Choose pots and planters

Size the container for the mature plant

Use the size the crop will reach to plan its eventual container, and the current rootball—the roots and surrounding growing material—to choose each pot during staged repotting. A lettuce seedling fits in a small tray, but the mature plant needs enough medium to hold water and nutrients through several harvests. The RHS container growing guide advises using the rootball as a guide and choosing a pot only slightly larger than it, because an overly large pot holds excess medium that can stay waterlogged and cause roots to rot.

Small pots are easy to place on a shelf or sill, but they change moisture quickly and demand more frequent checking. Larger containers hold more water and nutrients, dry out more slowly and are generally less work to look after. Where space allows, fewer larger containers are usually easier to manage than many small ones.

Match volume and depth to the crop

For leafy greens and beans, the suggested container depth is six to eight inches (approximately 15 to 20 cm). The N.C. Cooperative Extension container vegetable guide recommends this depth for leafy greens and beans, with replanting every two to three weeks for a continuous harvest.

Root crops need depth as well as width. The Extension guide suggests a container about one and a half times the length of the mature root: an eight inch (approximately 20 cm) carrot wants a container at least twelve inches (approximately 30 cm) deep. Choose shorter varieties when depth is limited.

The same Extension guide gives the following crop measurements. Metric equivalents are approximate; measurements and plant counts not supplied are marked “Not stated”. Crop Depth Volume Stated plant count Leafy greens and beans 6–8 inches (15–20 cm) Not stated Not stated Carrot with an 8-inch (20 cm) mature root At least 12 inches (30 cm) Not stated Not stated Tomatoes Not stated At least 5 US gallons (19 litres) Not stated Peppers and aubergines Not stated At least 3 US gallons (11 litres) Not stated Brassicas Not stated 5 US gallons (19 litres) 1 Brassicas Not stated 15 US gallons (57 litres) 3 Potatoes At least 12 inches (30 cm); 18–24 inches (45–60 cm) preferred Not stated Not stated For fruiting crops such as tomatoes and peppers, consistent watering in a container of adequate size also reduces blossom end rot.

Choose the material for the room

Container material changes how fast the medium dries and how the pot behaves in the room. The RHS guide summarises the main options: unglazed terracotta is porous, so it dries out more quickly than other materials and can crack in cold conditions; plastic is lighter, holds moisture better and is cheap, but can degrade in strong sun; metal heats up quickly in warm conditions and gets very cold in cold ones; wood is attractive and sturdy but rots over time unless lined or treated.

Indoors, consider room conditions and available space when choosing containers. A grow light warms the air above the canopy, so a porous pot under a lamp may dry faster than the same pot on a cool sill. Heavy pots of clay or filled with wet medium can strain a shelf or table, so check the support before filling. Fabric grow bags are light and can dry quickly, but drying depends on material, lamp heat and room conditions. Check the root zone before watering, using the watering and feeding guide linked below. Whatever the material, use a stable base and a tray to collect runoff—the water draining from the pot—and empty the tray after watering to protect furniture and floors.

Make drainage work

Drainage holes in the base of a container are essential, not optional. The RHS guide states this as a basic requirement. Before reusing a container, check whether adequate drainage holes already exist and are clear. If they do not, use a suitable container with drainage holes or add holes only if the container can be drilled safely. Water that cannot escape keeps the root zone saturated, excludes air from the roots and encourages rot, especially in the slow, low-light months.

Give the runoff a safe route. Place pots on stable trays or saucers, empty the trays after watering, and never leave a plant sitting in standing water. If water stops draining freely, the RHS maintenance guide suggests checking the drainage holes for blockage and the medium's structure, because decaying organic matter becomes dense and soggy. Tall, narrow containers can tip, particularly with a tall plant in them; choose a stable base, check that the supporting surface can bear the filled pot, and position it away from where it can be knocked over. Keep drainage holes clear.

Repot and refresh as the roots fill the pot

Plant roots eventually fill a container, and growth often slows when they do. The RHS maintenance guide notes that container plants then need to be moved to a bigger pot or the medium refreshed in the same pot, because the medium also loses its structure over time. Watch for roots circling the inside of the pot, roots emerging from the drainage holes, a pot that dries out much faster than before, and a plant that stops growing while the environment stays the same.

When roots fill the pot, repot into a container only slightly larger than the current rootball, working towards the crop's eventual container size. Refresh the medium around the roots, return the plant to its usual light, and check the root zone to guide watering. For a long-running setup, assess root crowding, declining medium structure and poor drainage to decide when repotting or refreshing is needed. Check containers for cracks, unintended leaks and cleanability, and replace those that are damaged or cannot be cleaned.

Avoid the common mistakes

Do not grow in a container without drainage, and do not rescue a decorative pot by lining it with plastic that seals the base. Do not plant a small crop in a very large pot and expect the extra medium to help; the surplus can stay wet and sour. Do not reuse containers that have held chemicals, or containers with unknown glazes, for edible crops, and keep feed and treatment containers clearly separate from growing pots. Finally, remember that plants in containers need more frequent watering and feeding than plants in the ground, because they have only a small volume of medium to draw on. The watering and feeding guide explains how to read the root zone before changing the routine.

Sources and review basis

1. Growing plants in containers — Royal Horticultural Society
2. Container maintenance — Royal Horticultural Society
3. Easy container vegetables and how to grow them — N.C. Cooperative Extension

Drainage requirements, pot sizing relative to the rootball, material behaviour and repotting advice follow the RHS container guides; crop-specific volumes and depths follow the N.C. Cooperative Extension container vegetable guide. US gallon figures are converted to litres approximately, and indoor conditions such as lamp heat and shelf load are working generalisations for home setups, so adjust to your space and to the manufacturer's instructions for a specific container.