Sales of self-watering planters peaked this heatwave summer, but there’s an important check you need to make before fall rains arrive. Not all self-watering pots and raised beds come with an overflow hole to drain excess rainwater, so it’s essential to check ahead of storm season, and add one if needs be. Roots sitting in permanently soggy soil will quickly rot and even reliably cold-hardy plants will struggle in overly moist conditions as temperatures plummet.

Some planters with water reservoirs for easy container gardening feature an overflow hole as standard, but many have a removable drainage plug or adjustable drainage system. Plenty more require you to punch out or drill an overflow hole, and this is especially true for planters designed for both indoor and outdoor use.

As we've all looked to fresh ways to cut watering time in the heat and drought of this El Niño summer, chances are you have a new self-watering planter sitting on your patio. So, add this essential task to your list of September gardening jobs to ensure plants in self-watering containers aren't swamped by late-season rainstorms.

How to Check for an Overflow Hole

Most self-watering containers have a double-base system with the soil sitting above a lower water reservoir, so an overflow hole will look very different to a drainage hole on a standard planter. Look for a small, intentional hole on the outer wall of the planter, fairly low down. If you can see the level of the reservoir, then the overflow hole may sit just above or below this line.

You might find that the overflow hole is blocked by a rubber or plastic plug, in which case remove it. Some raised beds also have adjustable drainage systems, so test to check that the overflow hole is open.

If you don't find an overflow hole then you will need to add one yourself.

How to Add an Overflow Hole

Many self-watering containers require you to punch out the overflow hole, and you'll usually find a slightly indented circle in the right spot. While it’s straight-forward to knock punch-out drainage holes through when a container is unplanted, it’s best to drill these once planted up.

If the overflow spot isn’t marked, create the hole just above the top of the water reservoir so the bottom layer of soil stays well-drained. Once you’ve identified the right position, stick a strip of masking tape over the target spot to help the drill bit catch and prevent the outer container wall from getting scratched.

For Plastic Containers

The easiest way to create a hole in most plastics is to carefully heat the tip of an old screwdriver over a flame and push it through the material. For strong, rigid plastic, drill a hole using a wood bit, and preferably a brad point bit that has a sharp, needle-like tip. Use a low speed and light pressure to avoid the plastic melting or cracking.

For Fiberstone, Concrete or Terracotta Planters

Use a masonry bit, keep the drill speed low and the pressure light, and let the drill do all the work.

For Ceramic Planters

For glazed ceramic, you’ll need a diamond-tipped drill bit – the same type you’d use for drilling tiles – and the same slow-and-steady approach. A splash of water, once you’re through the hard glaze, helps to keep the bit cool and speeds progress.

For Metal Containers

Use a cobalt drill bit or a high-speed steel (HSS) bit with a titanium coating, designed for use with metal. Always use protective safety glasses when drilling any material, but especially metal.

Most self-watering pots designed primarily for indoor use are fully sealed. A planter designed for houseplants is unlikely to be frost-proof so is better kept for indoor use, in which case the lack of an overflow hole will protect windowsills and floors from water damage. Do buy a cheap moisture meter such as this long-probe device from Amazon, though, so you can accurately judge whether or not a plant needs more water.

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