Blue rug juniper (Juniperus horizontalis 'Wiltonii') is a drought-tolerant groundcover that craves good soil drainage. This juniper is a practical solution for sunny slopes where water runs off so quickly that plants are apt to go thirsty.
Blue rug juniper is a hardy evergreen plant with silvery-blue foliage that can take on a purplish tone in the winter months. Unlike the straight species, Juniperus horizontalis, 'Wiltonii', being a cultivar, often does not produce dark blue berries, which take two years to mature.
With a moderate growth rate, it reaches less than a foot tall but can spread several feet wide, forming a dense mat. The best time to plant blue rug juniper is in the cooler temperatures of fall or early spring.
|Common Name||Blue rug juniper, creeping juniper, carpet juniper|
|Botanical Name||Juniperus horizontalis 'Wiltonii'|
|Plant Type||Shrub, groundcover|
|Mature Size||3-6 in. tall, 6-8 ft. wide|
|Soil pH||Acidic, neutral, alkaline|
|Hardiness Zones||3-9 (USDA)|
|Native Area||North America|
Watch Now: How to Grow and Care for Blue Rug Juniper
Blue Rug Juniper Care
Blue rug juniper is certainly a low-maintenance plant. If you are growing it as a groundcover for a large site, space plants four to six feet apart. The individual plants will form a mat that's thick enough to crowd out weeds and provide four seasons of visual interest.
Once the plants are mature, they will require occasional watering and shaping. They typically don't have issues with pests or diseases and are resistant to deer and air pollution. Just make sure the plants have adequate airflow and drainage, as most of their issues are caused by overly damp environments.
Blue rug junipers prefer to grow in full sun, meaning at least six hours of direct sunlight on most days. These plants can tolerate some shade and actually prefer protection from hot afternoon sun to prevent foliage burns. However, if located in too much shade, blue rug juniper will grow thin rather than lush.
Blue rug juniper is tolerant of a range of soil types, though it prefers to grow in dry, sandy soil with a pH range of 5.0 to 8.0. Excellent soil drainage is key to healthy growth. Furthermore, this plant is tolerant of rocky soil, as well as salt from saltwater spray or road salt.
Until a blue rug juniper plant matures, it's necessary to keep the soil evenly moist (but not soggy). After becoming established, it prefers a moderate amount of soil moisture and can tolerate some drought. However, soil that's too wet can kill the plant. So provide supplemental water to a mature plant only in extended dry spells or drought.
Temperature and Humidity
These plants can grow in a variety of temperatures and humidity levels. They can tolerate wide fluctuations of day-to-night temperatures as well as snow and frost. They're best planted in the fall or early spring when the temperature is regularly below 60 degrees Fahrenheit. Planting in cooler weather enables the root systems to become established before hot and dry weather arrives. Moreover, particularly humid or wet weather can cause some stem dieback or root rot.
Blue rug juniper grows well in average soil and is tolerant of somewhat poor soil. Fertilizer typically isn't necessary, but to give mature plants a boost, you can use an organic fertilizer in the fall if you wish. For the amount to use, follow the product label instructions.
Types of Creeping Juniper
In addition to blue rug juniper, here are several other varieties of Juniperus horizontalis:
- 'Bar Harbor': This variety has blue-green foliage turning reddish purple in winter. It has a slow to medium growth rate.
- 'Blue Forest': A compact variety that spreads one to five feet in ten years. Its foliage changes color from lavender in winter to blue in spring.
- 'Emerald Spreader': This variety has bright emerald green foliage throughout the year. Although low-growing, it spreads to six feet wide or more.
- 'Green Acres': As its name suggests, this variety grows lush dark green foliage.
- 'Monber': Also known as the 'Ice Blue' juniper. This variety features dense silvery-blue foliage that sometimes turns purple in the winter.
Blue rug juniper doesn't require pruning unless growth has become unruly or portions of the plant are damaged or diseased. You can prevent the juniper from spreading outside of its boundaries by keeping it trimmed. Light pruning can also encourage robust growth and branching to create more fullness. In the spring, clean out any garden debris or trash caught in this dense groundcover.
Trim the juniper in the late winter or early spring when weather is dry. Remove any dead or damaged portions, and cut back long branches to shape them but by no more than a quarter of their length. Don't prune all the way to bare stems growing in the center of the plant because this wood might be too old to put out new growth.
Propagating Blue Rug Juniper
You can propagate blue rug juniper from stem cuttings.
- Any time between late July through October, or late summer to early fall, use a sharp knife to take a 4- to 6-inch cutting from a healthy stem.
- Remove the needles from the bottom third of the cutting, dip the end in powdered rooting hormone, and bury the cut end in a pot filled with moistened potting mix.
- Place the cutting in bright, indirect light in a warm room, and keep the soil moist at all times. Root growth can take several months, so don't transplant your cutting outdoors until next spring.
Propagation from seed is not recommended, as this is a cultivar and its seeds produce a plant that is different from the parent plant.
Potting and Repotting
Blue rug juniper is a groundcover with considerable spread so it is not suitable to be grown in pots.
The plant is native to Alaska, Canada, and the northern United States and, as such, is well-adapted to cold winters up to USDA zone 3. It does not need winter protection.
Common Pests and Plant Diseases
Blue rug juniper is not bothered by serious insect or disease problems. However, wet soil can lead to root rot, and in wet springs, the plant can get tip and twig blight, which causes younger needles to turn brown and drop off. It can also be a host plant to cedar-apple and other rust diseases. Sometimes it attracts aphids, bagworms, webworms, scale, and spider mites.
As part of the natural aging process, the centers of mature plants can die back in the center and become unsightly.
Is creeping juniper invasive?
The cultivar Juniperus horizontalis 'Wiltonii' is a natural mutation that was found in 1914 in Maine by Jacob van Heiningen from South Wilton, Connecticut, hence the name. As a native plant, it is not considered invasive.
How quickly does creeping juniper spread?
It grows about 7 to 10 inches per year.
Is blue rug juniper soft?
It plume-like texture is finer and softer than that of other junipers but it is still recommended that you wear gloves when handling it, as it can cause skin irritation.
Juniperus horizontalis 'Wiltonii'. Missouri Botanical Garden.
Basic Principles of Pruning Woody Plants. University of Georgia Extension
Juniperus Horizontalis 'Wiltonii'. Missouri Botanical Garden
Juniperus horizontalis 'Wiltonii'. Missouri Botanical Garden.
Juniperus horizontalis 'Wiltonii'. American Conifer Society.