Drip irrigation for a vegetable garden: how much water each crop needs and how many minutes that is

The trouble with fixed rules

Drip guides for vegetables like simple rules: 30 minutes a day in summer, a gallon per tomato. They are a fine start, but they ignore the two things that can double or halve the water: the weather that day and how much space each plant takes. Thirty minutes is not the same amount of water with 0.5 GPH and 1 GPH emitters, and a 100°F day in July is not an overcast day in June.

So this guide starts from the math. With two public numbers, reference ET for your area and the coefficient for each crop, you get gallons per square foot for each bed and the drip minutes that go with them.

The formula: local ET × crop coefficient

Reference evapotranspiration (ETo) is the water a well-watered reference grass loses in a day through soil and leaves. It depends on temperature, humidity, wind and sun. Each crop uses more or less than the reference; the ratio is its crop coefficient, Kc, and the crop water need is ETo × Kc. This is the FAO-56 method that irrigation advisers use; background in /enWhy Evapotranspiration Is the Foundation of Modern Irrigation: the FAO-56 Paper Explained.

You do not have to guess ETo. Many states run weather networks that publish it daily, CIMIS in California being the best known, and extension services publish it in season; smart controllers pull it automatically. Summer values range from modest in cool, humid regions to very high in the desert Southwest, which is exactly why a schedule copied from another region rarely works.

The table: gallons per 100 sq ft per day, crop by crop

The table takes FAO coefficients for common vegetables and converts them to gallons per 100 sq ft per day for two example days: 0.16 in ETo, a typical moderate summer day, and 0.24 in, a hot day. Values include drip losses (efficiency 0.9) and apply at peak use, when plants cover the bed well.

From gallons to minutes, and what the one-gallon rule is worth

The controller only knows minutes. To convert, you need how many gallons per hour your drip puts on each square foot. Example: a 3 ft wide bed with two drip lines, 0.5 GPH emitters every 12 in. Each foot of bed has two emitters, 1 GPH over 3 sq ft, or 0.33 GPH per sq ft. Tomatoes in full production on a 0.24 in day need about 0.19 gal per sq ft: 0.19 ÷ 0.33 × 60 = about 34 minutes. On a 0.16 in day, about 23 minutes. The formula works for any layout: minutes = gallons per sq ft ÷ GPH per sq ft × 60.

The same math puts the one-gallon rule in context. A tomato on about 5 sq ft in full production needs about 1 gallon on a 0.24 in day and about 0.7 gallon on a 0.16 in day. So a gallon fits a hot week, not a cool one or a young plant. The most reliable final check: 4 to 6 in down at the base of the plant, the soil should be moist, not wet.

Crop stages: do not water seedlings like mature plants

The coefficient changes over the season, and so does the water. A just-transplanted tomato has a Kc of 0.6, about half of the 1.15 it reaches in full production. Running August minutes from day one is the fastest way to drown young roots. At the end of the season, as the last fruit ripens, Kc drops again (0.70 to 0.90 for tomatoes).

In practice, revise the minutes at least three times per crop: after transplanting, when plants cover the bed and start producing, and at the end of harvest, plus after any big weather change. On sandy soil split the daily water into two shorter runs; on clay one longer run every day or two works better (/enHow long to run sprinklers: minutes per cycle by soil and head type, with the math behind them).

Raised beds and containers

Raised beds dry out faster than in-ground beds: sun and wind reach the sides, and they are cut off from the soil moisture below. They usually need at least the table values, at shorter intervals. A drip line looped through the bed in two or three runs, or a ready-made raised bed kit, on its own zone or a hose timer, is the simplest setup.

Containers are their own world: small soil volume, fast drying, one emitter or more per pot sized to the pot and plant. Put them on a separate zone from in-ground beds, because they need water more often and in smaller doses.

The setup in brief

For vegetable rows, drip line with built-in emitters every 12 in or so is the most practical, laid along each row and covered with mulch; on sand, closer spacing, on heavy soil emitters can be farther apart. Pressure-compensating emitters keep output even on long rows and slopes.

Two parts are not optional: a filter at the head of the zone, because emitters clog on a grain of sand, and a pressure regulator, because drip is rated for far less than house pressure. And the vegetable garden gets its own zone, separate from lawn sprinklers. If you run drip from an outdoor faucet with a hose timer, the faucet needs a vacuum breaker for backflow protection. How to add a drip zone to an existing system: /enSprinklers or drip? How to choose zone by zone, and how to add drip to an existing sprinkler system.

Drought rules and rain barrels

During shortages many US cities limit sprinkler days and hours; drip and hand watering of vegetables are often treated more leniently, but the rules vary, so check your utility. Rain barrels are a good fit for vegetables where they are allowed: the table tells you how far they go. A 100 sq ft bed of tomatoes needs about 19 gallons on a hot day, so a 55-gallon barrel covers about three hot days.

Well-calculated drip remains the most water-efficient way to grow vegetables: water goes to the roots, not to the paths or the leaves. And dry foliage is one of the best defenses against the leaf diseases that spread on wet tomato leaves.

Maintenance

Clean the filter at the start and end of the season, more often with hard or well water. Once a year, open the end caps with the zone running for a few seconds to flush out sediment. Emitters clogged by mineral deposits often recover after soaking in a mild acid such as vinegar or citric acid. Before the first freeze, drain the lines: /enHow to winterize a sprinkler system: drain or blow out, protect the backflow preventer, and restart in spring.

Frequently asked questions