How to choose the right size pot: a practical guide with real measurements

The question we get asked more than any other at the nursery is some version of "is this big enough?" Usually someone is standing next to a pot with a photo of their patio on their phone, trying to picture a lemon tree in it. We have had that conversation thousands of times, and the honest answer is that most people buy a size too small. The pot looks big in the yard, it looks fine in the boot, and then eighteen months later the plant is root-bound and thirsty every afternoon in the Centurion heat.

So this is the guide we wish everyone read first. Real numbers, in millimetres and centimetres, for the plants South Africans actually grow in pots. We will also give you the actual dimensions of the pots we cast here, because "large" means nothing until you can hold a tape measure against it. If you want a shortcut, our size chart lays every pot out by height and diameter, and the Garden Planner will match a plant to a pot and a spot without you doing any sums at all.

Start with the plant in two years, not the plant in the bag

The nursery bag is a terrible guide to pot size. A 4 litre bag of Cape honeysuckle is a stick with three leaves, and it will be a 1.5 m shrub by the second summer. So before you look at pots, look up the mature size of the plant, or at least the size you intend to keep it at with pruning. Our Plant Library lists this for the common pot plants, and it is worth two minutes.

The University of Illinois Extension puts the general principle simply in its notes on container size: larger plants need more room for roots, the volume of the pot decides how often you will be watering, and square pots are the most stable in wind. All three matter more here than in Illinois. A small pot on a Highveld patio in January can hit soil temperatures that cook fine roots by mid afternoon, and a shallow one will need water twice a day. Volume is insurance.

The one-third rule for trees and big shrubs

For anything that grows on a trunk, an olive, a citrus, a bay tree, a clipped Searsia or a small frangipani, the rule of thumb we use is that the pot should be about a third of the finished height of the plant. A tree you want to hold at 1.5 m goes into a pot roughly 500 mm tall. A 2.4 m tree, which is about as tall as most people want on a stoep, wants something near 800 mm. This is partly about roots and partly about physics, because a tall tree in a shallow pot is a sail, and the first October thunderstorm will put it on its side.

In real sizes, that means our Cube at 500 mm is a proper home for a lemon or a young olive held at about 1.5 m, and the Egg at 770 mm suits a tree you want closer to two metres and a bit. Concrete helps here in a way plastic never will. The weight of the pot is the ballast that keeps a tree upright in wind, and our Fibre Concrete Mix is cast to sit outside in full sun, frost and rain for years without spalling. The RHS guide to growing citrus notes that most citrus stay under 1.5 m and can live in their original pot for several years before moving up, which brings us to repotting.

When repotting, go 5 to 10 cm wider and stop there

People assume that the fastest route to a big plant is a big pot. It is usually the opposite. A small root system in a huge volume of soil cannot use the water in the mix, so it stays wet, the roots sit in that wetness, and you get rot before you get growth. When you move a plant up, choose a pot 5 to 10 cm wider across the top than the one it came out of. For a big shrub or tree already in a 400 mm pot, 10 cm is fine. For a small houseplant, 5 cm or even less.

The exception is when you are buying the final pot and never want to repot again. Then you can jump further, but only if you plant with the drainage sorted and you are careful with watering for the first season. Put a layer of Pot Filler in the base of anything over 600 mm so the bottom is not one wet slab of soil, and read our large garden pots guide before you fill it, because a filled Egg is not something you move on your own.

Depth for vegetables and root crops

Vegetables are where depth matters more than width. Lettuce, spinach, rocket and most herbs are happy with 20 cm of soil. Carrots, beetroot and spring onions want 30 cm so the root has somewhere to go, and a carrot in shallow soil forks and stays stumpy. Tomatoes, peppers, brinjals and anything that fruits over a whole summer want 40 cm or more, and a tomato in a 30 cm pot on the Highveld will need water twice a day by February. The RHS guide to growing tomatoes is blunt about it: large containers, in a warm, sunny, sheltered spot. Our summers are warmer than theirs, so if anything go bigger.

