Germination temperature
Cool. After treatment, seed germinates at about 20 °C (68 °F) or with alternating 3-15 °C (37-59 °F).
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Mountain juniper is a tough, low-growing form of common juniper, but its seed is among the slowest of all conifers to germinate. This guide explains the warm-then-cold treatment it needs and why seedlings may keep appearing for two or three years.
Cool. After treatment, seed germinates at about 20 °C (68 °F) or with alternating 3-15 °C (37-59 °F).
About 6 mm (1/4 in).
Slow and uneven: usually the second spring after sowing, with stragglers in the third year.
Seed is covered; seedlings like light shade in their first season.
Essential: 2-3 months warm, moist stratification, then at least 3 months cold.
Sow in pots or a nursery bed outdoors; transplant after 1-2 years.
Juniper seed has deep double dormancy: a hard, thick coat and a dormant embryo, and the pulp of the berry-like cone contains germination inhibitors. Viability also varies widely; one review cited by the University of Alberta reports 0-80%. Soak dried seed for a few hours and rub off any remaining pulp before treatment.
Not usually done at home. Acid treatments are reported for other junipers but not for common juniper; the long warm phase below softens the coat instead.
Two stages. First keep the seed in moist sand or peat at about 15-25 °C (59-77 °F) for 60-90 days (USDA Woody Plant Seed Manual) or 14 weeks at 15 °C (59 °F) (Polish study). Then chill at 3-5 °C (37-41 °F) for at least 90 days. In the Polish study, drying the seed for 72 hours after 4-8 weeks of the warm phase raised laboratory germination to as much as 76%.
Only to clean the seed. Soak for a few hours to soften pulp; soaking alone does not break dormancy.
Germinate treated seed cool: about 20 °C (68 °F), the ISTA test temperature, or alternating 3-15 °C (37-59 °F) as in the Polish trial. Sow stratified seed in spring before air temperatures exceed 21 °C (70 °F). In India this suits hill nurseries; in the hot plains the seed rarely succeeds.
Cover with about 6 mm (1/4 in) of firmed soil or sand, as recommended in the USDA manual.
A gritty, free-draining mix such as seed compost with sand or fine grit. Use deep pots, because the trays may need to stay in place for two years.
Keep evenly moist through every season. A pot that dries out in summer can push seed back into dormancy.
Light is not needed for germination. Mulch outdoor beds over winter and remove the cover as soon as seedlings appear.
Once dormancy is broken, the USDA manual lists germination over 7-75 days. In practice, fall-sown seed usually comes up in the second year, and in the Polish nursery trial some seed was still emerging in the third year.
The simplest method is to sow fresh seed outdoors in late summer or autumn and let the seasons provide warm and cold treatment. Fresh, undried seed sown within a week of extraction can be sown untreated. Protect pots from mice and birds with mesh and do not discard them after the first year.
Leave seedlings in the seed pot or bed for their first season. The USDA manual transplants junipers after one or two years, lifting them early in spring and keeping the roots moist.
Give light shade and steady moisture for the first summer. Young plants grow slowly and form a prickly, spreading mound.
Cause: normal deep dormancy. Fix: keep the pot moist, outdoors and protected for at least one more winter; many seeds germinate in the second or third spring.
Cause: empty or non-viable seed, which is common in junipers, or skipping the warm stage. Fix: sow plenty of seed and give both the warm and the cold phase.
Cause: frost heaving. Fix: mulch beds and press seedlings back in after a thaw.
Cause: cedar blight (Phomopsis juniperovora), which the USDA manual calls the most serious juniper nursery disease. Fix: remove affected seedlings, avoid overhead watering in the evening and keep good spacing.
Cause: warm weather straight after chilling can push seed back into dormancy. Fix: as the USDA manual advises, sow stratified seed in spring before air temperatures exceed 21 °C (70 °F), and keep pots out of hot sun.
Mark the sowing date clearly and keep pots for three seasons. Juniper rewards patience more than any special trick.
Cuttings of common juniper root easily but grow slowly, a useful backup when you need a particular plant.
Mountain juniper grows on exposed rocky slopes. Plant it in full sun in well-drained, even stony soil and avoid rich, wet beds.
It is very cold-hardy but dislikes long hot, humid summers. In India it suits gardens in the hills rather than the plains.
Sow mountain juniper in autumn and be patient: hardy, evergreen seedlings follow over the next two springs.
This is normal. Common juniper seed has deep double dormancy and usually germinates in the second spring after sowing, sometimes the third. Keep the pot moist and outdoors.
It is not recommended. Cold treatment alone leaves most seed dormant; the warm phase of 2-3 months comes first, followed by at least 3 months of cold.
It struggles in hot plains. It is a mountain and northern plant that prefers cool summers, full sun and well-drained soil.
The blue cones of common juniper are used as a spice in small amounts. Seed for sowing should be treated as seed, not food.