation zone as a "reserve" of moisture for the plant roots.
The container size and height also play a role in the "location" of the Saturation Zone.
Taller, narrow containers will increase the upper zone while shorter, wide container will decrease the upper zone, the relative height / depth changes but the saturation zone actually remains the same.
Since the PWT remains constant regardless of the size or height of the container, it’s important to choose your container wisely.
The key takeaway should be, PWT is a matter of physics and occurs in all containers and cannot be eliminated.
What you can do is minimize its negative effects.
And adding rocks or gravel isn’t as effective as you might have thought.
Why Adding Rocks in Pots to Improve Drainage is Dumb
Let me admit it.
A few years ago, I fell in love with container gardening. Ever since that time, I’ve been planting several herbs in pots and experimenting with different potting mixes.
And guess what I’ve always relied on to aid in drainage, gravel!
Everything was working well until I realized that some of my plants were dying out because of root rots. But I later made an interesting observation – plants in a porous potting mix were performing better than those in the normal soil.
Despite the fact that I had put gravel in both containers!
That’s when I started to research the role of gravel in improving the drainage. As I later discovered, gravel elevates the perched water table widening the saturation zone.
I also learned that when it comes to pot drainage, the choice of growing media and the planter used is more important than adding gravel.
Key takeaway lessons
- Rocks don’t improve drainage; instead, they elevate the perched water table closer to your plant’s roots. The plants end up sitting in too wet soil, increasing the chances of plant root rots and you’ve wasted valuable pot space with gravel that’s doing no good.
- It’s better to focus on choosing the perfect potting mix that is porous enough.
- Taller and wider pots are recommended since they have a lower perched water table.
- To improve drainage, add compost to your soil and mix properly before planting. The organic matter in the compost acts as a sponge, holding water in the soil where the plants can reach it. For poorly draining soils it opens up the porous structure allowing water to drain through.
- If you MUST use gravel or rocks, do so outside the bottom part of the pot. Alternatively, you can place inside a decorative pot.
- Instead of using rocks to prevent the potting mix from escaping through the drainage holes, (which by the way is the other reason why gardeners add gravel at the bottom) use a landscaping fabric.
For many decades, gardeners have relied on adding rocks in containers to improve drainage.
However, being popular doesn’t mean it’s effective. This explains why very few gardeners ever enjoy long term success in container gardening.
Gravel or rocks at the bottom of the container doesn’t necessarily help.
To be blatant, it’s ineffective at best. At worst, it’s the reason for most drainage problems.
I could be wrong but I have science to back up my claims. Let us know your thoughts in the comments.