How many ollas do you need for a garden bed?
An olla (pronounced "oy-ya") is an unglazed clay pot buried up to its neck and filled with water. Water seeps through the porous clay into the surrounding soil, and roots grow toward the pot to take it up. The University of Arizona Cooperative Extension explains that the seepage continues until the nearby soil reaches field capacity, the most water it can hold against drainage.
Because each olla wets only the soil around it, the number you need depends on how far apart they can be and still keep the whole bed within reach. That spacing is the key input, and published guidance on it is limited and approximate.
Olla spacing guides
| Guide | Olla size | Spacing | Calculator uses |
|---|---|---|---|
| UC Agriculture and Natural Resources | Smaller ollas | 2–3 ft | 2.5 ft (0.76 m); radius about 1.25 ft |
| UC Agriculture and Natural Resources | 2 gal or more | 3–4 ft | 3.5 ft (1.07 m); radius about 1.75 ft |
| University of Arizona rule of thumb | Any; based on body width | wets a circle about twice the olla's width | spacing = 2 × body width; radius = body width |
The two guides give quite different answers. By the Arizona rule, an 8-inch olla waters plants within a circle about 16 inches across; the Arizona publication describes this as anecdotal and notes that research on ideal spacing is limited. The UC guidance spaces ollas several feet apart. Closer spacing means more ollas but each one serves less soil, so fills last longer. Whichever you use, check the soil midway between ollas once plants are growing, and add ollas if it stays dry.
UC Agriculture and Natural Resources also points out that ollas are inefficient for densely planted annual crops, because even coverage takes so many pots. For a full bed of closely spaced vegetables, compare the count here with a drip or soaker hose layout.
How this olla watering calculator works
The calculator lays the ollas out on a square grid at your spacing, then shares the bed's daily plant water use among them. Dividing each olla's capacity by its daily share gives the days until it's empty.
1. Number of ollas
With length and width
Ollas per row = Length ÷ Spacing, rounded up
Rows = Width ÷ Spacing, rounded up
Ollas = Ollas per row × Rows
Rounding up keeps every part of the bed within half a spacing of an olla. The actual gap is then Length ÷ Ollas per row, slightly less than your spacing.
With area only
Ollas = Area ÷ Spacing², rounded up
Spacing by olla width
Spacing = 2 × Olla body width
2. Daily water use per olla
Plant water use
Daily use (depth) = ETo per day × Plant factor
Area per olla = Bed area ÷ Ollas
US: Gallons per olla per day = Area per olla (sq ft) × Daily use (in) × 0.623
Metric: Liters per olla per day = Area per olla (m²) × Daily use (mm)
ETo per week is divided by 7; ETo per month is divided by 30.4. 144 ÷ 231 ≈ 0.623 gallon per square foot per inch.
The plant factor scales ETo, the water a reference grass surface would use, to your plants. The low, moderate and high classes use the middle of the WUCOLS ranges (0.2, 0.5 and 0.8). The vegetable option uses 1.0, a typical figure: FAO-56 lists mid-season crop coefficients near or above 1.0 for many vegetables, up to 1.15–1.20 for staked tomatoes, peppers, beans and cucumbers. Young plants use much less. For crop-by-crop needs, the vegetable garden water calculator goes further.
3. Days between refills
Refill time
Days until empty = Olla capacity ÷ Water use per olla per day
Water per refill round = Ollas × Olla capacity
Rounding
The olla count is a whole number and isn't rounded further. Days until empty are shown to one decimal below 10 days and as whole days above that, with "about" because they rest on estimated weather and plant use. Daily water per olla is shown to three significant figures, and spacing to two decimals.
When this calculator isn't the right tool
It's built for in-ground garden beds of herbaceous plants. For other situations, the garden and tree calculators have dedicated tools:
- Raised beds and containers, where the soil volume is limited. See the raised bed watering calculator and the container plant watering calculator.
- Trees and shrubs. The University of Arizona publication advises against ollas for woody plants, whose roots can break the pots. The tree watering calculator covers their needs.
- Densely planted beds, where a soaker hose or drip line may be simpler than many ollas.
