3 Rules of Mile High Gardening

How to garden with attitude at altitude

Posted by Steve Graham | Jul 25, 2010
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Gardening at altitude is all about attitude — and smart watering, according to Alison O’Connor, a horticulture agent for Larimer County Cooperative Extension in Colorado. Affiliated with Colorado State University, the Colorado Cooperative Extension program is a national leader in research and information about gardening above one mile in elevation.

Many assume gardening at elevation is a difficult struggle against rocky and snowbound terrain. Instead, a wide variety of plants thrive throughout the mountain states. To be sure, high-altitude gardening has challenges, but it brings a few advantages.

“It’s a great place to garden,” said O’Connor. “We don’t have a lot of the disease and the fungus pressure.” The arid climate that dries out plants and soil also minimizes water-related plant diseases. Also, even though summers may get hot, temperatures are significantly lower in the shade. This is as advantageous for many plants as it is for humans.

On the other hand, an extreme and unpredictable climate brings new challenges with every season and year. O’Connor says even experts sometimes struggle and have gardening failures. Just keep an open mind, she said, and don’t get too discouraged when plants don’t survive or grow as quickly as you might like.

Aside from keeping a positive attitude, here are O’Connor’s three core rules for mile-high gardening.

1. “Water is a big reality.”

Much of the Rocky Mountain West is categorized as a high desert because it is so arid. Many high-elevation areas get less than 15 inches per year of precipitation, compared to a national average of 36. Humidity may be just 20 percent on sunny summer days. Moreover, high winds exacerbate the lack of water, particularly in spring.

This dryness translates to the need for supplemental watering in most gardens. O’Connor’s office encourages and researches xeric plants that need minimal water. However, even dry, native plants need plenty of water to become established in the first season or two.

  • Trees and shrubs are often overlooked while watering, but O’Connor said they should be thoroughly watered at least once a month, including during dry winters. To avoid damage from broken limbs during heavy spring storms, keep trees pruned properly at a safe distance from the home.
  • All plants should be watered with drip irrigation systems or other watering systems that directly water the base and roots of plants, rather than overhead water that can be lost to evaporation and runoff.
  • Mulch is also important for trapping and retaining soil moisture. O’Connor recommends four inches of organic mulch, such as shredded bark or even grass clippings. Rocks may work to retain moisture, but they can also get too hot and stress out the plants.
  • Lawns are often frowned upon in arid mountain climates, but they can work with the right xeric grasses and an efficient irrigation system. Many cities in high, arid areas offer free irrigation audits that help determine the most efficient system and schedule.
  • While sprinklers must be shut off in the winter to prevent frozen pipes, plants should be occasionally but thoroughly watered on dry warm days outside of the main growing season. “Winter watering has done wonders to make plant material not only come out better in the spring, but reduce the drought-related stress,” O’Connor said.

2. “Know what kind of soil you are dealing with.”


Water isn’t the only important factor. O’Connor said some transplants from other dry areas assume that gardening in a high desert is essentially the same as gardening in a low desert. However, they don’t realize the soil may be very different.

Soils in Colorado and many neighboring states tend to be hard, heavy and rocky clay, preventing healthy drainage and root growth. A lack of water also discourages richer soil, and ancient mountain glaciers scraped away loamy topsoil and left rocks in their wake.

O’Connor recommends getting a soil test before starting any home garden. It is most important to know the organic content and pH level of your soil. Most high-elevation soil is alkaline. Mile-high soil also doesn’t drain well, and most gardeners have seen the requirement for “well-drained soil” on virtually every seed packet and plant tag.

Adding humus, compost and other organic matter can combat all these soil problems, and a few shovelfuls of sand will improve drainage.

3. “Realize the limitations on what we can grow.”

High-altitude water and soil conditions contribute to limitations on growing some plants. Winter weather — fast-changing temperatures, drying winds and intense sunlight — all inhibit the growth of rhododendrons and other broad-leafed evergreens. Blueberries and some other fruits also struggle at elevation.

However, O’Connor said the limitations have not inhibited her gardening since moving from the Midwest several years ago. “I haven’t found any of the plants that I enjoyed that haven’t done well here,” she said.

Even though March is often the snowiest month, early season crops such as lettuces and peas can thrive in early spring. Vegetables and other plants may not grow and flower as early, large or long as in other areas. Tomatoes and other vegetables require nighttime temperatures above 60 degrees, and can be killed by a late frost or a summer hailstorm. Denver’s average last frost date is May 3, but many experts suggest mothers’ day — the second Sunday in May — as the spring planting start. O’Connor pushes it even later, though it means a later crop.

“They really do better if you wait until June 1 to get them in the ground,” she said.

Denver’s first frost in the fall comes on Oct. 3 on average, but that doesn’t mean harvest season ends in September. Soil temperatures are just peaking in October.

The higher the elevation, the more limited the flora. Leadville, which is above 10,000 feet, has just 26 frost-free days in the average year — not the ideal climate for growing heat-loving tomatoes. However, with care, even Leadville homeowners can grow flowers and vegetables.


All these warnings about water, soil and weather shouldn’t scare away gardeners. O’Connor just suggests being flexible and open-minded.

“Don’t get frustrated, learn from your mistakes, and utilize resources of people that have lived here a long time,” she said.

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