A couple of weeks ago, I gave a presentation at the 2012 Peak to Prairie Landscape Symposium (“So You Think You Want to Convert Your Kentucky Bluegrass Lawn?”). I started off with a discussion of the precipitation and water situation in our area. People were especially interested in my chart with the precipitation for the Apr – Oct growing season (for lawns anyway) for each year over the past twenty years. I was hoping to send people home with the idea that we certainly cannot expect our “average” annual precipitation (approx 16″ with 12″ coming from rain) each year and that they needed to be more active in deciding how and when to water their yards and gardens. And of course, I wanted to nudge them to consider lower water using lawn options.

Precipitation is of HUGE interest to gardeners, and the following chart illustrates the challenge we are up against in our area.

Here is the data:
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Wow – some wild variation from year to year, isn’t there? Yet, many homeowners just set the ol’ sprinkler timer in spring and let it run automatically all season until it gets shut down in October. So some years, it is very likely they are overwatering their lawns (and possibly gardens, if they are included in the sprinkler zones). It may seem possible that they might also be underwatering at times, but what usually happens is they set the sprinkler timer so that the lawn is always green. So if the lawn is getting enough water in July to stay green, that same amount of water is being applied all during the growing season. But plants have different water needs during different times of the growing season (don’t you find yourself a wee bit thirstier in July than in May?).

Some other observations…looking at 2011, we see 14.02″ of rain for the six month season. Not too bad, right? If we got just over 2″ of rain each month, we only had to give our lawns just a bit of extra water most months (using the popular theory that Kentucky bluegrass needs about an inch of water per week during the warmest part of the growing season here). However, nearly FIVE INCHES of that rain came in a 24 hour period in September! This is a perfect example of our “feast or famine” precipitation pattern. Looking at 1999, we see 25.79″ – wow! But nearly 10″ of that came in a three day storm in April! (By the way, 1999 received our all-time highest annual overall precipitation on record.) Looking at 2002, egads. That was a memorable one, and we saw the introduction of our first ever water restrictions (though they were fairly generous).

So, what this data really shows is that we can never really know what each season is going to bring. Averages are nice, interesting numbers, but we can’t really do much planning based on them. And we certainly can’t go with the same automatic sprinkler program each year.

Contributed by Carey Harrington – Certified Colorado Gardener; Colorado Native Plant Master