This is why we point vegetable growers at troughs. The Curo Trough is 1160 mm long, which gives you room for three tomatoes with basil between them, or a whole salad bed along a wall. Soil depth is consistent along the length, so root crops do as well at one end as the other, and the mass of concrete keeps root temperatures steadier than a row of small pots would.

Wide bowls for plants that spread

Some plants grow out, not down. Agapanthus, clivias, bulbines, aloes, echeverias and most succulents have shallow, wide root systems and look wrong in a deep pot, and they also sit in cold, wet soil at the bottom of it in winter. For these a bowl shape, wider than it is tall, is the right answer. A 600 mm bowl only 250 mm deep will carry a good clump of agapanthus for years, and a shallow bowl of mixed succulents on a sunny table is one of the easiest pots anyone can keep alive on a balcony with water restrictions.

SANBI's PlantZAfrica page on Agapanthus praecox says all the species make good pot plants and only need lifting and dividing every four years or so to keep flowering, so a bowl holds a clump for a long while. Fill a bowl with our Succulent Mix for the succulents, or Bark Outdoor Potting Soil for the agapanthus, and top with white pebbles to keep the soil surface cool.

Tall pots for height at a door or corner

At a front door or in a dead corner you are not solving a root problem, you are solving a proportion problem. You want something at roughly eye height, and a tall pot lets you get there with a modest plant rather than a tree. A pot around 800 mm to 1000 mm tall with a 1 m plant in it gives you a 1.8 to 2 m feature that reads well against a door frame. Our Nevada goes up to 1800 mm tall in its biggest size, which is a full column on its own, and even the mid sizes make a small plant look intentional.

A word of caution with tall narrow pots: they hold less soil than they look like they should, and the soil column dries from the top down. Plant them with something forgiving, a Strelitzia, a clipped Searsia, a Sansevieria in a sheltered spot, and set them on concrete pot feet so the drainage hole never seals against a paved floor.

Measuring your space before you shop

Take the tape measure outside. Measure the width of the spot, then subtract at least 150 mm on each side so a pot does not touch a wall or a door swing. Measure the height of any overhang or the underside of a balcony rail, because a tall pot plus its plant can end up jammed under an eave. If you are placing a pot on a step or a paved edge, make sure the base fits fully on flat ground with room for feet. Then compare those numbers to the size chart, not to how a pot looks in a photo, which is where most sizing mistakes begin.

One more thing. A pot always looks smaller outdoors than it did in the showroom or on your screen. If you are torn between two sizes for an outdoor spot, take the bigger one. Nobody has ever come back to us saying their pot was too large.

Every pot we make has its real dimensions listed, and the collection can be sorted so you are only looking at sizes that fit the numbers you wrote down. When you know the plant, the depth it needs and the space it has to sit in, the right pot is just the one that matches all three, and you can browse the full concrete pot range by size to find it.

Frequently asked questions

What size pot do I need for a lemon tree?

For a lemon tree you intend to keep at about 1.5 m, use a pot around 500 mm tall and wide. If you want it closer to 2 m, go up to about 750 to 800 mm. Concrete gives the weight needed to keep it upright in wind.

How much bigger should a new pot be when repotting?

Go 5 to 10 cm wider than the current pot. A much larger jump leaves a small root system sitting in wet soil it cannot use, which leads to root rot rather than faster growth.

How deep does a pot need to be for vegetables?

Lettuce, spinach and most herbs need about 20 cm of soil, carrots and beetroot about 30 cm, and tomatoes, peppers and brinjals 40 cm or more. Deeper pots also dry out more slowly in summer heat.

Is a wide pot or a deep pot better for succulents and agapanthus?

Wide and shallow. These plants have spreading root systems and sit in cold wet soil at the bottom of a deep pot. A bowl about 600 mm across and 250 mm deep suits a full clump of agapanthus.

How tall should a pot be at a front door?

A pot around 800 mm to 1000 mm tall with a 1 m plant gives a feature near eye height that reads well against a door. Very tall narrow pots hold less soil than they appear to, so plant them with something forgiving.