Germination temperature
Warm-season sowing; seed ripens in April-May, just before the monsoon in India.
Tree growing guide
Red silk cotton, semal or salmali is a tall deciduous tree that covers itself in red flowers on bare branches before the monsoon and grows fast from seed. This guide covers collecting seed from the floss, nursery sowing, when to transplant and the shoot borer that damages young trees.
Warm-season sowing; seed ripens in April-May, just before the monsoon in India.
Shallow: cover the small seed lightly with fine soil.
About 5-18 days for good seed.
Full sun for seedlings; semal is a tree of open sites.
None needed; separate the seed from the cotton floss.
Raise in nursery beds and transplant at 10-15 cm (4-6 in), or sow directly in lines.
Semal seed ripens in April-May (TNAU). The capsules open to release seeds wrapped in silky floss; dry the fruits in the sun, then pull the seeds free of the cotton before sowing. A kilogram holds roughly 21,000-38,000 seeds (TNAU gives 21-38 seeds per gram; PROSEA about 28,000 per kg). PROSEA reports that seed can be kept in closed containers for 1-2 years, but fresh seed of the same season gives the most reliable result.
Not needed. Seed tested in Gujarat germinated at 95% without treatment.
Not needed. Semal is a tropical and subtropical tree with no cold requirement.
Optional. In a Nepalese study, washing seed under running tap water for 48 hours raised germination only from 57% to 63%, so a soak is not essential.
Sow in warm weather. No exact germination temperature is published for this species; in India the natural sowing window runs from seed fall in April-May into the early monsoon. In cooler countries sow indoors in a warm spot.
Cover the seed lightly with fine soil, just enough to keep it moist and in place. TNAU sows in drills 23 cm (9 in) apart with seeds 5 cm (2 in) apart in the drill.
A free-draining nursery soil or seed mix. In containers, use deep pots or root trainers, which suit the strong taproot.
Water after sowing and keep the bed moist; weed regularly once seedlings appear.
Give full light once seedlings emerge. Semal is a fast, open-site tree and becomes weak and drawn in shade.
Sources differ. PROSEA gives about 90% germination in 6-18 days; a Gujarat study (Indian Journal of Plant Physiology, 2014) recorded 95% within 5 days. A study in Nepal found only 57% for its untreated seed and noted that April-collected seed did not germinate in May and June, so if a sowing fails, resow later in the season.
TNAU transplants nursery seedlings when they are 10-15 cm (4-6 in) tall, lining them out 25 x 20 cm (10 x 8 in) apart. Field planting at 3 x 4 m or 3.7 x 3.7 m (10-12 ft) uses pits about 30 cm (12 in) across, with a ball of earth in drier areas; in high-rainfall areas bare-root plants are used.
Foresters also plant 'stumps': 1-2-year-old plants cut back to a short shoot and a long taproot. TNAU specifies a collar diameter of 1-2 cm, about 4 cm of shoot and 25-30 cm of root; PROSEA describes a fleshy taproot of 50-60 cm. Container plants have developed slowly after planting in Indian trials, so stumps or bare-root stock are often preferred.
Growth is fast: in the Nepalese study six-month-old seedlings averaged 87 cm (34 in) tall. Keep plantings weeded for up to three years (PROSEA).
Cause: the semal shoot borer (Tonica niviferana), which has caused extensive damage in plantations (TNAU). Fix: cut out and destroy bored shoots, and train a single new leader.
Cause: red cotton bug (Dysdercus cingulatus), whose nymphs and adults feed on young growth. Fix: remove clusters by hand and clear fallen floss and seeds where they breed.
Cause: seed not separated from the floss, sown too deep, or a lot that is temporarily slow to germinate. Fix: clean the seed, sow shallow and resow if needed.
Cause: too much shade. Fix: move pots or beds into full sun.
Semal becomes a very large, buttressed tree. Plant it well away from buildings, walls and drains.
In India, direct sowing in lines 45-60 cm (18-24 in) apart after harvesting a field crop gave very good results (PROSEA).
Semal is deciduous: it drops its leaves in the dry season and flowers on bare branches, so leaf fall then is normal.
Young stems carry sharp conical prickles; plant away from paths and children's play areas.
Sow red silk cotton seed in the warm season for a fast-growing flowering tree.
Yes. Pull the seeds free of the silky floss before sowing so they make good contact with the soil.
Good seed usually germinates in about 5-18 days. One study in Nepal found that freshly collected seed did not germinate for its first two months, so resow later if a first sowing fails.
Yes. Salmalia malabarica is an older name; the accepted name is Bombax ceiba.