The transition from summer to fall in Colorado requires adjusting the lawn watering schedule. Cool-season grasses like Kentucky Bluegrass enter a period of reduced growth and energy storage in autumn. The goal of fall watering is to hydrate the turf’s root structure so it survives the dry, cold winter months. Timing the end of regular irrigation balances preparing the turf for winter dormancy with avoiding damage to the irrigation system from an unexpected freeze.
Climatic Indicators for Ending Regular Watering
The decision to cease regular lawn watering should be guided by environmental signals rather than a fixed date on a calendar. The primary indicator is the sustained drop in air temperature, which signals the turf to slow its metabolic processes. Once daytime high temperatures consistently remain below 60 degrees Fahrenheit, the growth rate of cool-season grasses decreases significantly, meaning they require substantially less water for maintenance.
Shorter photoperiods, or the reduced hours of daylight, also act as a natural trigger for the turf to prepare for winter dormancy. This reduction in solar energy intake decreases the grass’s need for transpiration, further lowering the water demand. For most of the Colorado Front Range, this shift generally occurs between late September and mid-October.
The final deadline for stopping regular watering is the prediction of the first hard freeze, typically defined as temperatures dropping to 28 degrees Fahrenheit or lower for several hours. This event usually occurs in the late October to early November timeframe in lower elevations, but it can be earlier in mountain communities. Watering past this point is ineffective because the ground begins to freeze, making water absorption difficult, and it introduces a significant risk of damage to the underground irrigation system.
Gradual Reduction and Tapering Schedule
Stopping irrigation is a gradual tapering process that helps the turf acclimatize to winter conditions. Beginning in early to mid-September, transition from the summer schedule of frequent watering to a deep, infrequent routine. The goal is to reduce the frequency of watering while maintaining the deep soaking duration to encourage strong, deep roots.
If you were watering three times a week in the summer, you should reduce this to once or twice a week as temperatures drop into the 70s. The turf still requires about \(0.5\) to \(0.75\) inches of water per week in September and October to recover from summer stress and build carbohydrate reserves for winter. This adjustment prevents the grass from becoming overly dependent on surface moisture.
Over the final four to six weeks before the anticipated hard freeze, continue to reduce the frequency until you are watering only once every seven to ten days. Keep the root zone slightly moist, but not saturated, until the ground permanently freezes. A moist soil profile helps insulate the roots and prevents desiccation, or “winterkill,” which is a common cause of turf damage in Colorado’s dry winters.
Irrigation System Winterization
Once the final tapering application is complete and before the first hard freeze, the mechanical process of winterizing the irrigation system must be executed. In Colorado’s climate, water left in the pipes will expand as it freezes, leading to burst lines and damaged components. This makes the “blowing out” procedure necessary for protecting the system.
The blowout involves using a large volume, low-pressure air compressor to force residual water out of the main lines, lateral pipes, and sprinkler heads. This process requires connecting the air compressor to a designated fitting, opening each zone valve sequentially, and running compressed air through the system until only a fine mist is ejected. Use the correct pressure, usually less than 80 pounds per square inch for commercial systems and under 50 PSI for residential systems, to prevent fracturing the PVC components.
After the system is completely drained of water, the main water supply to the system must be shut off, and the backflow prevention device should be fully drained. Leaving the ball valves and test cocks on the backflow device in a half-open position helps to prevent any trapped water from freezing and causing a split. Once the system is winterized, any supplemental watering during a dry winter thaw must be done manually with a hose, typically when air temperatures are above 40 degrees Fahrenheit.