Which pot size do you need? How to choose the right plant pot size

Choose a plant pot 2 to 4 centimetres wider than the one your plant is in now, measured at the top rim. That size gives the roots room to grow, while the potting soil dries out quickly enough to prevent root rot.

At Masr Potten & Vazen we get this question a lot: which pot size suits my plant? The answer is more precise than many people think. A pot that is too small slows growth, but a pot that is too big is, surprisingly, often more harmful. In this guide you will read how to choose the right size, how to measure and when it is time to repot.

Which pot size suits my plant?

The rule of thumb is simple: take a new pot with a diameter that is 2 to 4 centimetres larger than the current pot. So for a plant in a 12 centimetre pot, you choose a pot of 14 to 16 centimetres. For large houseplants, from around 30 centimetres in diameter, the jump can be a bit bigger: 4 to 6 centimetres.

A few rules of thumb in a row:

• Small plants up to 15 centimetres: new pot 2 to 3 centimetres wider.
• Medium plants from 15 to 30 centimetres: new pot 3 to 4 centimetres wider.
• Large houseplants from 30 centimetres: new pot 4 to 6 centimetres wider.
• Decorative pot around an inner pot: allow 1 to 2 centimetres of room all around.

Why is a pot that is too big a problem?

It sounds logical to buy a big pot straight away, so you do not have to repot for a while. Yet it backfires. A pot that is too big holds far more potting soil than the roots can use. After watering, that soil stays wet for weeks, and roots need oxygen as well as water. If they sit in wet soil for a long time, they suffocate and start to rot: the notorious root rot.

The material of the pot plays a part in this. Terracotta is porous: the pot breathes, moisture evaporates through the wall and the soil dries faster. Plastic, on the other hand, holds moisture longer, which makes the risk of overly wet soil greater. A drainage hole at the bottom, possibly with a thin layer of clay pebbles, lets excess water drain away and so prevents root rot.

How do you measure the size of a plant pot?

Pot sizes refer to the diameter of the top rim, measured on the inside. So measure the opening of the pot, not the belly or the base. If your plant is in a nursery pot, look at the size stamped on the bottom, for example 14 or 17.

If you use a decorative pot without a hole as a cover for an inner pot, keep 1 to 2 centimetres of room all around. That way you can easily lift the inner pot out to water it. Watch the height too: the root ball has to fit fully inside the pot, with a watering edge of a few centimetres left free at the top.

When is it time to repot?

You repot most houseplants every one or two years, ideally in spring when the plant starts to grow. Your plant tells you itself when the time has come. Watch for these signs:

• Roots grow out of the drainage hole or visibly circle around inside the pot.
• The soil dries out within just a few days after watering.
• The plant barely grows anymore, not even in the growing season.
• The pot stands wobbly or the root ball pushes itself up out of the pot.

How do you repot without damaging the plant?

1. Water the plant a day in advance, so the root ball comes loose more easily.
2. Tilt the pot and gently pull the plant out of the old pot by its base.
3. Loosen the roots on the outside a little and set the root ball on a layer of fresh potting soil in the new pot.
4. Fill in around it with potting soil, press down lightly and water straight away so the soil settles against the roots.

Which pot size do you choose for outdoors?

Outdoors the same rules of thumb apply, but you can choose a little more generously: a larger pot dries out less quickly on a sunny terrace and stands more firmly in the wind. For an eye catcher next to the front door or along the driveway, a pot of 60 to 120 centimetres tall looks best, especially in pairs or in a rhythm of three.

Do watch the material outdoors. Not all terracotta is frost resistant: moisture that seeps into the wall and freezes can crack the pot. For year round outdoor use, choose a frost resistant pot with a drainage hole, and set it just off the ground so water can always drain away. You can read more about that in our guide on overwintering mosaic pots.

Torn between two sizes for a lovely spot indoors or outdoors? Then take the largest, it almost always looks better. In the collection of handmade pots and vases from Masr Potten & Vazen you will find sizes from compact bowls to amphoras of 120 centimetres, each one unique. Have fun choosing and repotting!