Choosing Pots and Container Sizes

The single biggest mistake new container growers make is choosing a pot that is too small. A pot that looks generous on a garden-centre shelf often turns out to be far too cramped once a plant gets going, and a cramped plant dries out, sulks and crops poorly. Get the size and the drainage right, and almost everything else becomes easier.

This guide covers the two things that matter most — size and drainage — then runs through the common pot materials, gives you a quick minimum-size table by crop, debunks the "crocks in the bottom" myth, and flags the weight question if you are growing on a balcony.

Why size and drainage matter most

A container is a closed world. In the open ground, roots can range for water and food; in a pot, they have only what you give them and only the room you provide. That makes two things decisive: how much compost the pot holds, and whether spare water can escape.

Size is really about water and root room. A bigger pot holds more compost, so it holds more moisture and dries out more slowly — the difference between watering once a day and three times a day in a July heatwave. It also gives roots space to fill out, and a plant with a strong root system feeds itself far better than one strangled in a small pot. As a rule, when you are unsure, size up. A tomato in a 40cm pot is a calmer, more forgiving plant than the same variety squeezed into a 25cm one.

Drainage decides whether roots live or drown. Compost in a pot with no escape route stays sodden after rain or over-watering, the air is pushed out, and roots suffocate and rot. Every container you plant into needs holes in the base — no exceptions. We will come back to drainage in detail below, because there is a popular myth wrapped up in it.

For the wider picture of how pots, compost and watering fit together, start with our guide to growing food in containers, which this article supports.

Pot materials: pros and cons

There is no single "best" pot — each material trades off price, weight, looks and how fast it dries out. Here is how the common choices stack up for a UK grower.

Terracotta (clay). Handsome, heavy and stable, which makes it good for tall plants that might blow over. The catch is that unglazed clay is porous: it wicks moisture out through its walls, so terracotta pots dry out faster than any other type — a real chore in a hot, breezy spot. Cheap terracotta can also crack in a hard frost if left full of wet compost over winter. Lovely for a sunny patio if you are happy to water often; less forgiving if you are out all day.

Plastic. The workhorse of container growing. Light, cheap, frost-proof and excellent at holding moisture (no porous walls), so plants in plastic need watering less often. It looks less refined and degrades in UV light over several seasons, but for productive veg it is hard to beat on practicality. Recycled and recyclable options are widely available. If you want maximum crop for minimum fuss, plastic is usually the sensible default.

Fabric pots (grow bags / "smart pots"). Felt-like fabric containers that have become very popular for spuds, tomatoes and root veg. They are cheap, fold flat for storage, and "air-prune" roots — roots that reach the breathable wall stop and branch rather than circling, giving a denser root system. The trade-off is that they dry out quickly (air gets at the compost from all sides), so they need attentive watering in summer. Brilliant for potatoes in particular.

Ceramic (glazed). Glazed ceramic gives you terracotta's good looks without the porosity — the glaze seals the walls, so it holds moisture like plastic and dries out slowly. The downsides are weight and price: a large glazed pot full of wet compost is genuinely heavy, and they are the dearest option. Check the base before buying, as some decorative glazed pots are sold with no drainage hole at all. A good choice for a permanent, smart-looking patio display if weight is not a concern.

A quick rule for our climate: if your spot is hot, sunny and you are short on time, lean towards plastic or glazed ceramic that hold water. If you have shade and are around to water, terracotta and fabric are fine.

Minimum sizes by crop

Container size is usually quoted by diameter (across the top) and sometimes by volume in litres. The figures below are sensible minimums for the UK — bigger is always fine, and often better. Where a crop is a hungry, thirsty one, err on the larger end.

If you are weighing up which crops give the best return for the space, our guide to the best vegetables for containers ranks the reliable performers.

The drainage and crocks myth

You may have read the old advice to put a layer of "crocks" — broken pot pieces, gravel or stones — in the bottom of every container to "improve drainage". For decades this was repeated as gospel. It is wrong, and trials have shown it can actually make things worse.

Here is what really happens. Water does not move freely from a fine material (compost) into a coarse one (gravel) until the compost above is already saturated. So a gravel layer doesn't drain the pot faster — it just raises the level of the soggy, waterlogged zone, leaving roots in wetter compost than if you had filled the pot with compost all the way down. The thing that drains a pot is the holes in the base, not a layer above them.

So skip the crocks. Fill the container with compost top to bottom and make sure the base has plenty of drainage holes. The one genuinely useful trick is a small square of old window-mesh, fleece or even a coffee filter over each hole — it stops compost washing out without blocking the water, and keeps slugs from sneaking in through big holes.

Getting the compost right matters just as much as the pot. Garden soil is too dense and dries to a brick in a container, so use a proper potting mix — our guide to the best compost for containers explains what to buy and how to keep it healthy through the season.

Weight on balconies and roof terraces

If you are growing above ground level, weight is not a detail — it is a safety matter. Wet compost is heavy: a 40cm pot can weigh 20kg or more once watered, and a row of them quickly adds up to a serious load on a balcony or terrace.

A few sensible steps keep things safe and manageable:

• Favour light materials. Plastic and fabric pots weigh a fraction of glazed ceramic or stone when full. Keep the heaviest pots for the ground, not the railings.
• Use peat-free, lighter mixes and consider blending in some perlite to reduce dead weight while keeping moisture-holding good.
• Keep weight near the wall, over the supporting structure, rather than out at the edge of a cantilevered balcony. Never hang heavy pots off a railing unless it is rated for it.
• Check before you load up. If you are unsure what your balcony can safely carry — particularly in a flat or a rented building — ask your building manager or a structural engineer. It is a quick question that avoids a real hazard.
• Group pots in trays or saucers to catch run-off, so you are not soaking the flat below every time you water.

If you are planning a balcony from scratch, our balcony vegetable garden guide covers layout, wind and weight together, and the container growing hub gathers all the small-space guides in one place.

Choose pots that are big enough, make sure every one drains freely, and match the material to your spot and your watering habits — do that and your containers will carry a genuinely worthwhile harvest, garden or no garden.