Light
The rocky mountain juniper prefers full sun but will take to partial shade. Like all junipers, it is pretty adaptable.
Soil
The ideal soil for this tree is a loose, inorganic mix of a neutral pH. Adding vermiculite, perlite or a pumice mixture is perfect if you are looking to amend your soil to give it texture to help your juniper.
Water
The tree does not require much watering and is very drought tolerant. This plant is perfect for xeriscaping or other waterwise gardens.
Temperature and Humidity
This plant prefers warm dry temperatures, but it is cold hardy to -35 degrees Fahrenheit.
Fertilizer
The rocky mountain juniper does not require feeding.
Types of Rocky Mountain Juniper
In the United States, the rocky mountain juniper is a favorite for the bonsai enthusiast. To create a bonsai using a rocky mountain juniper, there are many dwarf or miniature cultivars available that are perfectly suited for bonsai creations. Juniperus scopulorum ‘Snow Flurries’ is an especially nice, variegated cultivar. You need to transplant it into a pot just large enough for its roots with a premixed bonsai soil that will consist of inorganics such as pumice, vermiculite, and crushed lava. Occasionally, pine bark is also included. Once established for a few years, you can re-pot to a larger pot by removing the soil from the roots and trimming and shaping the tree into your own unique work of art. If you are looking for a tall slender cultivar for the garden, you might want to choose Juniperus scopulorum ‘Blue Arrow’, this cultivar is often used as windscreens and hedges.
Pruning
Rocky mountain juniper should be pruned around a central leader, a hardy stem that grows upright. Prune it in early spring, after the last chance of frost has passed but before new buds set on the stems. Cut any dead, damaged, or diseased stems down to the trunk. Cut out tips of branches if it might open up the tree and allow light to touch more branches in the center. In total, try to cut back only 20 percent of the tree each season.
Propagating Rocky Mountain Juniper
Juniper can be propagated via cuttings, seed, or grafting. Grafting is rather difficult and requires an experienced gardener to make it work well. Therefore, creating a cutting for propagation, or using seeds, will be the most common routes for the typical home gardener. To create the proper cutting, choose a branch that is healthy and green. Do this during the winter or early spring, as you want to use only established growth. Separate the cutting from the main branch with sharp shears, so that you have a “heel” or “node” on one side and lively growth on the other. Peel the bottom half of the cutting, up to about 1 inch, then dip it in rooting hormone. Place this in damp peat moss and tamp the medium down thoroughly. Use pots that are at least 5 inches deep to allow for better root growth. It might take up to six months for the cuttings to develop their root systems.
How to Grow Rocky Mountain Juniper From Seed
Collect seeds in the fall, clean them well, and place them in a paper bag. Keep this bag in a cool place, such as a garage, through the winter. In early spring, sow the seeds in 5-inch pots, with about 1 inch between each seed. Cover them lightly with potting soil and water thoroughly. Though a greenhouse is best for growing these tiny saplings, a transparent lid can help raise the humidity enough if the pots are in sunlight. Look for temperatures above 75 degrees Fahrenheit and humidity at above 75 percent. Expect the first shoots to appear within two weeks.
Potting and Repotting Rocky Mountain Juniper
Rocky mountain juniper can’t be grown in pots unless it is a variety specifically meant for bonsai enthusiasts. Allow other varieties to grow in a pot only until ready to plant outdoors. To plant your tree, pick your location knowing the size, and the characteristics of your tree. Consider the utilities and any infrastructure under the plant and think about future improvements. Once a site is chosen, dig a hole twice as wide as your tree’s root ball, or container, and just as deep. Remove the tree from the container or burlap and set it in the hole. Maintain it in an upright position as you fill the hole and compress the soil. Lightly mulch to a depth of 3 inches to the dripline of the tree making sure that no mulch touches the trunk of the tree itself. If your tree is in a windy spot, consider staking it to ensure that it remains upright.
Overwintering
These trees thrive in even very low temperatures. Young, immature trees might benefit from mulch over the roots if the temperatures dip below 10 degrees Fahrenheit. Adult trees are cold hardy and don’t need protection through the winter.
Common Pests and Plant Diseases
Spider mites and bagworms are common pests that make their home in the rocky mountain juniper. Botanical oils and insecticidal soaps can help with the problem. Bagworms can be removed by hand and killed by dropping into a bucket of warm, soapy water. Rocky mountain juniper is vulnerable to Cercospera blight; copper-based fungicides can help with this issue. The tree might also develop Botryospaeria stevensii canker, especially when growing in humid areas. Cutting out the affected area is usually the best and easiest solution. Keep in mind that canker usually takes hold only if the tree is already stressed, so pay closer attention to proper watering, pruning, and soil amendments.
