Ollas: slow watering with a terracotta pot

An olla (also spelled oya) is an unglazed terracotta pot buried next to plants and filled with water. The water slowly seeps through the porous wall and moistens the soil around the pot, right where the roots are. It is a very old irrigation technique, simple and with no power needed.

How it works

Terracotta is porous. When the soil around the olla is dry, it draws water through the wall; when the soil is moist, the flow slows down on its own. The olla therefore gives water mostly when the soil needs it, without waterlogging it.

The water stays below the surface: it hardly evaporates and never wets the leaves. Roots grow around the pot, deep down, which makes plants more resistant to heat.

Making an olla from two pots

You need:

• two identical, unglazed terracotta pots (no glaze inside or outside);
• a waterproof sealant or glue that withstands moisture (bathroom silicone sealant or epoxy glue, for instance);
• a small piece of broken pot, a cork or a coin to plug a hole;
• a saucer, a slate or a flat stone to cover the opening.

Steps:

1. Plug the bottom hole of one pot: lay the piece of pot or the coin over the hole on the inside and cover it with sealant. This pot is the bottom of the olla.
2. Run a bead of sealant along the whole rim of the bottom pot.
3. Turn the second pot upside down and set it on the first one, rim to rim. Its bottom hole, now facing up, is where you fill the olla.
4. Smooth the joint on the outside and let it dry for the time given on the product (often 24 hours).
5. Check the joint: fill the olla with water outdoors. The wall should sweat slowly, but no water should run out of the joint or the plugged hole.

The sealant only covers the joint: water goes through the terracotta, not through the glue.

Installing the olla

1. Soak the olla in water for an hour: dry terracotta first absorbs a lot of water.
2. Dig a hole slightly larger than the olla, in the middle of where the plants will grow.
3. Bury it up to the neck: only the filling opening sticks out of the ground.
4. Plant around it, at a distance that depends on the size of the pot: a small olla waters an area about 30 cm across, a large one up to about 1 m. The exact shape of that area depends on your soil.
5. Fill it, then cover the opening with the saucer or the stone: this limits evaporation and stops mosquitoes from laying eggs in it.

Using it well

• Refill it before it runs dry. Depending on the heat, the size of the pot and the soil, this can range from every day in a heatwave to once a week.
• Water young plants as usual for the first days: their roots have not reached the olla yet.
• In pots or window boxes, a small olla works very well too.
• Combine it with mulching: covered soil stays moist longer and the olla empties more slowly.
• In winter, empty it or bring it indoors: water freezing in the terracotta can crack it.

Ollas suit vegetable gardens (tomatoes, courgettes, lettuce…), flower beds and newly planted shrubs. They are not suited to a lawn.

What about water restrictions?

Filling an olla is watering: it counts towards your water use and remains subject to your commune's drought decree. Some decrees treat drip irrigation separately, with more flexible hours; ollas are not always named in them. When in doubt, read your commune's decree; if it bans all watering, do not fill it.

Above all, an olla lets you water much less: the water goes straight to the roots, with no loss to evaporation or run-off.