Just to make sure we’re all on the same page: you don’t need to put rocks in a planter. I detail why in this article.
The gist is that they don’t do anything. At all. Other than take up space.
Rocks in the bottom of planters do NOT aid drainage. If there are rocks in the bottom of the pot and it has a drainage hole, they take up space. If you put rocks in a pot without a drainage hole, you’re just creating a perched water table, which can cause root rot, the very issue you’re probably trying to avoid.
That being said, there are some (ok, two) legitimate reasons that you might want to put rocks in your planters. I can provide you with a few alternatives that will fix your issues. But don’t eschew drainage holes. It’s a couple of minutes work with a drill.
Why use rocks in a planter?
There are two main reasons you might want to add rocks to your planters:
- Your pot is too big
- Your pot is too light.
Rocks can help with both of those things because you can add rocks to either the top or the bottom of your pot and it’ll both weigh it down and stop it from being blown away by the wind/tipped over by a mischievous cat.
However, we don’t all have casual access to rocks. We don’t for example, enlist our poor, long-suffering girlfriend to go down to the river banks and take rocks that we need for our aquarium (Dave, if you’re reading this, it YOU).
There are other stuff you can use that you might have, or can get more easily If depends on where you live and your budget as to what works best. Oh, and whether you’re willing to, for example, nick rocks from local rivers.
Sand
Sand is a great option for filling up plant pots that are too big. It’s not massively heavy, so it’s not going to be safe in high winds, but should be heavy enough that it’s less fun/more effort for your cat to push about.
Sand doesn’t retain any water, so if you’re worried that your pot is too big and you’ll end up with root rot, then adding sand will lower the volume of water that the soil can retain. I wouldn’t mix it in with your soil, I’d try to add it around the edges/bottom of the pot. That way, if you come across an appropriately sized pot, you should be able to pick up the pot and the sand should be left behind. In theory. these things never quite seem to work like that.
Wood
Again, if you have a plant that’s too small for it’s pot, packing around the edges with orchid bark will help to reduce the amount of water that’s retained by the soil. It is, however, really light, and will probably be LOT of fun for a cat to play in.
A couple of years ago we used wood to fill up our raised beds. Raised beds are often at about wait height to make them easy to work in, but you need a tonne (probably literally, but tbh I have no idea how much a tonne of doil is) of expensive topsoil, compost, perlite and all that fun stuff to fill them up.
Don’t waste your money. It’s totally unnecessary to completely fill your raised bed with growing substrate. Instead, pack it full of logs. Chop up old wooden furniture, ask friends for errant tree branches, do whatever you need.
We filled about half of our raised beds with old tree stumps. Then we piled in everything that was in our compost bin (whether it had been composted or not) and we needed two-thirds less soil than we originally thought we’d need. And got rid of all the tree clippings that we’d hidden behind the shed to deal with at a later date.
Polystyrene
I wouldn’t normally recommend using polystyrene to fill up plant pots, but we get so much in packaging nowadays that…we may as well use it, rather than sending it straight to landfill.
Obviously, it’s not going to do anything to weigh your pot down, but it can take up space in your pot. It doesn’t absorb water, and it can aerate the soil a bit. If it makes you feel better, break it up really small and pretend it’s perlite.
Oh, and if you’re the kind of person that HATES the look of perlite, make skip putting polystyrene in your plants. It has the same *awesome* habit of floating to the top and gathering a little coating of algae and grime.
Final thoughts
Rocks don’t equal drainage. Have I said that enough?
And yes, thousands of people keep their plants perfectly healthily in hole-less pots with rocks in the bottom. But I honestly have no idea how they do it. I’m guessing they measure the amount of water they use?? If I did it, all my plants would have root rot in two months.
