Container drainage — getting that right can mean the difference between success and failure for your patio plantings. If we’ve heard it once, we’ve heard it a thousand times: To improve the drainage in your containers, you need to add something in the bottom.
Whether it’s small stones or marbles, foam packing peanuts, or maybe even crushed aluminum cans, the theory is that we need to create air space at the bottom of the container so the water can move through it more easily.
But in the real-life application, does that really work?
To show you what’s really going on inside those containers, with and without added drainage, we set up this video to demonstrate how water really passes through or doesn’t.
If you were able to look through into the container walls to see what’s really happening, you might be surprised.
Using clear plastic cups as our simulated planting containers, we added the same about of drainage holes in the bottom of both.
In the first clear plastic container, we added some stone, just as you’ve been taught to do. Next, we filled the cup with soil.
In the second cup, we used straight soil. Nothing was added to the bottom for drainage.
The next step was to add about the same amount of water to each container. Then wait a couple of minutes to see what happens.
In the container where stones were added, a few small air pockets formed in the soil. But for the most part, nearly all the soil is saturated. Beyond the initial flush, the water has stopped moving through to the stones.
It’s similar to a sponge that you pull from a bucket of water. Once the initial excess water flushes from the sponge, it is still completely saturated.
In the other container where nothing was added, the saturation point is well towards the bottom of the cup.
And that is typical of what really happens. Water will move through to the point where the drainage or the natural exit point occurs. In the case of the container of pure soil, the water has continued to move towards the bottom and is only saturated near the drainage holes.
Back to the cup where we added stones, the water is still present and holding at the bottom of the first substrate level, where the soil and stones meet, but not towards the bottom of the cup.
The science behind why this happens is that water does not pass easily between two substrates of different pore sizes. In this case (and whenever bulkier drainage material is added to a planting container), water is going to collect in that area that is denser – the soil.
The practical application of when we plant into a container where we’ve added material for drainage, plant roots are going to sit in an area where the soil is more saturated. We definitely don’t want that because our plants can drown.
However, in the case where we add nothing but the soil, there’s more area where the roots aren’t sitting in saturated soil. That’s what we want, more air space.
Even though water is stalling below the roots, they still have access to water through wicking and capillary action. Yet roots won’t be sitting in water to potentially drown.
The bottom line is this; the next time you want healthier container plants and improve the drainage, don’t add anything to the soil other than straight high-quality container mix or potting soil.