Colorado blue spruce, also called blue spruce (Picea pungens) grows at a slow to medium rate of less than 12 inches per year and up to 24 inches per year when young. It can take 35 to 50 years for a Colorado blue spruce to grow 30 to 50 feet. Its mature size of 50 feet tall and 20 feet wide in most gardens is smaller than its size in the wild, where it can reach 135 feet tall and spread 30 feet wide. It grows in U.S. Department of Agriculture plant hardiness zones 2 through 7.
While many fast-growing trees are short-lived, the slow-growing Colorado blue spruce can live at least 150 years and as many as 600 years.
To grow best, soil at the site should be well-draining, and the soil pH should be mildly acidic, with a pH of 5.0 to 7.0. Growth of Colorado blue spruce slows in heavy, clay soils, and in soils with a pH of 8.0 or higher, notes Montana State University Extension.
Water is important for Colorado blue spruce, particularly during the first year when the tree's root system is becoming established. Regular water can make the difference between 3 inches of growth in a year or 12 inches. Use drip irrigation if you're automating the process.
Applying fertilizer to Colorado blue spruce in spring may increase its growth rate, but it's generally not needed and it's best to leave the tree to get its nutrients from the soil. If you decide to fertilize, choose a balanced fertilizer that has near-equal NPK numbers of 9 or less. All fertilizers have a three-number NPK number on the bag, which stands for nitrogen, phosphorous and potassium. Do not use fertilizer where the first number is higher than 10. If the tree trunk has a diameter of less than 6 inches, apply 1/6 pound of actual nitrogen. For larger trees, the maximum suggested amount is 1/3 pound actual nitrogen.
To calculate actual nitrogen, divide the percentage of nitrogen on the package into the number of pounds to be applied. For example, if the numbers on the fertilizer bag are 6-2-0, 0.06 percent of the bag is actual nitrogen. Divide 0.15, which is 1/6 pound, by 0.06. The result is 2.5 pounds of fertilizer. Distribute it under the tree's canopy and water well after.
'Fat Albert' Colorado blue spruce (Picea pungens 'Fat Albert,' USDA zones 3 through 7) grows about 12 inches per year, reaching a height of 10 feet and width of 7 feet in 10 years. Its mature size is 10 to 15 feet tall and 7 to 10 feet wide.
'Corbet' Colorado blue spruce (Picea pungens 'Corbet,' USDA zones 3 through 7) is a dwarf tree growing 3 feet tall and 2 feet the first 10 years, maturing to 8 feet tall and 6 feet wide.
Another dwarf, 'Colonial Gold' Colorado blue spruce (Picea pungens 'Colonial Gold,' USDA zones 3 through 7), grows 4 feet tall and 3 feet wide within the first 10 years, eventually becoming 15 feet tall and 10 feet wide. It has blue-gray needles tinged with yellow.