How to Winterize Your Sprinkler System: The Complete Blowout Guide

The short version is: winterize your sprinkler system before a hard freeze, shut off the irrigation water, and remove water from the lines, valves, and exposed backflow equipment. For many St. Charles County systems, a professional compressed-air blowout is the safest and most complete method. Gravity drains can leave water trapped in low spots and valve bodies.

If you are in Wentzville, O’Fallon, St. Peters, St. Charles, Lake St. Louis, or nearby, do not wait for the first ice on the driveway. Irrigation companies get busy when the forecast turns cold, and the first freeze can arrive before a convenient appointment.

When should you winterize a sprinkler system in St. Charles County?

Aim to finish winterization before the first sustained freeze, not after it. In practice, that often means scheduling the work in October or early November, while watching the forecast and your lawn’s watering needs.

You do not need to shut the system down just because the calendar says fall. But once overnight temperatures are expected to stay near or below 32°F, leaving water in exposed equipment becomes a gamble. A single freeze can crack a backflow preventer, split a valve, or damage fittings underground.

A good local provider will look at the actual system rather than apply the same date to every yard. A shaded property in St. Peters may stay colder than an open yard in Wentzville. Irrigation installation, pipe material, backflow type, and soil grade all affect the safest timing.

Why does sprinkler winterization matter?

Water expands when it freezes. That expansion has nowhere to go inside a closed sprinkler line, valve, sprinkler body, or backflow assembly. The result can be a crack that does not show itself until spring startup, when the system runs and water begins pooling in the yard.

The most vulnerable parts are often the components above ground:

• Backflow preventers and exposed valves
• Short sections of pipe near the water connection
• Sprinkler heads and risers
• Zone valves and fittings
• Low spots where water remains after a basic drain

Replacing a sprinkler head is annoying. Replacing a cracked backflow device or repairing several damaged zones is a much bigger spring expense. Winterization is preventive maintenance, not just one more fall chore.

Inspect the system before you drain it

Run each zone briefly before shutting the system down. Walk the yard and look for broken heads, bubbling soil, poor spray patterns, leaks, and areas that never pop up correctly.

Make a note of anything that needs repair. Winterizing a damaged system does not fix the damage, and a leak can make it harder to tell whether the spring startup is working properly. If a head is broken or a valve is leaking, ask whether the repair should happen before or after the blowout.

Also locate these parts before the appointment or DIY work:

1. The irrigation water shutoff valve
2. The backflow preventer
3. The dedicated blowout connection, if the system has one
4. The controller and zone map
5. Manual drain valves or low-point drains

If you cannot identify the shutoff or blowout port, that is a good reason to stop and call someone who works on irrigation systems.

Shut off the water and set the controller

The irrigation shutoff is separate from the valve that supplies water to the house. It may be in a basement, crawlspace, valve box, or near the water service. Close the irrigation supply slowly. Do not force a stuck valve.

After the water is off, relieve pressure according to the system design. Set the controller to system off, rain mode, or another winter setting that prevents scheduled watering while keeping the program available for spring. The correct setting depends on the controller and whether the system has a pump.

Do not assume unplugging the controller is always best. Some manufacturers and extension guidance recommend leaving the controller powered while keeping it from activating irrigation zones. If your system includes a pump, follow the pump manufacturer’s winter instructions rather than guessing.

For controller options and freeze-related features, see our smart irrigation controller review.

Is draining enough, or do you need a sprinkler blowout?

There are three common approaches:

Method	How it works	Main limitation
Manual drain	You open drain valves and let water leave by gravity	Water can remain in low spots, valves, and sprinkler bodies
Automatic drain	The system releases water when pressure drops	It only works if the system was designed, installed, and maintained for it
Compressed-air blowout	Regulated air pushes water out through each zone	Requires the right connection, air volume, pressure, and safety procedure

A blowout is generally the more thorough option for an electric-valve residential system. Colorado State University Extension notes that air volume matters as much as PSI. A small compressor that reaches a high pressure may still move too little air to clear a zone properly.

Do not connect a compressor randomly to a hose bib or force air through a backflow preventer that is not designed for it. The backflow device may need to be drained separately. Pressure vacuum breakers and reduced-pressure assemblies have specific internal parts and procedures; a qualified irrigation professional should handle them if you are unsure.

