Working in a nursery, I obviously encounter people needing guidance on how to pot a plant or plants into their lovely new containers. It’s a basic task to any of us who have done it, but I regularly run into people who have never once done so, and they ask for pointers. One thing they seem to suggest all the time is that they need rocks in the bottom of their container for drainage. I even hear it among people within the industry on occasion. Rocks in the bottom of a pot do absolutely zero for drainage. It’s false information, and in many many cases is ultimately responsible for the failure of the planting! Let me clarify.
Generally speaking, it’s harmless enough. The pot has a big hole in the bottom (or several), so no matter how many rocks are there the water is going to drain out because that’s its nature. The rocks aren’t helping any, but no harm no foul, right?
Let’s talk about water and soil for a sec. Soil holds water in the spaces between the bits and grits. Like a sponge. Naturally, gravity pulls water down (and out). You’ve noticed how the top inch of soil can be visibly dry, but it’s wet just below that. It gets wetter and wetter the deeper you go. Saturate a large sponge and hold it flat. Momentarily it’ll stop dripping as the sponge reaches its holding capacity. Turn the sponge onto its side, and yet more water will drain until equilibrium is reached and the top edge dries out a bit. Stand the sponge on end, and you’ll have even more water draining out until the bottom half is wet and the top half is drier. Now, keep that visual in mind with soil.
Back to potting. The biggest reason not to use drain rocks is that the rocks are stealing space that could be soil that the plants could actually use. Makes sense, right? Plus, if there is a hole, the water will drain. No rock is going to aid that. Sometimes, in fact, the rocks can plug the drain hole altogether, and then it’s a fast downhill slide for the plant.
At Flora Grubb Gardens where I work, we carry lots of cachepots (pots with no drain hole that are usually intended to be a fancy saucer for the plant left in its plastic nursery pot). These can actually be direct-planted in certain cases, like with succulents. This is one of those situations where drain rock will actually be the death of the plants. People will put in rocks “for drainage”. Then, when they water it, the water that drains through the soil is trapped in a rock layer. There is no soil touching it, so the water is difficult to wick back up to the roots. It stagnates from lack of air and light, rot sets in, plants die. Plain and simple. It’s better to use just soil, so that any overwatering will at least have a chance to get wicked up to the plants. Ideally, you wouldn’t water enough to let it saturate to the bottom of the container, at least in the case of succulents.
You can also use water plants easily in a cachepot. Carnivorous bog plants, for example, you can water to the point of saturation because they are used to standing water on their roots. In this case, it’s a good idea to have a small layer at the bottom of charcoal. Not for drainage, but to help keep the bottom from turning into a stagnating stink-bowl.
There are actual legitimate reasons to use a layer of rock or filler, though. Visualize that wet sponge again. Say you have a small plant with shallow roots. If you used a shallow pot, the soil would be like that flat sponge. The wettest part of the soil would be right there at root level where the plants could access it. But you want to plant this in a pot that’s four feet high, say, and the roots are only able to reach one foot deep. You’ll have to water that pot more frequently, because the soil will act like that standing sponge: the wet part will be at the bottom of the pot, out of the reach of the roots. If you fill the bottom three feet of the container with rocks (or some filler, even polyfoam shipping peanuts), that will “elevate the water table” to where the roots will be able to get to that water. In the case of succulents, where you want the roots to be able to dry out between waterings, a tall container with no filler actually makes it easier to avoid overwatering, since even frequent watering will sink the to bottom of the pot and leave the root zone dry. Usually, we’re using tall pots for deeper-rooting plants so this would be detrimental or just unnecessary, but there are those cases where rock can help. Just not for drainage, like people think.
The water table is also the reason you don’t want to “pot up” a plant into too big of a container too fast. The bigger the container, the deeper the water table will be inside it. If you’re taking a shrub from a 4″ little pot into a huge 15-gallon pot, you’ll have to water it frequently – even several times a day in some cases. That’s why in nurseries plants are moved up from pot to pot in stages: from a 4″ to a 1-gallon, then a 5-gallon, then a 15, then a 24″ wooden box, and so on.
I hope I was able to clear up the Great Myth of Drainage Rocks for you! 🙂