Deb Babcock: Planting over hardpan
July 29, 2012
If you have plant and garden questions, Master Gardeners are on hand every Saturday at the Mainstreet Steamboat Springs Farmers Market. Stop by their table with your question or just to say, “Hi.”
Steamboat Springs — In answer to almost every question about plants asked of Master Gardeners, one part of our reply often deals with improving the soil. The reason for that is the vast majority of plant problems originate with soil issues. And one major soil problem here in the mountains is our dense clay soil.
Unless your home-builder was extra careful in the excavation and recovery of the soil surrounding your home, you probably have a layer of hardpan under your topsoil near the house. Hardpan is a rock-hard layer of soil often caused by construction equipment driven over the damp, clay subsoil. The hardpan also can be created by people walking on the damp soil and compacting it. When this soil dries out, it has the consistency of hard brick.
The problem with hardpan is the inability of plant roots to penetrate the soil. Even water cannot penetrate this layer, creating an environment where plant roots are stunted and immersed in soggy, undrained soil if the porous top layer of soil is only a few inches deep. A shallow layer of topsoil dries out quicker, too, in our hot, windy summers. The roots then cannot grow deeply into the soil to extract water, and the plant often dies.
If the layer of hardpan in your yard or garden is fairly thin, you might be able to simply till it under to a depth of 12 to 18 inches. If tilling doesn't work, you might need to use a soil auger to drill through it when planting, add a significant amount of porous top soil over the hardpan, or bring in an irrigation contractor to install drainage system. You can also consider planting your garden or yard using raised beds filled with porous soil that drains well.
If you are able to plow under a layer of hardpan, improve it by adding organic matter and mixing it in well with the plowed soil. If you have long-term plans for the piece of ground being worked, consider planting some long-rooted grasses or plants with long tap roots (alfalfa — a perennial — or safflower — an annual) and then till those into the soil in a subsequent year to add more organic material.
Other recommended soil amendments include tree bark, aged wood chips, aged sawdust and certain aged manures or composts.
If you're hoping to plant grass in the improved soil, be sure to check into cool season grasses or some of the native varieties of meadow grass. Because of the low amount of summer rain here and the summer temperature extremes, large swaths of lawn are difficult to maintain without a lot of water and care.
Perennials, annuals, shrubs and trees also should be chosen with an eye toward their hardiness in our climate and water needs.
Most plants come with a USDA Hardiness Zone rating and many also have an AHS Heat Zone rating to help you choose those plants most likely to thrive here. Generally, you're looking for plants with a Hardiness Zone of 4 and a Heat Zone of 4, but your elevation, microclimates within your garden and protection from elements can affect which zone-rated plants will be successful for you.
Deb Babcock is a Master Gardener through the CSU Extension Routt County. Call 970-879-0825 with questions.
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