What size pot to use

Slide the plant out and look at it. A dense web of roots that holds the shape of the pot means the plant has run out of room. Loose soil with a few roots running through it means it goes back into the same container.

The signs below let you judge without unpotting, and any one of them is enough to check properly.

Spring and early summer are the easiest time to move a plant up, because new roots grow into the fresh mix quickly then. In winter, repot only when something forces it, such as soil that stays wet or roots that have gone soft.

• Roots coming out of the drainage holes or circling on the surface.
• Water running straight through with the mix taking up little of it.
• A pot that feels light again two days after watering.
• A plant that tips over in its own pot.

The next pot goes about an inch wider across for a small plant and roughly two inches for a large one. The fresh mix then forms a narrow ring around the existing ball.

Depth follows width. A pot much deeper than it is wide holds a wet zone at the bottom that most houseplant roots never reach.

Skipping a size to save work later gains nothing. A plant does not grow faster in a larger pot, because the pot is not what limits it.

There is one deliberate exception. A plant you want to keep at its current size stays in a snug pot with regular feeding, and simply needs water more often.

In a pot far too large, most of the mix has no roots in it. Water goes in, the roots take what they can reach, and the rest of the pot stays wet for days or weeks.

Roots sitting in mix that never dries are where rot starts, and by the time leaves yellow the damage is already below the surface.

That wet top layer also suits fungus gnats, which lay their eggs in soil that stays wet.

A plant already sitting in a pot several sizes too big does not have to be repotted the same day. Water less and more slowly, check that the hole in the base is clear, and pour off anything standing in the saucer. If the mix is still dark and heavy after a week, move it into a smaller pot.

Terracotta breathes through its wall and dries the ball from the sides as well as the top. Plastic and glazed ceramic hold that water in for longer.

The same plant in the same size pot needs water at different intervals in the two materials. After a switch, check the soil by feel for a few weeks instead of keeping the old rhythm.

A wide shallow pot dries faster than a tall narrow one of the same volume, which suits succulents and cacti.

Whatever the material, the pot needs a hole. A container without one has no drainage, and a layer of pebbles in the bottom does not create any.

Most plants arrive in a plastic grow pot with holes, standing inside a decorative outer pot. Size the grow pot to the roots and the outer pot to the grow pot.

An outer pot about an inch wider than the grow pot and roughly the same height works best, so the grow pot sits level with the rim rather than sunk out of sight.

Pour the outer pot out about ten minutes after watering. Water collects there where you cannot see it and keeps the bottom of the ball wet.

With a tall outer pot you cannot look into, stand the grow pot on an upturned saucer or a layer of clay pebbles so it sits above whatever arrives at the bottom.

A moth orchid goes into a pot that sits close around its roots, filled with coarse bark rather than soil. A clear pot lets you see the roots, and silvery roots are the sign to water.

Cacti and succulents stay in snug pots with a mineral mix, so the whole pot dries quickly after each watering.

Peace lily, spider plant and other fast growers fill a pot within a season and take a size up more often than most.

Snake plant and ZZ plant push against the wall of a plastic pot before they look crowded from above. A pot bulging at the side is the sign to move them.

The new pot changes the watering rhythm before anything else. Fresh mix holds more water than the old rooted ball did, so the gap between waterings usually gets longer.

Check by feel two to three inches down for the first weeks rather than keeping the schedule you had before.

Leave fertilizer alone until new growth starts, since fresh mix comes with a supply of its own.