When planting into pots, the most important thing is good drainage.
Without fast, free-flowing drainage water can build up in the pot, harming plants and potentially damaging the pot itself. We recommend raising the pot on pot feet, as well as sealing the inside surface of the pot.
The most common result of poor drainage in pots is that your plants drown. It’s not always obvious why plants die, but drowning and root rot are the main causes for potted plants.
The first way poor drainage can damage a pot is by causing efflorescence. Water dissolves salts and minerals from potting mix and fertiliser. This mineral-laden water can slowly leech through porous pot walls, and can even pick up more minerals from the pot itself on the way through. When it reaches the outer surface it evaporates, leaving the minerals behind. It’s the same process that forms stalactites in limestone caves. Some people like the whitish, streaky, patchy look this causes—the efflorescence—but a lot of people think it’s ugly.
To help prevent efflorescense, we recommend sealing the inside surface of your pots with water-based bitumen sealer. Sealing the pot stops water seeping from inside the pot into the walls, which stops the main cause of efflorescence in its tracks.
While it is still possible for water to enter the walls from the outside, potentially bringing minerals from within the pot walls to the outer surface, we have found the efflorescense on sealed pots to be minimal. If you absolutely cannot countenance the possibility, we can only recommend a painted finish, paired with sealer.
The other, more serious way in which poor drainage can cause damage to a pot is pressure-induced cracking. Unlike potting mix, water is a fluid and presses outward almost as hard as it presses down. This isn’t a big problem in most cases, but if the pot has large flat surfaces, the water pressure can cause those flat surfaces to bulge outward, like the sides of a 2 L milk bottle. Pot walls are clearly stronger than milk bottles, but they’re also less flexible, so enough pressure can cause cracking.
Not all pots are equally prone to this issue. Round pots, for example, don’t bulge because the pressure is evenly distributed, and ceramic pots don’t bulge because they’re much thicker and even less flexible.
There’s really only one way to prevent this bulging: make sure the water can escape quickly & easily.