How to Choose the Right Pot Size for a Houseplant

The short answer: 1 or 2 inches wider

Choose a pot that is 1 or 2 inches larger in diameter than the one the plant is in now, and not much deeper. That is the range given by Clemson Cooperative Extension, Mississippi State University Extension and the Royal Horticultural Society, which states it as 2.5 to 5 cm. University of Illinois Extension names the upper figure, two inches. A plant coming out of a 4-inch nursery pot therefore goes into a 5-inch or 6-inch pot, and one in a 6-inch pot goes into a 7-inch or 8-inch pot.

Measure the diameter across the top of the current pot and buy against that number. Judging by the size of the leaves leads to pots that are too large, because foliage can spread far wider than the roots beneath it.

For a plant with no pot to measure, such as a division or a bare root gift, Mississippi State offers a proportion to check against. The diameter of the container should be about one-third of the height of the plant, measured from the top of the foliage. A plant 18 inches tall would sit in a pot about 6 inches across.

Why a pot that is too big causes trouble

The limit on pot size comes from water. Mississippi State puts it plainly: containers that are too large for small plants can have too much soil and, therefore, too much water for proper root growth.

The Royal Horticultural Society calls the problem overpotting and describes the mechanism. The large volume of new compost added at potting sits wet for a long period, and that reduces aeration around the roots. Roots need air as much as water, and a small root ball cannot draw down a big mass of wet mix quickly enough to let air back in.

The symptoms the RHS lists are easy to mistake for other problems: leaf yellowing, leaf browning, leaf fall, wilting, stunted growth and compost that stays soggy on the surface. Wilting is the confusing one, since it tempts people to water again.

If a plant is already overpotted, the RHS gives two ways out. Leave it in the large pot and water extremely carefully. Or tip it out, let the loose mix fall away, and move it down into a container only just larger than the root ball. The second option removes the wet mix that is causing the symptoms.

Signs a plant has outgrown its pot

A plant needs a larger pot when its roots have used up the space, and the roots themselves are the evidence. Clemson lists three signs. Roots form a solid mass around the root ball. Roots grow out of the drainage hole. Or the plant requires frequent watering and wilts shortly after it is watered, because there is too little mix left to hold a supply.

To check the first sign, tip the pot on its side and slide the root ball out. Mississippi State describes what a pot-bound plant looks like: a crowded mass or network of constricting roots. If you see mostly potting mix with a few roots reaching the edge, slide the plant back in. It does not need a new pot yet. The guide to signs a houseplant needs repotting goes through each sign in more detail.

As for how often this comes up, the RHS notes that plants in pots need potting up every year or two. Treat that as a reminder to look, since a slow grower may show none of the signs after two years and a fast one may show them within one.

Depth, drainage and what goes in the bottom

The 1 to 2 inch rule is about width. On depth, the RHS wording is that the new pot should be not much deeper than the old one. Extra depth below the roots is one more zone of mix that stays wet with no roots in it.

Whatever the size, the pot needs a hole. Mississippi State explains that a container with one or more drain holes lets water drain out, which allows excess salts to leach away and air to reenter the mix.

Do not add gravel to make up for a pot that is too deep or drains poorly. Clemson advises against a layer of clay pieces or rocks because it slows water movement through the pot, and Illinois says a layer of gravel in the bottom does not help drainage. Clemson’s alternative is a single small piece of broken clay pot laid over the hole to keep mix from washing out. The reasoning is covered in the guide to rocks in the bottom of a pot.

Clay or plastic changes how the same size behaves

Two pots of the same diameter can dry at different speeds, and the material is the main reason. Mississippi State explains that clay pots are porous and allow moisture to evaporate through the sides, so the mix dries out faster and needs more frequent watering. Plastic, glass, metal, wood and glazed pots hold their moisture longer.

That difference can be used when you are between sizes. If the only pot on hand is at the top of the range, 2 inches wider, unglazed clay offsets some of the extra wet mix. If you tend to forget watering, plastic or glazed ceramic at the same size gives a longer interval. Watering from below works with either material, and the method is covered in the guide to bottom watering houseplants.

How to move the plant into the new pot

Timing comes first. Clemson recommends repotting only as needed, during spring and summer while the plant is actively growing. It advises against repotting plants that are ailing, dormant or beginning to flower.

1. Water the plant an hour before repotting, as Illinois suggests, so the root ball is hydrated.
2. Slide the plant out and break up any pot-bound or circling roots.
3. Put enough dry potting mix in the bottom of the new pot that the top of the root ball sits within 1 inch of the container top. Illinois gives the same figure, a one-inch gap between the top of the mix and the top of the pot. That gap holds water while it soaks in.
4. Set the plant at the same height it was growing before. Clemson warns against adding soil above the original level on the root ball.
5. Fill around the sides, press the mix gently with your fingers, and water well so the excess drains out of the pot.

Clemson adds that the watering should be repeated many times, especially if the mix contains peat. After that, go back to checking the mix before each watering. A plant in fresh mix has more to draw on than it did, so it will need water less often than it did when it was pot-bound. For feeding after the move, see the guide to fertilizing container plants.