Japanese garden juniper

Identification and origin

Juniperus procumbens belongs to the cypress family, Cupressaceae. It is native to Japan, Korea and the Nansei-shoto (Ryukyu) islands, and was long treated as part of Chinese juniper under the name Juniperus chinensis var. procumbens. Its epithet means “prostrate”: its long, stiff branches spread parallel to the ground, just above it.

It is an evergreen shrub 6–18 in tall whose trailing branches gradually form a dense mat 10–15 ft wide. Growth is slow: it takes about ten years to cover roughly 10 ft. Its short needles (up to about 0 in), stiff and sharp-pointed, grow in whorls of three; they are blue-green to gray-green and may take on a purplish tinge in winter. The fleshy seed cones, small blackish berries when ripe, are rare on cultivated plants.

The cultivar ‘Nana’, introduced in 1922, is a dwarf, less vigorous form: a compact cushion 6–12 in tall and 4–6 ft wide, with intertwining branches, turning purplish green in winter.

A groundcover for hot, dry spots, it covers slopes (which it helps stabilize), rock gardens, borders and foundation beds, or cascades over a wall. It also grows in containers and is sometimes grafted onto a standard.

Light

Full sun, at least six hours of direct sun a day, in an airy spot. In shade, growth thins, the interior goes bare and shoot diseases gain ground.

Watering

After planting, keep the soil evenly moist until the plant is established: a young groundcover should never wilt. Check the soil around the root ball rather than following a schedule, and water when it starts to dry at depth, not just on the surface.

Once established, it handles heat and drought and only needs water during extended dry spells. Too much water is far more dangerous than too little: soggy soil leads to root rot. Water at the base, ideally early in the morning; avoid overhead sprinklers and evening watering, which encourage tip blights. In containers or as bonsai the mix dries much faster: see the bonsai guide.

Potting mix

Good drainage comes first. It adapts to clay, loam, sandy or shallow rocky soils, acidic, neutral or alkaline, and tolerates poor soil, urban pollution and salt. It prefers a rather dry, sandy soil; only waterlogged soil is fatal.

Before planting a groundcover, remove perennial weeds and loosen the soil deeply, then mulch about 2 in deep until the mat closes; on a steep slope, coarse netting holds the soil. Once established it needs almost no fertilizer: excess fertilizer or water encourages late-season tip blight.

Temperature

Very hardy in USDA zones 4a–9b, meaning winter lows down to about -29 °F in the ground. In containers, roots are far more exposed to freezing than in the ground.

Rainy springs and mild, humid weather around 70–81 °F favor tip blights, while conifer spider mites are active in the cool seasons, spring and fall.

Repotting

In the ground it transplants easily. Fall is the best planting season, thanks to cooler weather and rain. Space plants according to their mature spread, in a staggered pattern: air moves better and repeated pruning becomes unnecessary.

In containers, use a pot with drainage holes and a very free-draining mix, and repot in early spring once roots fill the container.

Pruning

On junipers, new growth only comes from tips that still carry needles: a branch cut back to bare old wood will not regrow and leaves a permanent gap. It tolerates heavy pruning as long as every shortened branch keeps green growth at its tip.

• When: early spring, before new growth starts. Avoid hard freezes and late summer.
• How: rather than shearing, shorten branches one at a time back to a side branch. Repeated shearing creates a dense shell that shades out the interior.
• Precautions: prune in dry weather and not too much; some diseases enter through wounds. Thin out clumps that grow into each other.

An overgrown, bare plant recovers poorly: it is often better to replace it with a compact form such as ‘Nana’.

Propagation

The species is propagated by seed. To keep a cultivar such as ‘Nana’ true, it is grown from cuttings taken from mid-fall to midwinter:

1. From a healthy plant, cut young stems 4–6 in long, still green but slightly firm.
2. Strip side shoots from the lower third and dip the base in rooting hormone.
3. Insert them one-third to one-half of their length into a pot with drainage holes filled with equal parts peat and finely chopped bark.
4. Keep the mix moist but not soggy, under a slit plastic bag, in bright light without direct sun.

Roots form in late winter, but new growth often does not show until early summer; then move each rooted cutting into its own small pot.

Common problems

A single symptom can have several causes: a browning branch may point to disease, mites, scale or suffocated roots. Check the foliage and the drainage before treating.

• Phomopsis tip blight: young shoots turn dull red or brown, then ash gray, and die; tiny black specks appear afterward. It strikes mostly in spring, in wet weather. In dry weather, cut well below the dead part, disinfect the blade between cuts and destroy the debris.
• Kabatina tip blight: it attacks twigs one year old or more, enters only through wounds and shows up in late winter. Same treatment.
• Root rot: poorly drained or waterlogged soil.
• Spruce spider mites: fine yellowish-green stippling, then bronzing and needle drop, sometimes with fine webbing. Shake a branch over white paper: tiny moving specks reveal them. A strong jet of water, repeated several days in a row, knocks them off.
• Juniper scale: tiny white shields; in heavy infestations the plant looks dusted with snow and weakens. Horticultural oil in late spring, when the crawlers hatch, controls it well.
• Other pests: aphids, bagworms, twig borers and webworms.
• Rusts: cedar-apple rust and related rusts spend part of their life cycle on junipers.
• Bare center: on older plants, foliage sometimes dies back in the middle of the mat.

Toxicity

Contact with the foliage can cause dermatitis: the stiff, prickly needles irritate skin. Wear gloves and long sleeves when pruning, wiring, taking cuttings or repotting it. As with any ornamental plant, keep children and pets from chewing on it.