- Hand or hose watering of beds. Use the garden watering calculator.
Olla watering calculation example
A 12 ft by 4 ft vegetable bed will use 1.5-gallon ollas at the UC spacing for smaller ollas, 2.5 ft. The plants are at full size, and the local weather network reports ETo of 0.20 inch per day.
- Ollas per row: 12 ÷ 2.5 = 4.8, rounded up to 5.
- Rows: 4 ÷ 2.5 = 1.6, rounded up to 2.
- Ollas: 5 × 2 = 10 ollas in 2 rows of 5, about 2.4 ft apart along the bed and 2 ft between rows.
- Area per olla: 48 sq ft ÷ 10 = 4.8 sq ft.
- Daily use: 0.20 in × 1.0 = 0.20 in per day.
- Water per olla per day: 4.8 × 0.20 × 0.623 ≈ 0.598 gallon.
- Days until empty: 1.5 ÷ 0.598 ≈ 2.5 days. Refilling all 10 takes 15 gallons.
The same kind of calculation in metric
A 3 m by 1.2 m flower bed of moderate water-use plants will use 4-liter ollas that are 30 cm wide, spaced by the University of Arizona rule. ETo is 5 mm per day.
- Spacing: 2 × 30 cm = 0.6 m.
- Ollas: 3 ÷ 0.6 = 5 per row; 1.2 ÷ 0.6 = 2 rows; 5 × 2 = 10 ollas.
- Area per olla: 3.6 m² ÷ 10 = 0.36 m².
- Daily use: 5 mm × 0.5 = 2.5 mm per day, so 0.36 × 2.5 = 0.9 L per olla per day.
- Days until empty: 4 ÷ 0.9 ≈ 4.4 days. Refilling all 10 takes 40 L.
With the UC spacing for smaller ollas (0.76 m) instead, the same bed needs 8 ollas (4 per row, 2 rows). Each would serve 0.45 m² and last about 3.6 days. Fewer ollas means more frequent refilling.
How often to refill an olla
The University of Arizona publication notes that refill frequency varies with soil, weather and the plants. The calculator's figure is a starting point; your first few fills tell you the real rate.
Check, then adjust
For the first couple of weeks, look into each olla daily and note how far the level drops. Compare that with the calculator's daily use per olla. If yours empty much faster, the plants are using more than estimated or water is escaping; if much slower, the soil may already be moist or the roots haven't reached the pot yet. Top up before an olla runs dry, since plants relying on it get no water once it's empty.
Install for even seepage
UC Agriculture and Natural Resources recommends burying ollas with 1–2 inches of neck above the soil to keep dirt and mulch out, and packing soil firmly around the body, because air pockets stop water from moving into the soil. In very deep soil, a saucer under the olla helps push seepage outward rather than down. Keep the opening covered to cut evaporation.
Plan for new plantings
Seedlings and fresh transplants can't reach the olla yet. The University of Arizona notes they need supplemental watering until their roots find the pot, which can take a month or more. Plant within each olla's reach, not at the edge of it.
Filling from other sources
Ollas can be filled by hand or kept topped up by gravity from a tank through tubing, which the Arizona publication notes works with collected rainwater. The rainwater irrigation supply calculator shows how long a barrel or tank would keep your ollas full.
Common mistakes
- Spacing too wide for small ollas. A small pot at large spacing serves a big area and empties fast, and the soil between may stay dry.
- Ignoring the weather. A fill that lasts a week in mild weather may last only a few days in a hot spell. Update the ETo figure as the season changes.
- Glazed or sealed pots. Only unglazed clay lets water through.
