The Guide to Rocky Mountain Vegetable Gardening (Cool Springs Press, 2009) is a vegetable gardening book that addresses the unique growing conditions and challenges of the five states of the Rocky Mountain region. Authors Bob Gough and Cheryl Moore-Gough include advice on everything from starting your garden from seed, to planning your garden with helpful space-saving techniques. Helpful charts will outline when to plant and when to harvest cool and warm season vegetables. The following excerpt comes from chapter one, “The Lay of the Land: Climate and Soils.”
You can buy this book in the MOTHER EARTH NEWS store: The Guide to Rocky Mountain Vegetable Gardening.
Of all the information that we will share with you in this book, the most important thing for you to know is the specific climate and soil type of your area. Our five-state area covers more than a half million square miles from the flats of the western Great Plains west beyond the alpine peaks of the Rockies. Wow! But what you’ll need to know to get started can be found in our state-by-state general discussions of the soils, precipitation, temperatures, and storms that will have an influence on your garden. Ours is a region of extremes, and gardening here can be a challenge. The most important issue is elevation, for elevation has the most significant impact on temperature, precipitation, and local weather conditions. Sometimes there’s as much as 30 to 40ºF of difference between daytime and nighttime temperatures, and the intense sunlight can burn plant foliage and rapidly dry soil and leaves. Soils become thinner at higher elevations and hence are less suitable for vegetable gardens. You’ll quickly learn to amend and enrich your garden soil to be successful, but it’s important to know the type of soil in your garden to determine what additions may be needed.
Gardeners can expect about an 11°F reduction in temperature for every 3,300-foot increase in elevation. But even small differences in elevation can cause marked differences in growing conditions. For example, we used to live six miles south of Bozeman, Montana, where our garden was about 200 feet higher than Bozeman proper, yet our growing season was about a month shorter than that in town. Part of the reason was the heat sink effect (heat retaining effect) of towns, but another was the small difference in elevation.
We’ve included tables in the Appendix containing elevation, average length of the growing season, and the range in the length of the growing season of twelve cities for each state included in the Rocky Mountain region. The length of a growing season is based on the average number of days between the first and last frosts (that is, 32.5°F temperatures). Using these figures you can estimate the length of your garden’s growing season pretty closely.
Our extreme variability in growing season length is the result of the highly variable climatic conditions that make our gardens such a challenge. There may be multiple sets of data for the same town, depending upon whether the information was collected at the airport, in the town center, or at the local agricultural experiment station.
Colorado is the highest state in the United States, with an average elevation of 6,800 feet. About 40 percent of the state is the High Plains, which slope gently higher as you move westward from the eastern border at elevations of 3,350 to 4,000 feet, through the Front Range. The High Plains are usually hot on summer mornings and cool during an afternoon thunderstorm. Those thunderstorms can sometimes be severe and the hail they contain can destroy gardens in minutes. The daily maximum summer temperature is about 95°F at elevations below 5,000 feet but cools at higher elevations to the west. There are wide variations in temperatures within short distances. For example, the difference in average annual temperature between two gardens only 90 miles apart can be equivalent to differences in temperature between Florida and Iceland. About 85 percent of its annual precipitation falls in summer. Gardeners in the northeastern areas enjoy a respectable growing season of about 140 days; those in the southeastern areas, an even longer season of about 160 days; and the fortunate few in the extreme southeastern corner of Colorado relish their 180-day seasons.
About 200 miles west of the eastern border lie the Foothills, with elevations of about 7,000 to 9,000 feet. Gardening here becomes challenging, with an average July temperature of 60°F and daily highs in the 70s and 80sºF. Nights are particularly cool all summer long, which will limit your vegetable selection. Cool-season crops will do well but warm-season crops will be more of a challenge. Beyond the Foothills lie the mountains, rising from 9,000 to 14,000 feet, and beyond them is the high plateau that extends to the western border at elevations above 10,000 feet. While there may be some gardens in the low western valleys, much of the mountainous area simply does not have a growing season long enough to make it worthwhile. Nights are so cool above about 8,000 feet that many folks simply do not garden. Gardeners in the valleys of the Gunnison, Dolores, and Colorado Rivers enjoy especially long growing seasons, with the area around Grand Junction having up to 221 frost-free days in some years. Summers are wet in the eastern areas of the state but they are pretty dry in the western areas.
