How to water container plants: how often, how much, and how to tell

Why containers dry out so fast

A container holds a small amount of mix with warm air on every side of it, so water leaves from the surface and through the walls of the pot at the same time. Roots cannot go looking for more the way they can in a bed. A plant in a 3-gallon pot on a sunny balcony can use everything in the pot in a single hot day.

How quickly a pot dries depends on a handful of things, and knowing them tells you which of your containers to check first:

• Size. Small pots dry fastest. A 1-gallon pot may need water twice a day in July; a 15-gallon one might go two or three days.
• Material. Unglazed terracotta and fabric grow bags lose water through their walls. Plastic, glazed ceramic, metal and wood hold it longer.
• Position. Full sun, wind and a hot wall all speed things up. Wind dries pots faster than sun does.
• The plant. A big, leafy tomato or cucumber in full growth uses far more than a young seedling or a rosemary. Fruiting crops in midsummer are the thirstiest.
• The mix. Peat- and coir-based potting mixes hold water well. Mixes with a lot of bark or sand drain fast and dry fast.

This is the reason every crop guide on this site pushes you toward a roomier container. A larger pot is easier to keep evenly moist, and even moisture is what most plants want.

The two checks that tell you when

Schedules do not work well for containers, because the same pot needs a different amount in May, July and September. Two quick checks do:

The finger test. Push a finger into the mix to the first knuckle, about an inch. If it feels dry at that depth, water. If it feels damp, wait. The surface often looks dry when the mix underneath is fine, so the depth matters.

The lift test. Once you know what a freshly watered pot weighs, lifting or tipping it tells you straight away. A light pot needs water; a heavy one does not. This is the quickest way to check a row of small containers, and it becomes second nature within a couple of weeks.

Wilting is a less reliable signal than it looks. Plants wilt when they are dry, but also when they are waterlogged, when their roots are too hot, and in the afternoon sun even with plenty of water. A plant that wilts at 3 p.m. and is fine by evening usually needs shade on the pot, not more water. A plant that is wilting in the morning needs water now.

How much to give

Water the whole root ball, not the surface. Pour slowly until water runs out of the drainage holes, pause, and if the pot was very dry, go round again. A splash over the top dampens the first inch and encourages roots to stay up there, where they dry out first.

If a pot has dried out completely, the mix can shrink from the sides and water runs straight down the gap without soaking in. When that happens, stand the pot in a tray or bucket of water for twenty minutes so it soaks from below, then let it drain. Adding a little liquid dish soap to a watering can, just a drop or two, helps very dry peat re-wet.

Do not leave pots standing in saucers of water for long. A full saucer for an hour after watering is fine; a pot sitting in water for days rots the roots.

When to water

Morning is the best time. The plant has moisture available through the warm part of the day, the leaves dry before evening, and anything that splashes onto foliage has time to evaporate, which helps keep fungal problems down. In a heat wave, check again in the late afternoon and water small pots a second time if they need it.

Evening watering is better than no watering. If mornings do not fit your day, water in the evening and aim at the mix rather than the leaves.

Making pots easier to keep watered

A few things reduce how often you need to water, and they all help at once:

• Mulch the surface. A half-inch to an inch of bark, straw, gravel or even dry leaves slows evaporation and stops the surface crusting over and shedding water. This is the cheapest improvement you can make.
• Size up. Moving a plant from a 3-gallon pot to a 5-gallon one roughly doubles the water it has to draw on.
• Group pots together. Pots standing in a group shade each other's sides and create a slightly more humid pocket. Put the thirstiest ones in the middle.
• Shade the pot, not the plant. A dark pot in full sun cooks its own roots. A lighter-colored pot, a board leaned against the sunny side, or a pot-in-pot arrangement keeps the root zone cooler.
• Use a water-retaining mix. Potting mixes with a good share of peat or coir hold more than bark-heavy ones. Water-retaining crystals help for a few weeks and then stop; mulch does the same job for free.
• Self-watering planters. A reservoir under a false floor wicks water up as the plant needs it. They suit tomatoes and other crops that want even moisture, and they stretch watering to every few days even in summer. In July you will still be filling the reservoir regularly.

Watering while you are away

A container garden can get through a long weekend without much trouble, and a week with some preparation:

1. Water everything thoroughly the morning you leave.
2. Move pots out of direct sun and into a group, ideally against a shaded wall.
3. Mulch anything that is not already mulched.
4. Stand small pots in trays with an inch of water. For a few days that is fine.
5. For longer trips, a drip kit on a tap timer is the reliable answer. A basic kit for ten pots costs less than replacing a season of tomatoes.

A neighbor with a watering can is still the best system there is. Leave the can by the tap and tell them to water until it drains.

Signs you are overwatering

Overwatering is less common in outdoor containers than under-watering, but it happens, especially in spring and in shade. The signs are a mix that stays wet for days, yellowing lower leaves, a sour smell from the pot, fungus gnats around the surface, and a plant that wilts even though the mix is damp. The fix is to check the drainage holes are clear, raise the pot on feet so it drains freely, and let the top inch dry before watering again. A plant that has sat in waterlogged mix for a while may need repotting into fresh mix.

Common questions