Bigger is not better

It feels generous to give a small plant a big pot so it has room to grow. In practice this is one of the most common ways houseplants die.

A small root system can only drink from the mix it reaches. In an oversized pot, most of the mix has no roots in it, so after watering it stays wet for a long time. Roots growing into that cold, soggy mix run short of air, and the plant is set up for root rot, even if you water carefully.

So step up gently. Colorado State University Extension suggests a new pot only 1 to 2 inches (about 2.5 to 5 cm) wider than the old one, and Brooklyn Botanic Garden gives the example of moving from a 3-inch pot to no more than a 5-inch pot. The RHS gives a wider range of 5 to 10 cm. When in doubt, choose the smaller step.

The reverse is useful too. If a plant has lost roots to rot, move it into a smaller pot that matches the roots it has left, not a bigger one.