Ask SAM: A green spring without the rain

This column is provided by Larimer County Extension's Small Acreage Management (SAM) Program to assist rural residents.

Dear SAM,

The plants on our place seemed to grow better this year than the past few years. I checked the newspaper and saw we have had below average rainfall. Why are the plants so green if they aren't getting rain?

Joan

Dear Joan,

You have made an interesting observation. Grass growth was good this spring and plenty of wildflowers were blooming, as well. Yet from January to mid-June, we received only 4.70 inches of precipitation north of Fort Collins. This is about 40 percent below our average of 8.05 inches for that time period. How did the plants grow without the rain?

Rainfall is only one of the environmental factors affecting plant growth. Temperature, soil moisture and wind also influence when plants show signs of drought.

The average temperature this year from March through June, when plants were growing, was 51 degrees. This matches our 100-year average temperature for this period. So the 2007 spring season hasn't been cooler than normal. It has just been cooler than the past few years. In 2006, for example, the average temperature was 53 degrees for the same period. Two degrees might not sound like a big difference, but it feels a lot different.

Above-average precipitation last October followed by good winter snowfall helped this spring's plant growth. Many parts of the county received deep snows that melted slowly. The fall rains and melting snow filled the soil with moisture. Grasses were able to use that moisture this spring. The plants were busy drinking even though we didn't see the rain filling their cups.

Evapotranspiration, or ET, is the amount of soil water lost to evaporation and plant use. ET is affected primarily by temperature and wind. By mid-June, ET for grass fields was 12 percent below the 2002-2006 average.

Adequate soil moisture and moderate temperatures combined with reduced ET gave plants enough water to grow, even though we were short on rain.

As we enter late June and this newspaper heads to press, temperatures are rising and our afternoon clouds aren't developing. Soils are drying and cool-season grasses are going to seed.

Expect to see a rapid brown-up during the next two weeks. Pastures will become less palatable. Fire danger will increase. And rivers and lakes will be filled with two-legged mammals looking for relief from the heat.

SAM

Have a question about rural living? Write to Ask SAM, Larimer County Cooperative Extension, 1525 Blue Spruce Drive, Fort Collins, CO 80524 or e-mail emarx@larimer.org.