Productive Colorado soils, like those throughout the lower elevations of our region, have low acidity, which can cause some nutrient deficiencies. Front Range soils tend to be heavy clays that need amendments. Adding coarse sand equal to about 50 to 80 percent of the top eight inches or so of garden soil will go far to amend what you have.
The soils along river valleys are most productive, as are soils in the moister northeastern parts of the state. Drier soils on the plains of southern Colorado and on the mountain slopes and plateaus are thin and can be relatively unproductive.
Idaho’s elevation rises from north to south, with the lowest location at the confluence of the Clearwater and Snake Rivers (738 feet) and the highest at Mt. Borah in Custer County, (12,655 feet). Large parts of the state, especially northern areas, are strongly influenced by Pacific Ocean air, though eastern Idaho is not. Temperatures are highest at the lower elevations of the Clearwater and Little Salmon River basins and along parts of the Snake River Valley from Bliss to Lewiston. Gardeners in Swan Falls enjoy the highest annual average temperature for the state (55°F) while those in Obsidian, at 6,780 feet, experience the lowest (35.4°F). Daily temperature fluctuations are most extreme in the high valleys and the semi-arid plains of the Snake River. In fact, the daily temperature from July to September can vary by more than 30°F at Boise.
Idaho precipitation patterns are complex. Average valley precipitation is greater in the southern sections, with large areas of the Clearwater, Payette, and Boise River basins getting 40 to 50 inches or more per year. On the other hand, large areas in the northeastern valleys, much of the Upper Snake River plains, the Central plains, and the lower elevations of the southwestern valleys receive fewer than ten inches per year. In the northeastern valleys and the eastern highlands less than half the rain falls between April and September, while in the Boise, Payette, and Weiser River drainage basins less than a third falls in those same months. Low relative humidities throughout the state mean dry air and rapid drying of soils and plants.
Wind throughout the state can be highly destructive, and savvy Idaho gardeners plant in protected areas.
As in other states, the growing season varies greatly depending on elevation, soil type, topography, and vegetation cover. Lewiston and its immediately surrounding areas have the longest seasons in the state. The central Snake, and lower Payette, Boise, and Weiser River basins enjoy about 150-day seasons, while upstream areas of the Snake near Pocatello and Idaho Falls have about 125-day growing seasons. Some high valleys have no growing season at all.
The most productive Idaho soils are the desert soils along the Snake River and the prairie soils in the western part of the state around Lewiston and Moscow. In general, the rest of the state has relatively poor soil.
Montana has great climatic variations. The western part of the state is mountainous while the eastern two-thirds is part of the Great Plains. Elevations vary from a low of 1,800 feet in the northwestern part of the state where the Kootenai River enters Idaho to 12,850 feet at Granite Peak near Yellowstone Park. About half the state lies above 4,000 feet. Land west of the Continental Divide enjoys a modified northern Pacific Coast climate, with milder winters, more even distribution of annual precipitation, cooler summers, stronger winds, more cloudiness, higher relative humidity, and shorter growing seasons than those of eastern Montana. In western Montana hot spells are rare in summer and of relatively short duration, though temperatures can sometimes top 100°F in the low valleys. Above 4,000 feet it is almost never “very hot.” Eastern Montana has a more extreme climate with average July temperatures of 74°F in southern areas. Midsummer days are warm but nights cool into the 50s and 60sºF. Miles City is one of the warmest parts of the state, having a July minimum of 60°F and an average maximum temperature of 90°F.
Precipitation is highly variable. The western mountains are the wettest area and nearly half of the annual precipitation falls from May to July. Heron is the wettest location, receiving 34.7 inches of rain on average each year. North-central Montana is the driest part of the state, although the absolute driest spot is near Belfry along the Clark Fork of the Yellowstone River in Carbon County. Belfry receives an average annual precipitation of only 6.59 inches.
Summer storms are frequent, with hailstorms in July and August causing about five million dollars of crop damage annually.
The average growing season for Montana is about 130 days. Most of the agricultural areas enjoy a growing season of more than 120 days, while the middle Yellowstone River Valley in the area around Miles City can expect a 150-day season. The higher valleys of western Montana have no growing season at all.
Soils in the eastern parts of the state are rich and can be quite productive, as can be the soils along major rivers like the Yellowstone, the Milk, and the Missouri.
