Ollas

Olla gardening

by Curtis W. Smith, NMSU Extension Horticulture Specialist

Irrigation of plants by means of ollas, unglazed pottery jars, is an ancient practice. It was brought to the American Southwest by Spanish settlers and adapted to local gardens by Native American gardeners as well as by the Spanish settlers.

Over time, modern systems were adopted, but these modern systems are not as efficient as irrigation by seepage from buried ollas. Modern systems, even surface drip irrigation systems loose more water to evaporation and more likely to clog than ollas. When ollas are used properly, plant roots will proliferate around the moist clay jar, intercepting water before it can move through the soil by capillary action. This water intercepted by plant roots will then be used in the plant transpiration stream. This results in almost 100% of applied irrigation water being absorbed by the plants.

Olla irrigation solves problems for gardeners who cannot irrigate frequently, and is a boon for plants that should not be irrigated frequently. Ollas allow gardeners who travel as well as gardeners whose irrigation frequency is limited by water conservation ordinance to irrigate infrequently while still maintaining the health and beauty of their garden plants.

Proper plant and olla selection is important. Woody plants may break the pottery jars as their woody roots grow in diameter. Herbaceous plants are less likely to damage ollas. Olla porosity, size, and shape must be matched to plant water needs, root size and root distribution. Deeply rooted plants benefit from deeper ollas, shallow rooted plants are more efficiently irrigated with shallow ollas. The diameter of the olla may also be chosen to match the diameter of the plant cluster. Shallow, broad, ollas will provide adequate irrigation for clumps of grasses and annuals.

Olla plantings should be planted in clusters to maximize water use efficiency. While the planting group may be of one plant type, mixtures of grasses, annuals, biennials, and perennials may also be planted around a single buried olla. Mixtures of plant types may be used to create a more natural landscape. The olla clusters may themselves be clustered to create more expansive or linear plantings.

The olla pottery may become a decorative element in the landscape along with large rocks and flagstones. Portions of olla left exposed above ground should be glazed or treated to prevent evaporation.

To modernize these ancient irrigation systems, the jars may be recharged by a drip irrigation system, timed and sized to replace water lost from the ollas. Recharge of ollas may be done daily, or as frequently as allowed by water conservation ordinances.

For further information on ollas, see:

• Clay Pot Irrigation entry in the Continental Drift blog
• Ollas page at East Central Ministries,Inc.

Ollas can be ordered from:

• Arizona Pottery http://www.arizonapottery.com/
• Neo Gardens http://neogardens.com/catalog/ollas2.html

Curtis Smith, Extension Horticulture Specialist, asked the Santa Fe Master Gardeners to begin a new project with his guidance. He suggested we run an experiment, comparable to our herb garden, with one row of plants watered by ollas and one watered by a drip irrigation line.

On April 20, 2005, we covered our garden plot with black plastic. On April 25, we covered the plastic with 2-3 inches of shredded bark mulch to kill the weeds and grasses.

By June 15, most of the weeds were dead. We set up the drip lines—1 for the ollas and one for drip only—to compare how the plants responded.

On June 22, we dug holes for the ollas and filled them with water for the first time. They all leaked! We'd made our ollas of two 8-inch clay pots sealed with silicone. We'd placed a landscape fabric scrap over the bottom holes, also covered with silicone. The bottom holes had leaked, so we added silicone to seal them.

On June 29, we dug two bags of Back-to-Earth compost into each row. The soil is extremely heavy clay – almost heavy enough to make ollas! When we dug, we found worms in the olla row but not in the drip line row. We found two grubs in the drip line row.

On July 6, we planted both rows with the same plants: just like Noah's ark, two by two. We planted flowers, an ornamental grass, chile plants, runner beans; and vegetable seeds: Indian corn, zucchinis, and bush beans. We didn't expect to harvest any corn, but we thought we might get good growth comparisons.

Related information: Olla Irrigation? Oh Yeah! (NMWCA Summer 2004 Newsletter article, PDF)