How a safe sprinkler blowout works

If your system has a proper blowout port and you know the pipe and backflow setup, the general sequence is:

1. Close the irrigation water supply. Confirm the line is isolated from household water.
2. Connect through the dedicated blowout fitting. Use the fitting and adapter intended for the system.
3. Regulate pressure before opening the air. Common guidance is up to 50 PSI for flexible polyethylene pipe and up to 80 PSI for rigid PVC, but the system’s component ratings and manufacturer instructions control. Lower pressure is safer when you are uncertain.
4. Open one zone at a time. Start with the zone recommended by the system design, often the farthest zone from the connection.
5. Use short passes. Run the zone until the water changes to a fine mist, then stop. Repeat a short second pass rather than running high-velocity air through an already dry line for a long time.
6. Keep people and pets away from the heads. Debris can discharge at speed. Wear eye protection and never stand over a sprinkler head or valve.
7. Repeat for every zone. A zone-by-zone process is slower, but it is much easier to control.
8. Shut down the compressor before changing connections. Release pressure safely and follow the equipment instructions.

The two biggest blowout mistakes are using too much pressure and using too little air volume. More PSI is not a substitute for the right compressor. Excess pressure can damage pipe, fittings, sprinkler heads, and valves.

If you want to understand how pressure and flow affect coverage during the season, read our guide to lawn water pressure and sprinkler requirements.

Protect the backflow preventer and exposed parts

A blowout clears the irrigation zones, but it does not automatically protect every part of the backflow assembly. Follow the manufacturer’s procedure for draining the device, opening test cocks, and positioning ball valves. Many assemblies should be left partially open so trapped water has room to drain and expand.

Insulate exposed components with an appropriate cover after they have been drained. Do not block required vents or leave a cover in a way that traps water against the device. Above-ground pipe and shutoff fittings may need protection as well.

If you have a reduced-pressure backflow assembly, do not improvise. These devices may require separate draining of internal check zones. A local irrigation professional can identify the assembly and winterize it without putting the device or your drinking-water protection at risk.

Common sprinkler winterization mistakes

Waiting until after the freeze

Once ice has already formed inside a component, winterizing may be too late. Schedule ahead of the coldest forecast.

Assuming the controller is the shutoff

Turning off the timer does not remove water from the irrigation lines. You still need to close the water supply and drain the system.

Relying on a basic gravity drain

A system may drain most of its water while leaving enough behind in a low spot to cause damage. Know how the system was installed before choosing this method.

Using a small compressor at dangerous pressure

A homeowner compressor may have enough pressure but not enough volume. Raising the regulator to compensate can damage the system.

Blowing through the backflow device

The backflow preventer often needs its own procedure. Do not treat it like another sprinkler zone.

Forgetting the spring startup

Write down the winterization date, shutoff location, repairs needed, and any special backflow steps. In spring, open the water slowly and inspect each zone for leaks before returning to a normal schedule.

When should you hire a professional?

Hire an irrigation professional if you cannot identify the water shutoff, do not know whether the lines are PVC or polyethylene, lack a dedicated blowout connection, have a pump, or have a pressure vacuum breaker or reduced-pressure backflow assembly.

Professional help also makes sense for larger systems, steep properties, several zones, or any yard where a mistake could damage expensive equipment. Ask what method they use, whether they protect the backflow preventer separately, and whether spring startup or repair notes are included.

If you want help comparing local options, submit the details through Midwest Lawn Care’s St. Charles County request form. We can point you toward providers who handle the type of irrigation work your yard needs.

The bottom line

Winterize before a sustained freeze. Shut off the irrigation water, remove water from every zone, protect exposed equipment, and do not guess with compressed air. For many Missouri systems, a properly regulated professional blowout is safer than a rushed DIY job with the wrong compressor or connection.

A few minutes of inspection and the right fall service can prevent a much more frustrating spring.

Sources: Colorado State University Extension winter sprinkler guidance, University of Minnesota Turfgrass Science winter irrigation guidance, and Scotts sprinkler winterization overview.