Drip irrigation for containers: parts, emitters, timers and gallons-per-hour math

Why drip suits pots

Plants in containers dry out much faster than plants in the ground and are likely to need daily care in hot summer weather, so Kansas State suggests a small drip system on a timer to give container vegetables consistent moisture. UF/IFAS lists drip emitters for containerized plants and hanging baskets, Colorado State calls drip well suited to container plants, and EPA WaterSense names potted plants on balconies, porches and decks. Nebraska Extension describes tubing run out of sight along a railing or banister, with an emitter aimed into each container.

Drip delivers water to the root zone without wetting the leaves (Vermont). Colorado State puts its efficiency above 90 percent against 50 to 70 percent for sprinklers; Maryland says 90 to 95 percent of the water goes into the soil; EPA cites research showing microirrigation uses 20 to 50 percent less water than conventional spray sprinklers.

The parts, in order from the faucet

Colorado State gives the order for a permanent system as backflow prevention, control valve, filter, then pressure regulator. Clemson attaches a timer directly to the spigot, with the regulator and filter on its outlet side.

A patio kit described by a CSU Pueblo County Master Gardener apprentice held a filter, a ¼-inch hose adapter, a pressure regulator, connectors, emitter heads, ¼-inch tubing and stakes; a faucet splitter kept one outlet free for a garden hose.

Choosing emitters

Pressure-compensating or not. Ordinary emitters put out more water as pressure rises; pressure-compensating emitters hold a steady flow across a pressure range (Colorado State). Oklahoma State says they keep acceptable uniformity from 10 to 30 psi and need no regulator, but cost more. Height matters on stepped or stacked pots: each 2.31 feet of fall adds about 1 psi (Oklahoma State). Color codes differ between manufacturers, so Colorado State buys all emitters from one maker.

How many per pot. The Pueblo County apprentice uses one or two low-output heads in a small pot and, in large multi-plant pots, a ring of tubing with four 1-gph heads spaced around the pot plus one reaching into the middle. EPA says vegetable gardens may need one emitter per plant. Colorado State puts two emitters at a perennial's base so that one can fail without the plant going dry.

The gallons-per-hour math

gallons per watering = emitters × gph per emitter × minutes ÷ 60
run time (minutes) = gallons wanted ÷ (emitters × gph) × 60
zone flow (gph) = the sum of every emitter on the line
faucet flow (gph) = bucket size (gallons) ÷ seconds to fill × 3,600

Worked example. The Pueblo County apprentice estimates that 15 minutes on her large pot delivers about five quarts. Five 1-gph heads × 15 ÷ 60 = 1.25 gallons, which is five quarts. Oklahoma State's tree example runs the other way: four 2-gph emitters apply 18.8 gallons in 2.4 hours (18.8 ÷ 8 = 2.35 hours).

Will the faucet keep up? Colorado State measures a faucet by timing how long it takes to fill a bucket at full force, then plans to use no more than 75 percent of that flow. It gives about 200 gph as a conservative limit for a zone on ½-inch line (possibly 250 or more) and up to 480 gph on ¾-inch line; fifty 2-gph emitters already add up to 100 gph. Oklahoma State estimates that a standard outside hydrant supplies about 5 gallons per minute, enough for about 300 point-source emitters at 1 gph.

The default values are the Pueblo County large pot: five 1-gph heads and 1.25 gallons, which gives 15 minutes. No source here gives a per-pot drip volume for vegetables, so treat the gallons as a starting guess and tune it as below.

Tuning the run time by measuring drainage

Colorado State tunes drip by checking soil moisture at rooting depth the day after watering and adjusting run times; the Pueblo County apprentice runs a trial and checks that every pot gets enough water. Container nurseries measure it instead, as the leaching fraction: the share of applied water that drains out of the pot. The method is written for nursery crops, but the arithmetic works for any pot.

1. Catch what goes in. Collect water from an emitter like the pot's own in a lidded pail during one watering (UF/IFAS).
2. Catch what drains. Stand the pot in a tray or a snug bucket and let it drain for about 60 minutes (UF/IFAS) or at least an hour (NC State).
3. Work out the fraction. Drained ÷ applied × 100 = leaching fraction in percent.
4. Adjust. New run time = (100 − measured %) ÷ (100 − target %) × old run time. UF/IFAS's drip example cuts 12 minutes at 36 percent to 9.6 minutes for a 20 percent target.
5. Repeat. UF/IFAS tests 3–5 containers per zone on sunny or partly sunny days and retests every 2–4 weeks; NC State notes run time often has to rise as plants grow.

NC State's nursery guidance calls values under 20 percent good and says to shorten the run when drainage is above that. A pot that drains nothing at all gives no reading; the hand-watering advice in watering containers is to water until it runs from the drainage holes. Small and fabric containers dry faster (see container sizes and grow bags), so put pots with similar needs on the same line.

When to run it

• Do not run drip overnight (Colorado State); avoid midday, when heat evaporates water before it soaks in (EPA).
• For annual crops in sandy garden soil Clemson starts with daily watering in two 20-minute windows, mid-morning and mid-afternoon; in clay, 30 minutes mid-morning every other day. These are bed figures, not pot figures.
• For pots in high summer, the Pueblo County apprentice sets the timer to every 12 hours, a few minutes each time.
• Change the schedule as the weather warms and cools (Clemson) and with every change of season (EPA). Without a rain sensor, stop automatic watering until the soil dries (Nebraska).
• ATTRA notes that drip lets you fine-tune water applications, and that overwatering can leach nutrients, especially nitrogen, below the root zone.

Setting up a container line

1. Count the pots and emitters, add up the gph and check it against the faucet and supply-line limits above.
2. At the faucet fit the backflow device, timer, filter and regulator; a splitter keeps a hose outlet free (CSU Pueblo County).
3. Lay the line. Warm ¼-inch tubing in the sun for about 30 minutes so it handles more easily, cut generous lengths and pin them with stakes (CSU Pueblo County).
4. Fit the emitters in each pot; plug any misplaced hole with a goof plug.
5. Run a trial, check every pot and swap heads for higher or lower gph as needed (CSU Pueblo County).
6. Cover lines and heads with mulch (CSU Pueblo County; Clemson).

Maintenance checklist

• Spring: flush the supply line, clean the filter, cap and pressurize the system and check each emitter; soak and blow out clogged ones (Colorado State). Fit fresh timer batteries and check heads before the pots fill (CSU Pueblo County).
• Season: look at plants often for leaks and clogs (CSU Pueblo County); watch for wet or dry spots and test the system while it runs (UF/IFAS); clean the filter regularly (Clemson); flush after any repair (Colorado State) or by opening the line ends (Oklahoma State).
• Fall: flush standing water before the first freeze (EPA); store the backflow device, filter and regulator indoors and drain the line (Colorado State); take the batteries out of the timer and store it indoors (CSU Pueblo County).
• Storage: voles chew plastic tubing, so keep kits indoors or in a metal container (Maryland).

For railing and balcony layouts see balcony gardening; for crop needs, growing tomatoes in containers.

Where sources differ