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Expansive soils are found throughout Colorado including major population areas along the Front Range and Western Slope. Expansive soils, also called shrink-swell, welling soils or clay soils, shrink when dry and swell up when wet. The movement and pressure of these soils can crack sidewalks and driveways. Underground pipes are vulnerable as are foundations which can crack and basement floors which can crack and heave. In Colorado, expansive soils contain a clay called montmorillonite. This is sometimes referred to as bentonite clay. True bentonite is found in Wyoming and Nevada.
Not all expansive soils swell at the same rate. Soils should be tested before construction to determine the potential for damage to any structures. Soils that have a cracked or puffy appearance when dry and are sticky when wet will have a higher concentration of swelling clay. Soils that have been amended with topsoil can disguise the clay soil underneath and according to Colorado law, builders must inform home owners about expansive soils on their building site. It is very important that homeowners understand the steps taken by the contractor to deal with the problem so they can properly maintain the soil around the foundation.
The presence of expansive soils has implications for landscaping. Engineered fills are sometime used to restrict water around a foundation, so do not amend this soil to increase drainage. Make sure to maintain the slope away from the foundation when landscaping. If the soil around the building settles, build it back up with the proper grade. Down spouts extension might be needed to release roof water at least five feet away from the foundation. Sprinkler systems should also spray five feet away from the structure.
Do not plant flowers or shrubs closer than five feet to the building and trees should be at least 15 feet away. Plant roots that are too close to the building can draw moisture from the soil, adding to soil moisture fluctuations. Low-water ground covers or rock over a non-permeable liner can be used around the foundation. The goal is to reduce water fluctuations next to the foundation through long term maintenance of the soil slope, drainage and landscaping.
For further information on landscaping with expansive soils, refer to Fact Sheet #7.236 - Landscaping on Expansive Soils.
Q: I planted several young trees this year. What can I do to get them through the winter safely?
A: Trees, even when established, have a difficult time with the cold winds and temperature fluctuations of Colorado winters. Avoid sun scald damage by wrapping the trunks of smooth bark trees. Wrap trees about Thanksgiving but remember to unwrap it in the spring. Drought injury can occur in the winter from extended periods of low moisture and drying winds. Place five inches of organic mulch over the root zone to prevent water loss and to moderate soil temperatures.
Q: Now that winter is on the way, can I just quit watering until spring?
A: No, because of our arid climate you will need to water in the fall and during winter if there is little natural moisture. Lack of winter water will weaken plants causing them to die in late spring or even early summer. This is especially true of plants that have been in the ground for less than three years. Water early in the day when temperatures are above freezing to give the water time to soak into the ground. When watering in the winter, make sure to disconnect your water hose from the spigot when you are done to prevent frozen pipes and water damage in your home.
Autumn Weed Control: Broadleaf weeds in your lawn can be controlled with an application of herbicide this time of year. For effective control, weeds must be actively growing when the herbicide is applied. One example is Dicamba, or products containing dicamba. This herbicide can control bindweed, knotweed, mallow, spurge, thistle, wood sorrel, and yarrow. If you cannot identify the weed in question, contact the CSU Cooperative Extension office.
Spring Bulb Planting: Wait until the last week of September to plant your spring flowering bulbs. Bulbs planted too early can rot due to high soil temperatures. September could possibly be very warm and dry, so watch the weather patterns and plant your bulbs when the temperatures cool down.
The authors have received training through Colorado State University Cooperative Extension's Master Gardener program and is a Master Gardener volunteer for Larimer County.
Fact Sheets are available at the Larimer County Extension Office, 1525 Blue Spruce Drive, Fort Collins, Colorado, telephone (970) 498-6000, or contact us by e-mail at larimer@coop.ext.colostate.edu
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