Sprinkler startup: turn the system on after the frost window, not on a date

Winterization is about getting water out before the ground freezes. Startup is about putting water back in only after it stops — and doing it slowly enough that the pipes survive the transition.

Check your region first

The trigger is the frost window opening in spring, not a month on the calendar. We read soil temperature daily and mark when it is safe to pressurize buried lines again. See whether the window is open where you live.

Why waiting beats guessing

A sprinkler system that still has water in it when the temperature drops below freezing splits the same way it does in fall — pipes, fittings, valve bodies, and the backflow preventer. Colorado State University Extension wrote the winter guide because repairs at spring startup are the cost of doing it wrong in autumn.

The spring mistake is the mirror image: turning water on while overnight freezes are still routine. Warm afternoons do not protect buried pipe from a cold night if the system is already charged. If the lawn needs moisture before the frost window opens, a hose on the dry spots is the low-risk option — not the irrigation main.

The pressurization sequence

Spring order from CSU Extension: everything closed first, then water introduced slowly through the backflow assembly.

Close everything that was open for winter — drain valves, blowout ports, and test cocks on the backflow device.
Open the main shutoff slowly — partial turns with pauses, not one fast twist.
Pressurize the backflow assembly gradually — ball valves opened slowly after test cocks are closed.
Run each zone manually — one at a time, walking the yard for leaks before you trust the timer.

This is the reverse of winterization. Where fall is about expelling water with gravity or compressed air, spring is about reintroducing it without a shock. CSU Extension's guidance for a pressure vacuum breaker in spring: close the test cocks and ball valves, turn on the water supply to the device, then open the ball valves slowly to pressurize the rest of the irrigation system. Opening them quickly can damage the system through water hammer.

Before you turn water on

Close every drain that was opened for winter. Manual drains, automatic-drain bleed ports, and any blowout fitting valve should be shut. Water should have only one path in — through the main shutoff.
Close the backflow test cocks. CSU Extension notes specifically that if PVB test cocks were left open for winter, they must be closed before the system is turned on in spring.
Look at the backflow assembly. Cracked bodies, split test cocks, and fittings that weep before pressurization are winter damage. Rain Bird's spring checklist starts here for the same reason — this is the part that protects your domestic water supply, and local code may require a certified tester if it fails.
Confirm the controller has power. CSU Extension recommends leaving the controller energized through winter so transformer heat reduces moisture inside the enclosure. Reconnect or replace anything that got unplugged.

Pressurizing — slowly is the whole job

Open the main irrigation shutoff in partial turns, not one fast motion. Listen for rushing air in the lines — that is normal on the first fill — and pause if you hear hammering or see water surfacing where it should not. CSU Extension's language is explicit: opening ball valves slowly prevents damage caused by water hammer.

For a reduced-pressure backflow device, CSU Extension says the isolation ball valves should be fully closed when the system is pressurized in spring, and that ball valves should always be opened and closed slowly for the same water-hammer reason. If your assembly type is different from your neighbor's, the winter guide you followed in fall is the reference for which port opens in which order.

Test each zone before you set the schedule

Run zones one at a time on manual at the controller — the same way CSU Extension says to blow out one zone at a time in fall. Walking the yard while each zone runs, check for:

Heads that stay buried or do not rotate — often a cracked body or clogged nozzle.
Geysers at fittings — split lateral or loose clamp.
Low pressure on one zone only — partially closed valve or a break on that circuit.
Water running off the lawn — a sign to shorten run times, not to ignore.

Run times are not the factory default on the controller. The sprinkler run time calculator walks through the catch-can test extension services publish and scales it to the depth you want per zone. Startup is when you discover whether each zone can actually deliver that depth.

The house-side half of the same frost

Outdoor faucets were shut off on the same frost deadline as the sprinklers. Before you charge the irrigation system, confirm the hose bibs are ready for the season — indoor shutoff open, bleeder closed if you have one, hose reconnected only after hard freezes are done.

Bed mulch and warm-season seeding land in the same spring window. Refreshing mulch in landscape beds is a separate job from the irrigation lines, but it shares the criterion: soil warming, not a month name.

Where to stop and call someone

Hand it to a professional if:

The backflow device fails a visual check or your municipality requires annual certification before the supply valve opens.
You find a main-line break, a wet spot that grows while one zone runs, or pressure that never stabilizes — those are not controller problems.
The system has a pump that was drained and stored — CSU Extension's winter guidance is to reinstall it correctly or have someone who does it daily.
You cannot identify which valve controls which zone, which usually means the as-built drawing is gone and guessing under pressure gets expensive.

Questions people actually ask

When should I turn my sprinkler system back on?

After the hard-freeze window has passed for your area — not on a calendar date. A warm week in March does not mean a cold night will not still split a line that is full of water. Our regional pages track soil temperature daily and flag when the frost window opens in the other direction.

Why open the water so slowly?

Colorado State University Extension warns that opening ball valves quickly can cause water hammer — a pressure surge that damages pipes, fittings and valves. The spring startup is the reverse of winterization: pressurize gradually, with pauses, not in one motion.

What do I do with the backflow preventer first?

If test cocks were left open for winter, close them before turning water on. CSU Extension's spring procedure for a pressure vacuum breaker is to close test cocks and ball valves, turn on the supply, then open the ball valves slowly to fill the rest of the system.

Should I run all the zones at once?

No. Run one zone at a time on manual at the controller and walk the yard. You are looking for heads that did not pop up, fittings that hiss, and wet spots that were not there last fall — damage from winter often shows up only under pressure.

Can I skip startup and just set the timer?

The timer assumes the plumbing is intact. A cracked lateral or a stuck valve wastes water for weeks before anyone notices. The startup walk is the inspection; the schedule comes after.

Sources

Colorado State University Extension — Home Sprinkler Systems: Preparing Your Sprinkler System for Winter: spring pressurization of PVB and RP devices, closing test cocks before startup, opening ball valves slowly to prevent water hammer, keeping the controller powered through winter.
Rain Bird — Is Your Sprinkler System Ready for Spring?: backflow inspection after winter, opening the main valve slowly and gradually.

General information, not advice for your specific system. Local code and your equipment change the answer.