Watering your plants with an olla

Both systems aim to do the same thing — deliver water to the roots without wetting the foliage — but they are managed in completely different ways. A drip system delivers a set volume decided in advance, either by a timer or by you. It therefore irrigates just as much on a rainy day as on a hot, windy one, unless you constantly readjust it. A porous olla only releases what the soil draws from it, since it is the drying of the earth against the wall that pulls water outwards.

Drip irrigation has two clear advantages: it covers long bed lengths, and once connected to the mains it requires no manual filling. On the other hand it demands thorough filtration — emitter blockage remains its main weakness — and it deteriorates quickly in sunlight. An olla needs neither pressure nor electricity, but it does mean making rounds with a watering can, and it only irrigates a limited circle around itself.

In practice, the choice comes down to area. Over a few square metres of vegetable bed, containers, or planters, an olla almost always wins. Beyond around twenty square metres planted in tight rows, the number of ollas to fill becomes a burden and drip irrigation regains the upper hand. There is nothing to stop you combining both, reserving ollas for thirsty plants and for areas you keep the least eye on.

The published figures vary widely because they depend on what you are comparing against. Trials reported by the University of Arizona's agricultural extension place water savings at between 60 and 70% compared with watering by hand. A 2024 review paper records savings of around 69% against furrow irrigation, and 70 to 80% against buried porous pipes.

Focus on the mechanism rather than the percentage. Water never wets the surface, so it barely evaporates, does not run off, and does not feed weeds. Compared with a well-adjusted drip system, the difference narrows considerably, and any gains then come mainly from eliminating overwatering.

Restrictions apply to uses and times, not to equipment. Check the by-law in force in your area before filling your jars.

The buried pot is not a universal solution. Here are a few situations where it falls short.

• Seedlings sown in situ, whose seeds germinate in the very top centimetres, which remain dry.
• Grassed surfaces and ground cover, where water needs to be distributed evenly across the whole area.
• Very small pots, in which the pot takes up more volume than the roots.
• Very stony soils, where contact between the clay and the surrounding earth is incomplete.
• Established trees and shrubs, whose roots extend well beyond the wetted circle.

In these cases, occasional surface watering remains simpler and more reliable.

Start by measuring the wetted radius. The most widely cited observation, echoed by the University of Arizona extension service, is that the wetted circle reaches roughly twice the diameter of the pot, and maize trials quoted in the same document give spacings in the region of 20 to 30 cm between the plant and the source.

On a bed three metres long and 80 cm wide, three to four medium-sized pots therefore cover most of the area, provided plants are positioned close by. It is better to move plants closer to the pots than to add more pots — the most common mistake is planting too far away.

The calculation depends mainly on water costs and surface area, but two points are fundamental. A pot has no wearing parts, no seals, no timer, and it lasts several seasons provided it is never frozen when full or allowed to fur up with scale. A drip irrigation system, by contrast, is replaced section by section over the years.

That said, the cost per unit of covered surface remains higher for pots, since one needs to be buried every 40 to 60 cm. The real question is therefore not the raw return on investment, but the value you place on a kitchen garden that looks after itself while you are away.