Pots and drainage
Most container problems start below the surface. A pot that holds water at the roots, or one too small for the plant, causes more trouble than almost anything else on a balcony.
Every pot needs a hole
NC State Extension is blunt: drainage holes are necessary in every container, so roots do not stand in water and rot. If you love a decorative pot that has none, use it as an outer sleeve. NC State suggests double-potting: plant in a plastic pot with holes and set it inside the decorative one.
Oregon State Extension adds two practical touches: holes drilled in the side about half an inch (just over 1 cm) above the base also work, and raising pots on bricks or slats stops them sitting in water on a concrete floor.
Source: NC State Extension; Oregon State University Extension Service, checked 26 October 2026.
Skip the gravel layer
A layer of stones or broken pots at the bottom feels as if it should help water drain. University of Missouri Extension calls this a myth: a coarse layer under the mix shortens the column of potting mix, so water collects in the mix just above the gravel and roots meet soggy media. NC State goes further: never put a layer of gravel or rocks in the bottom of a container. Fill the whole pot with a free-draining potting mix instead.
Source: University of Missouri Integrated Pest Management; NC State Extension, checked 26 October 2026.
Clay, plastic, glazed or fabric?
- Unglazed clay and terracotta are porous. NC State says porous pots lose moisture faster and need watering more often. That suits plants that like to dry out: NC State recommends clay pots for aloe. Unglazed clay can crack if wet pots freeze.
- Plastic, glazed ceramic and metal are non-porous and hold moisture longer. Heavy-duty plastic keeps moisture well but can fade in sun; metal heats up.
- Colour matters in sun. Dark containers absorb more heat, so NC State advises keeping them out of full sun.
- Grow bags work as long as they drain, according to Oregon State, and are light enough to move.
- Self-watering pots draw water up from a reservoir by capillary action. NC State says they suit people who are away for weekends or holidays, but cost more and are heavy when full.
Source: NC State Extension; NC State Extension; Oregon State University Extension Service, checked 26 October 2026.
Saucers: catch the water, then tip it out
Saucers protect floors, but NC State advises emptying them after watering, because standing water leads to root rot, salt build-up and mosquito breeding. WHO’s dengue guidance is to remove anything that can hold water and to empty containers weekly. In the monsoon, check saucers after every heavy shower.
Source: NC State Extension; World Health Organization, checked 26 October 2026.
How big a pot?
Bigger is usually easier. NC State notes larger containers dry out more slowly and need less frequent watering. Extension services publish minimum sizes for vegetables; we convert them to litres with NIST’s factors (1 US gallon is about 3.79 litres, 1 quart about 0.95 litre).
| Crop | Minimum volume | Minimum depth | Basis |
|---|---|---|---|
| Tomato | about 19 litres | about 30.5 cm (12 in) | 5 US gallons, one plant, 12 to 24 inches deep (NC State Extension); N.C. Cooperative Extension, Onslow County, also gives 5 gallons |
| Chilli or capsicum | about 11.4 litres | about 30.5 cm (12 in) | 3 US gallons (N.C. Cooperative Extension, Onslow County; NC State’s table gives 2 gallons); 12 to 16 inches deep (NC State Extension) |
| Brinjal | about 19 litres | about 30.5 cm (12 in) | 5 US gallons, one plant, 12 to 16 inches deep (NC State Extension); N.C. Cooperative Extension, Onslow County, gives 3 gallons |
| Lettuce | about 1.9 litres | about 15.2 cm (6 in) | 2 US quarts, 6 to 8 inches deep (NC State Extension) |
| Small radish | about 1.9 litres | about 10.2 cm (4 in) | 2 US quarts, 4 to 6 inches deep (NC State Extension) |
A wide, shallow tray can hold plenty of litres and still be too shallow for roots, so the calculator below checks both. Sources differ for peppers and brinjal. Where NC State’s handbook table and N.C. Cooperative Extension give different minimums, we show the larger one: 3 gallons for peppers (NC State lists 2) and 5 gallons for brinjal (N.C. Cooperative Extension lists 3). Oregon State says tomatoes, peppers and eggplant need 5-gallon containers, so for chilli a bigger pot is the safer choice.
For other crops: N.C. Cooperative Extension suggests at least 6 to 8 inches (15 to 20 cm) of depth for leafy greens, beans and peas, and containers 1.5 times deeper than the root’s mature length for carrots and radishes. Oregon State suggests 10-inch (25 cm) pots for herbs and green onions.
Our picker groups pots as small (up to about 5 litres), medium (about 5 to 15 litres) and large (over 15 litres). The bands are only a rough fit: a 6-litre pot is medium but too small for chilli, and a 16-litre pot is large but too small for tomato or brinjal. The crop minimums in the table apply whatever the band, and the picker cards for these crops show them.
Source: NC State Extension; NC State Extension; NC State Extension; NC State Extension; N.C. Cooperative Extension, Onslow County Center; Oregon State University Extension Service; National Institute of Standards and Technology (NIST), checked 26 October 2026.
How many litres does my pot hold?
Measure across the inside of the rim, across the inside of the base, and the depth of mix, in centimetres. Leave the base blank for a straight-sided pot or grow bag. The result lists the crops in the table whose minimum volume and minimum depth your pot meets.
The calculation treats the pot as a cylinder, or as a cone with its top cut off when the base is narrower: volume = π × depth ÷ 3 × (R² + R×r + r²), where R and r are half the top and base widths. Real pots with rounded bases hold a little less.
Balcony safety
Wet potting mix is far heavier than dry. Before filling large tubs or hanging planters, check what your balcony slab, railing and hooks can carry, keep heavy pots on the inside of railings, and follow your building or housing society’s rules. Where water drains matters too: drips on the floor below can annoy neighbours.
General gardening information, not professional advice. Plant toxicity details are a starting point, not veterinary or medical advice: if a pet or child may have eaten a plant, contact a veterinarian or doctor promptly.