Choosing Pots and Potting Mix: A Practical Guide

Choosing pots and potting mix is a practical guide to matching container, drainage and mix to each plant so the roots stay healthy.

Pick a pot with drainage holes, big enough for the mature plant’s roots but not oversized, and fill it with a potting mix that holds some water while leaving about a fifth to a third of its volume as air. Garden soil does neither job in a container, because it compacts when confined and stays soggy or sets hard.

This guide is for anyone growing in pots, tubs, grow bags or raised containers. It covers pot materials, sizes, mix ingredients, recipes for different plants, and how to test whether a mix works. It belongs to the Plant Growing Guides guides and supports watering the right way and sowing seeds.

Pots and mix at a glance

Pot materials compared

Terracotta pots need a soak before planting, or they will draw water from the mix. In freezing climates, lift terracotta onto feet and move it to shelter, because water absorbed in the clay expands as it freezes.

Choosing the pot size

A pot that’s too large for the plant stays wet in the unrooted mix, and roots can rot. A pot that’s too small dries within hours and restricts growth. For repotting an established plant, go up by 2 to 5 cm in diameter (see Repotting Houseplants). For crops grown for one season, choose a container that suits the adult plant.

As a rough guide to volume, a round pot 15 cm across holds about 1.5 litres, 20 cm about 3.5 litres, 30 cm about 12 litres and 40 cm about 25 to 30 litres. Tall and narrow shapes hold less than their diameter suggests.

What goes in a potting mix

• Peat or coir: holds water and air. Peat-free mixes based on coir, composted bark and wood fibre are widely available, but they can dry and rewet differently, so check the surface more often at first. Rinse coir blocks if they are salty.
• Compost: adds nutrients and biology. Use well-rotted material, up to about a third of the mix (see Composting at Home).
• Perlite or pumice: light, stable air spaces that don’t break down. Pumice is heavier and better for tall pots.
• Bark fines: add structure and drainage, slowly breaking down.
• Coarse grit or sharp sand: improves drainage for succulents and alpines. Don’t use fine builders’ sand, which clogs pores.
• Vermiculite: holds water and nutrients, useful in seed mixes.

Simple recipes by plant type

All parts are by volume. These are starting points, as ingredients differ between regions and brands.

Test a mix before you plant

1. Squeeze a damp handful. It should hold together briefly, then crumble when poked. A soggy lump means too much fines; a mix that won’t clump needs more compost or coir.
2. Do a drain test. Water a filled pot slowly. Water should reach the holes within seconds to a minute, without pooling on top for long.
3. Check after two weeks. Good mix settles by about 10 per cent. Add a little on top if needed.
4. Weigh by lifting. Notice how heavy the pot is when freshly watered and when dry, and use that difference to judge when to water.

Drainage: what works and what does not

Every pot needs holes, at least one per 15 to 20 cm of width, and large tubs need several. A gravel or crock layer at the bottom is a persistent myth: it raises the saturated zone closer to the roots rather than lowering it. A piece of mesh or a single broken shard over the hole stops mix washing out, and that’s all. If you love a decorative pot without holes, use it as a cover and keep the plant in a plastic pot inside, lifting it out to water and drain. Raise tubs on feet so holes never sit in puddles.

Reuse, refresh and climate

• Reusing mix: ornamentals can take old mix topped up with 25 to 30 per cent fresh mix and a slow-release feed. Don’t reuse mix from plants that had wilt, blight or root rot.
• Feeding: many bagged mixes carry only 4 to 6 weeks of nutrients. After that, feed according to the product label.
• Hot-dry climates: choose larger pots in light colours, glazed or plastic, and mulch the surface.
• Cool, wet climates: use terracotta and gritty mix, keep pots off the ground and sheltered from prolonged rain.
• Humid tropics: use coarse mixes with bark and raise containers for air flow.

Depth and shape matter as much as width

Roots follow the shape of the pot. Carrots, parsnips and long radishes need 30 cm or more of depth, and short round carrots do well in 20 cm. Lettuce, spinach and most herbs have shallow roots, so a wide, 15 to 20 cm deep trough suits them better than a tall pot. Bulbs and succulents are happiest in shallow, wide containers, often called pans, because their roots spread sideways. Tall plants such as tomatoes, sunflowers and young trees need weight at the base or a wide stance, or wind will topple them when the mix is dry.

Troubleshooting container problems

Peat, peat-free and garden-soil blends

Peat is a good growing medium, but it comes from peatlands that store carbon and are slow to recover, so many gardeners now choose peat-free. Good peat-free mixes work well, but quality varies more between brands than it did with peat, so buy a small bag first and run the drain test above. Peat-free mixes generally hold water well when fresh but can be slower to rewet once dry, and some break down faster, so plan to top up or refresh them in the second year. For large raised containers, a mix of roughly half bagged potting mix and half good topsoil or compost is cheaper than filling with potting mix alone, but avoid it in small pots, where it compacts.

How pots and soil mix work together

Terracotta absorbs water through its walls and dries fast, so it suits succulent, rosemary and thyme, while a glazed or plastic pot holds moisture for fern, mint and basil. Perlite and grit improve drainage and stop the soggy base that causes root rot. Coir holds water and needs a little compost or fertiliser, because it carries almost no nutrients on its own.

Garden clay packs down in a container and blocks air, so you should never fill a pot with it. A saucer protects your windowsill but holds standing water, which encourages fungus gnat larvae and kills fine roots if it sits for days. Why does pot size matter so much? An oversized container keeps wet mix around a small plant, and that needs far longer to dry, so tomato and pepper do better when you step up one size at a time. Your local extension service can confirm which mix suits your climate.

Matching pots and potting mix to your roots

Drainage holes in a container let water escape and prevent the root rot that kills plants in a sealed pot. Perlite and grit keep air spaces open in the mix, and that aeration feeds oxygen to roots that would otherwise stall in wet compost. Coir holds water well and releases it slowly, but it carries almost no nutrients, so a handful of compost or a slow fertiliser improves a pure coir mix.

A pot only 2 to 5 cm wider than the root ball suits most plants, because a much bigger pot holds wet mix for weeks and encourages root rot. Terracotta absorbs moisture through its walls and dries within 3 days in summer, which suits lavender and rosemary but not thirsty fern. Plastic and glazed pots hold water longer, so mint and basil stay steady between drinks, while a saucer protects your sill and should be emptied within 1 hour. You’ll avoid compacted pots by leaving out garden clay and topsoil, because they set hard and block air in a container.

Fresh mix each spring prevents fungus gnat larvae and root rot from carrying over in a reused pot, and you can refresh tired mix with compost. Peat-free blends suit most containers, and the RHS explains how peat, coir and compost mixes differ for your plants. To test a mix, pour 1 litre of water through a filled pot and you should see it drain in under a minute, which shows that perlite or grit is doing its job. Once your container is filled, Watering Plants the Right Way shows how a dry pot and a soggy saucer control root rot risk.

Mistakes to avoid

• Using garden soil or cheap fill in containers
• Putting small plants in huge pots
• Pots without holes, or gravel in the bottom
• Using fine sand in place of grit
• Reusing diseased mix
• Choosing dark pots for hot, sunny patios

What to read next

Put your pots to work with Container Vegetable Gardening, learn to propagate from cuttings in the right mix, and see how to transplant seedlings into containers. Back to the Plant Growing Guides.