Chapter 02 · Pots
A pot dries from every side at once
A pot has no surrounding soil to wick from, a huge surface relative to its volume, and in peat-free compost a nasty habit of shrinking away from the wall as it dries. That is why hanging baskets die in a long weekend, and why a 30 cm tomato can drink a 10-litre pot dry in a hot day. Check daily in the growing season. Water thoroughly when it is dry 2–3 cm down. Do not sip.
Part of the watering guide. Also: pots · beds · lawns.
Water slowly at the base. Fill to the rim, let it soak, then repeat once and stop before water runs out of the bottom. That is the RHS container method: the roots and the compost around them are wet, without washing the feed out. Empty the saucer within a few hours. Leave a pot standing in water and the roots suffocate. Peat-free compost often wants watering a little less heavily, and a little more often, than the old peat mixes.
Terracotta breathes and dries faster than plastic. Fabric bags evaporate from the sides as well as the top. Grouping pots so they shade each other, and a mulch of grit or compost on the surface, cuts the work more than any gadget.
Winter rain is usually enough — except for evergreens in dry wind, and anything sat under eaves. Empty saucers in a freeze. Soggy compost and ice is a combination pots do not forgive.
By container
How often, by container
Summer frequencies for the UK, assuming full sun and peat-free compost. Cool, dull weeks stretch every interval. Heatwaves compress them. Finger first.
| Setup | Typical frequency | How much | Watch-out |
|---|---|---|---|
| Seed tray / modules | Daily, lightly | Bottom-water until the surface darkens | Seedlings collapse at compost level if it stays soggy (damping-off) |
| Pots under 5 L | Daily in warm weather, sometimes twice | Fill, soak, repeat once — stop before it runs out | Dry out fastest; worst in terracotta |
| 10–15 L pot (about 25–32 cm) | Every 1–2 days in summer | About 1 litre, then empty the saucer | Surface can be dry while the core is still wet |
| 30 L+ planter | Every 2–3 days, more in heat | 2–4 litres, slowly | Weight is more honest than the top centimetre |
| Hanging basket, 12 inch | Daily, twice in a heatwave | Until it is heavy; soak in a trug if it has dried out | Wind and exposure on every side |
| Grow bag | Daily once plants are large | Into a buried bottle or ring, not onto the plastic | Wet at the slit, dry at the far end |
| Fabric grow bag | Every 1–2 days | Generous — the sides evaporate too | Thirstier than plastic of the same volume |
| Self-watering pot | Top up the reservoir every 4–7 days | Reservoir only, plus a monthly flush from above | Salts build up if you never water from the top |
Dried-out compost
When water runs straight through
You water the pot. The water is out of the drainage holes in about two seconds. The pot feels no heavier. The middle of the roots is still dust-dry. More water is not the answer here. The compost has become water-repellent, and shrinkage has opened a channel down the inside of the pot.
- 01Water in stages. A third, wait a minute, then again. One flood mostly produces a puddle and a fast run-through.
- 02Stand the pot in a tray. Whatever runs through gets taken back up. Small pots: twenty minutes in 3 cm of water does more than watering from above.
- 03Press the compost back against the pot wall. Close the gap the shrinkage opened, or the next watering finds the same route out.
- 04Check by weight. A pot that has actually taken water feels heavier. If it does not, go round again.
- 05If it still runs straight through, mix a wetting agent at 2.5–5 ml per litre and repeat the staged soak. It changes where the water goes. It does not feed the plant, and it does not store extra water.
If water soaks in normally and the pot gains weight, there is nothing here you need to buy.
Hanging baskets
The worst case
Small, exposed on every side, and often packed with plants chosen to look full rather than to be watered easily. Take a dried-out basket down and stand it in a trug for half an hour — treating it in place mostly waters the patio. In a heatwave, morning and evening is normal, not fussing.
Grow bags
Water the roots, not the plastic
A grow bag is a shallow trough of compost in a plastic sleeve. Water poured on the plastic runs to the slit you cut and leaves the far end dry. Bury a bottle with the base cut off next to each plant, or use a watering ring. Feel along the bag. Moisture that swings between wet and dry is what brings on blossom end rot in tomatoes.
Seed trays and modules
Bottom-water, then drain
Stand the tray in 1–2 cm of water until the surface just darkens, then lift it out to drain. A heavy rose from above flattens seedlings and wets the stems. Compost left wet in still, crowded air is how damping-off takes a whole sowing overnight. Never let the surface dry out completely, and never leave them sitting in a puddle.
Starting volumes
About a tenth of the container
For anything under about 3 litres, make up a full can and work through a batch. The wetting agent column applies only where water runs straight through instead of soaking in.
| Container | Water to wet it through | Wetting agent, if water runs through |
|---|---|---|
| 1 L pot, about 13 cm | 100 ml | 0.25–0.5 ml |
| 3 L pot, about 19 cm | 300 ml | 0.75–1.5 ml |
| 10 L pot, about 25–28 cm | 1 L | 2.5–5 ml |
| Hanging basket, 12 inch | 1 L | 2.5–5 ml |
| 30 L planter | 3 L | 7.5–15 ml |
| Standard seed tray | 200 ml | 0.5–1 ml |
Need a starting volume? The calculator on the guide page works out a tenth of a pot, or litres per square metre for a bed soak.
Holidays
Holidays are when pots die
Group pots in the shade, water them thoroughly the morning you leave, and stand them in trays with a little water — not a bath. Self-watering pots and drip on a timer are what get hanging baskets through a week in July. A neighbour who will push a finger into the compost beats a timer you have never tested. Test the timer for a few days before you need it.
Keep reading
Back to the hub, or across to the other chapters
Finger tests, the month-by-month UK notes, and the problem table stay on the main watering guide. These three chapters are the deep dives.