Winterizing irrigation after turf: what still holds water before the hard freeze

01What still needs a blowout once the lawn is turf?

Walk the system from the house out:

The house end. CSU Extension starts at the stop-and-waste valve or drain between the water supply and the backflow device. It may be in a basement, a crawlspace or an underground box as much as 5 feet down, turned with a long key and never forced. Greeley’s water department adds draining the backflow assembly and the line back into the house (City of Greeley).

The backflow preventer. CSU says compressed air must never go through a pressure vacuum breaker (PVB) or reduced-pressure (RP) device, because its rubber seals can melt. The compressor connects at the blowout fitting downstream of it, and a device without that fitting needs one added before anyone blows the system out. Afterward CSU leaves the device’s ball valves and test cocks half open for the winter, Greeley turns them to 45 degrees, and Rain Bird’s homeowner guide says to insulate the device or use heat tape in extreme conditions (Rain Bird).

Capped lawn zones. These are the trap in a turf yard. Rain Bird describes a blowout as air pushing water through the mainline, valves and lateral pipes and out through the sprinkler heads, and says never to run the compressor without a zone valve open. A zone with every head capped gives the water no way out, so its pipe under the turf stays full. Its valve still holds water too: CSU says air is the only way to clear the water on top of an electric valve’s diaphragm, and Rain Bird’s answer for a diaphragm valve that isn’t blown out is to take it apart and drain it. Tell whoever does the blowout which stations are capped. Taking a lawn zone off at the valve for good is the lasting fix, and it is covered in the sprinkler guide.

Hose faucets. Greeley says to disconnect and drain outdoor hoses, because one hard overnight freeze can burst the faucet or the pipe.

02When does the hard freeze come in Northern Colorado?

The National Weather Service calls it a hard freeze when the air reaches 28°F or lower for long enough to kill most vegetation (NWS). The Colorado Climate Center maps freeze dates for Colorado’s weather stations from NOAA’s 1991 to 2020 normals, and its data file gives the first fall night at 28°F or colder at the stations nearest Northern Colorado’s towns:

The Climate Center’s own reading: only 1 year in 10 has a first freeze as early as the first column, and only 1 in 10 has one later than the last (Colorado Climate Blog). Greeley’s water department says the city usually gets its first freeze in October and recommends winterizing in mid-October. What that means for the calendar:

• On the plains, finish by mid-October. By then half of years have had a hard freeze at Loveland 2N and Longmont 2 ESE, and Fort Collins and Greeley follow within days.
• In Estes Park, finish in September. Half of years bring 28°F there by September 28.
• Don’t plan on the late column. The Climate Center says Front Range first freezes have shifted one to two weeks later since 1980, but that the shift doesn’t rule out an early cold front like the one that hit eastern Colorado in September 2020.

03How much air can drip, poly and PVC take?

Well under the 120 psi a high-pressure compressor can put out, and lower on drip than on the rest:

On a drip zone:

• Turn the compressor down first. Rain Bird says its dripline’s insert fittings are rated to 50 psi, so the air stays at 40 or less, and that the pressure regulator on the drip zone controls water, not air. The compressor’s own regulator is what holds the air down. Rain Bird also says to follow each maker’s steps for valves, filters and backflow devices; another brand’s tubing has its own limit, so ask for it.
• Open the ends. Rain Bird allows air through its dripline only with the flush valves open, run until no water comes out of them, then closed. Its guide puts each flush point in a valve box with a gravel sump, as far from the water source as possible, and says the flush point is needed to empty the zone for winter. For a tree ring under the turf, ask where that box is: a flush end buried under the turf can’t be opened without lifting it.
• Some drip only needs draining. CSU’s drip sheet calls drip winterizing minimal: the head assembly (a portable valve, backflow device, filter and regulator) goes indoors and the mainline is uncapped to drain, since poly mainline isn’t damaged by freezing. Lines with many elbows or with check valves still need air (CSU Extension).

On every zone, drip or spray, CSU says to blow one zone at a time, the way the system normally runs, in two brief passes and no more, because air moving fast through dry pipe heats it. Rain Bird’s limit is 2 minutes of air on a zone at a time.

04Should you blow it out yourself or hire it out?

It comes down to the compressor, and the sources size it three different ways:

• CSU Extension: a zone’s flow in gallons per minute divided by 7.5 gives the cubic feet per minute (CFM) of air it needs. Its example, a zone designed for 30 psi and 20 gallons a minute, needs 2.66 CFM at 30 psi.
• Rain Bird: a compressor that delivers 10 to 25 CFM. A small 2 hp shop compressor may not be enough, and a high-pressure, low-volume one (120 psi) shouldn’t be used.
• City of Greeley: at least 100 CFM, set no higher than 70 psi.

They disagree on the number, not the failure. With too little air, CSU says, the air rides over the water and leaves it to drain into low spots; Greeley says the lines then look clear when they aren’t.

Rain Bird says to call an irrigation contractor when the right compressor isn’t available, and that depending on the system and its equipment you may want a professional who is fully equipped for the job. Greeley says to hire a company if the process makes you uncomfortable, and to ask first whether it has a compressor that can handle the job, and for references.

Whoever does it, hand them a short list, because after turf the system no longer matches the lawn it was built for:

• which stations are drip, and whose tubing, so the air comes down for them;
• which lawn stations are capped, so the compressor skips them;
• where each drip flush point and the backflow preventer’s blowout fitting are;
• whether a dog will be in the yard, since CSU says the whistling can upset pets enough to run off.

CSU’s sheet ends with a caution for whoever runs the compressor: wear eye protection.

05How do trees and shrubs get water once the system is off?

From a hose. Greeley’s water department says to keep one handy for trees in a long warm, dry winter spell, then disconnect and drain it again. CSU Extension’s fall and winter watering sheet sets the terms (CSU Extension):

• When: air and soil above 40°F with no snow cover, at midday so the water soaks in before night, about once or twice a month through a dry spell.
• Trees: 10 gallons per inch of trunk diameter, measured 6 inches above the ground, spread over many spots under the canopy and beyond. CSU names birches, maples and lindens among the shallow-rooted trees that need it.
• Shrubs, October through March in a dry winter: 5 gallons twice a month for a newly planted shrub, 5 gallons a month for an established one under 3 feet, and 18 gallons a month for one over 6 feet, less after rain or snow.

Where to lay the hose in a turf yard, and what each town’s winter watering rules allow, is in watering trees after turf goes in.