What changes how many ollas you need and how long they last
| Factor | Effect | What to adjust |
|---|---|---|
| Olla size | Larger ollas hold more and, by UC guidance, can be spaced farther apart. | Choose the matching spacing guide and enter the capacity. |
| Spacing | Closer spacing means more ollas, each serving less area, so fills last longer. | Reduce spacing if soil between ollas stays dry. |
| Weather | Heat, sun, wind and dry air raise ETo and water use. | Update the weather choice or ETo as the season changes. |
| Plants and growth stage | Large, leafy plants in full growth use far more than seedlings. | Raise the plant factor as plants fill in. |
| Soil | The University of Arizona notes soil type affects both spacing and refill frequency. | Watch soil moisture between ollas and adjust spacing. |
| Salt buildup | Salts can build up in the clay and clog its pores, slowing seepage (University of Arizona). | If an olla drains more slowly over time, clean or replace it. |
| Cracks | UC ANR notes that needing to refill more often can mean a crack has formed. | Check the olla if its rate suddenly rises. |
The soil infiltration rate chart shows how quickly different soils take up water. If you have weather data but no published ETo, the evapotranspiration calculator estimates it.
Frequently asked questions
How many ollas do I need for a 4 by 8 ft bed?
It depends on the spacing you use. On a square grid, a 4 by 8 ft bed needs 8 small ollas at 2.5 ft spacing (2 rows of 4) or 6 large ollas at 3.5 ft spacing (2 rows of 3). Spacing by the University of Arizona rule of twice the olla's width gives more ollas, each serving a smaller area.
How far apart should ollas be?
UC Agriculture and Natural Resources suggests placing ollas 2 to 3 feet apart, and larger ollas of 2 gallons or more up to 3 to 4 feet apart. A University of Arizona publication cites a rougher rule that an olla wets a circle about twice its own width. Research on ideal spacing is limited, so check that the soil midway between ollas stays moist and add ollas if it doesn't.
How often do I need to refill an olla?
Divide the olla's capacity by the water the plants around it use each day. For example, a 1.5-gallon olla serving 4.8 square feet of full-size vegetables in weather with 0.20 inch of daily ETo lasts about 2.5 days. Refill frequency changes with weather, soil and plant size, so check the level often during the first few weeks.
What size olla should I buy?
Larger ollas hold more water and, by UC guidance, can be spaced farther apart, so you need fewer of them and refill less often. Smaller ollas suit small beds and tight spaces. Use the calculator to compare how many of each size a bed needs and how long each fill would last.
Can I use ollas for trees and shrubs?
The University of Arizona publication, citing New Mexico State University's Curtis Smith, recommends ollas for herbaceous plants such as flowers, grasses and many vegetables, but not for woody plants like trees and shrubs, because their roots are likely to break the pots.
For trees, see the tree watering calculator.
Do new plants need extra water with ollas?
Yes. Seedlings and new transplants need supplemental watering until their roots reach the olla. The University of Arizona notes this can take up to a month or more. Plant close to each olla so roots reach it sooner.
Why is my olla emptying faster than before?
Hotter, drier or windier weather and larger plants all raise water use. If the change is sudden and the weather hasn't changed, UC Agriculture and Natural Resources notes that the olla may have cracked.
Why is my olla emptying more slowly over time?
Cooler weather and moist soil both slow seepage. Over longer periods, the University of Arizona notes that salts can build up in the clay and clog its pores. Air pockets around a newly buried olla can also block water from moving into the soil, so pack soil firmly against it.
Who built and checked this calculator
Licensed Professional Engineer and Irrigation Association Certified Irrigation Designer with more than 20 years in irrigation design, system auditing and landscape water management. Full profile
How this page was checked
- Layout math, unit conversions and both worked examples verified by hand.
- Spacing values checked against University of Arizona and UC ANR guidance, and labeled as typical.
- Plant water use checked against FAO-56 and WUCOLS.
Sources
- Nickel, A. and Brischke, A. Irrigating with Ollas (AZ1911). The University of Arizona Cooperative Extension, 2021.
- UC Agriculture and Natural Resources, The Coastal Gardener. Olla Irrigation.
- Allen, R.G., Pereira, L.S., Raes, D. and Smith, M. FAO Irrigation and Drainage Paper 56. Crop evapotranspiration: Guidelines for computing crop water requirements, Chapter 1 and Chapter 6. Food and Agriculture Organization of the United Nations, 1998.
- California Center for Urban Horticulture, UC Davis. Water Use Classification of Landscape Species (WUCOLS).