How to winterize your irrigation system
Autumn maintenance prevents freeze damage. A short checklist can save expensive spring repairs.
Why winterization is not optional
Water expands by about 9% when it freezes. Residual water trapped in buried pipes, solenoid valve bodies, backflow preventers and sprinkler risers has nowhere to expand into — it cracks whatever is around it. HDPE pipe is fairly resilient, but solenoid valves and brass fittings crack reliably at or below −4 °C. Pop-up sprinkler bodies made of ABS plastic crack at −3 °C if filled with water.
The frustrating aspect of freeze damage is that it is invisible until spring: cracked valve diaphragms, split swing pipes and fractured riser threads all look normal while frozen. The system only reveals the damage when you repressurize in April and water starts appearing from below the surface in multiple locations. A single missed winterization step can generate €200–400 in parts and repair labour.
When to winterize
The trigger is consistently low night temperatures, not the first cold night. A single night at −2 °C is rarely enough to freeze buried pipe at 20+ cm depth, but a week of nights at or below zero can lower soil temperature enough to freeze lines at standard burial depth. In northern Italy (Po Valley, Alpine foothills), winterize by mid-October. In central Italy, late October to early November. In the south, late November is usually safe.
If your area has unpredictable late-autumn cold snaps, prioritise above-ground components: shut the supply isolation valve, drain the above-ground section and remove the backflow preventer for indoor storage. Underground pipe at 20–25 cm is rarely at risk until sustained freezing, but above-ground metal components can freeze overnight.
Manual drain procedure
Step one: close the supply isolation valve and turn off the controller. Step two: activate each zone manually from the controller for 30–60 seconds to release pressure in the lines. Step three: open any manual drain valves at low points. Step four: remove and clean inline filters and mesh screens — end-of-season cleaning prevents calcium build-up from hardening over winter.
For above-ground components: remove the backflow preventer and store indoors. If your system has a master valve, leave it open after draining so any residual water can drain rather than being trapped. Leave the controller power on (or on battery) until you are certain all zones have depressurized — some controllers have a multi-zone sequential drain function that activates each zone in turn.
Air compressor blowout for larger systems
A blowout uses compressed air to push residual water out of pipes — the most thorough winterization method for systems with many metres of buried line where gravity drainage is incomplete. Hire a compressor rated for at least 28 l/s (60 CFM) flow — pressure alone (bar) without adequate flow will not move water effectively. Connect to the system via a blowout adapter at the main line; keep working pressure below 3.5 bar for pop-up systems.
Activate one zone at a time, beginning with the farthest from the compressor. Run each zone for 20–30 seconds, pause, repeat 2–3 times until no water exits the sprinkler heads. Never blow out multiple zones simultaneously — the flow splits and neither zone clears completely. Important: always open a zone before connecting the compressor. Never blow through a closed solenoid valve; the pressure spike can damage the diaphragm.
Spring startup procedure
Before restarting: reinstall the backflow preventer, replace any filters cleaned over winter, and visually inspect all accessible above-ground components. Repressurize slowly by partially opening the supply valve for 30 seconds before fully opening it — sudden pressurization can stress connections and fittings that have been dry for months.
Activate each zone manually and walk the garden during each run. Look for: sprinklers that do not pop up (clogged or broken riser), sprinklers that do not retract (worn spring), pools of water appearing at the soil surface (cracked fitting below ground), and solenoid valves that do not open (frozen debris in diaphragm). Spring startup takes 20–30 minutes and catches damage early — before a week of incorrect operation turns a minor fix into a major excavation.
Winterization in the US and UK: professional blowout and freeze zones
In the US, professional irrigation winterization blowouts cost $75–150 for a typical residential system and are performed by licensed irrigation contractors using truck-mounted compressors (minimum 50 CFM). The service is standard across USDA zones 3–6 (the entire northern half of the country) and recommended in zones 7–8 where freeze events are infrequent but severe enough to crack pipes. Most US contractors complete blowouts in October–November. Anti-freeze fluid (propylene glycol) is sometimes used in backflow preventer bodies that cannot be fully drained; it is food-safe and approved for potable water systems.
In the UK, complete blowouts are less common because most residential irrigation systems are smaller and simpler. Hozelock and Gardena recommend manual drainage plus the "water stop" function on timers as the standard procedure for UK climates. Full blowout service is offered by irrigation specialists in Scotland and northern England where hard frosts are common; in London and the south-east, draining above-ground components and leaving HDPE pipes buried at 300 mm is generally sufficient. The Met Office winter forecast (available on the UK government website) is a useful guide for deciding whether to fully blow out or simply drain and cover.
SprinklerMap Team — Irrigation technical guides
Software development, garden design workflows and technical review on realistic residential cases. Our story →