Most of Utah is mountainous, varying from an elevation of about 2,500 feet in the Virgin River Valley in southwestern Utah to 13,498 feet at Kings Peak in the Uinta Mountains. Most of the state receives only light precipitation throughout the year.
The lower elevations generally are warmer than elevated valleys and mountains. In general, the southern counties are 6 to 8°F warmer than the northern counties. There are wide daily fluctuations in temperatures, and in winter on clear nights the cold air settles in the valley bottoms while the benches and foothills remain warmer. Experienced gardeners know that the best growing areas are the higher lands at the valley edges. Although there is no orderly or extensive zone of equal length growing season, most agricultural areas of the state enjoy 130- to 150-day seasons.
Precipitation is highly variable, ranging from fewer than five inches per year over the Great Salt Lake Desert to more than forty inches in some areas of the Wasatch Mountains. The annual average for agricultural areas is about ten to fifteen inches. Areas of the state below 4,000 feet receive less than ten inches. Northwestern and eastern Utah are also quite dry.
The loam soils in the narrow belt at the base of the Wasatch Range are highly productive, as are the dry soils and the gray desert soils in much of western and some parts of eastern Utah.
Wyoming’s elevation rises from north to south, with an average elevation of 6,700 feet. The lowest elevation is 3,125 feet near the northeastern corner of the state; the highest is the 13,785-foot Gannet Peak in the west central part of the state. Eastern Wyoming has an average elevation of 4,500 feet while the foothills to the west rise to 6,000 feet and more.
The entire state is relatively cool and areas above 6,000 feet rarely experience temperatures of 100°F. The average maximum temperature in July is 85 to 95ºF, though areas above 9,000 feet have an average July maximum of only about 70°F. The lower part of the Big Horn basin, the lower elevations of central Wyoming, and the northeastern and eastern sections along the border are the warmest. Summer nights are cool.
Late spring and early fall frosts are common. The average growing season in the main agricultural areas is about 125 days. Areas along the eastern border west to the foothills can experience growing seasons from 100 to 130 days, while Farson, near Sandy Creek off the Green River, has only a 42-day season. There is practically no growing season for tender plants in the upper Green River Valley, Star Valley, and the Jackson Hole area.
Elevations greater than 7,000 feet receive annual precipitation of up to thirty inches, with about a third of that falling during the growing season. Southwestern Wyoming at elevations of 6,500 to 8,500 feet receives 7 to 10 inches. Lower elevations at 4,000 to 5,500 feet in the northeastern parts of the state and along the eastern border can expect about 12 to 16 inches per year. The southwestern sections are very dry. The lower part of the Big Horn Basin, with 5 to 8 inches, is the driest. Seaver, at 4,105 feet, receives 5.5 inches. Worland, near the southern part of the basin, receives 7 to 8 inches; Thermopolis, 11 to12 inches; and Laramie, in the southeast corner of the state at 7,236 feet, about 10 inches. As an example of how quickly conditions change in the West, Centennial, only thirty miles west of Laramie but at an elevation of 8,074 feet, receives about 16 inches per year. The High Plains area receives about 10 to 15 inches per year, with 9 to 12 inches of that falling during the growing season.
Gardeners along the eastern border below 4,500 feet can expect a growing season of 130 to 150 days. The area from the eastern border to the foothills at elevations of 4,500 to 6,000 feet usually has growing seasons of 100 to 130 days, while elevations of 6,000 to 7,000 feet can experience seasons ranging from 80 to 100 days. Shorter seasons of about 80 days or fewer prevail above 7,000 feet. In some areas frost can occur every night. Both hail and wind can cause problems.
The lowlands of eastern Wyoming have some very fertile moist and dry soils but low precipitation and low temperatures limit their usefulness. You can modify these with careful irrigation and season extenders and you will have a great garden.
Putting It All Together
Gardening becomes increasingly more challenging the higher the elevation, and the cool summer nights, short growing seasons, and poor soils make gardening at elevations above 7,000 to 8,000 feet very difficult. Site characteristics vary so widely that you must understand your specific garden conditions, based upon the soils, precipitation, and climatic conditions within a few hundred yards of your garden. Pay close attention to your location and choose your varieties wisely. And remember, following some good neighborly advice will go a long way toward making you a successful gardener.