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AGM plants have been through a rigorous trial and assessment programme. They are:
This plant will provide nectar and pollen for bees and the many other types of pollinating insects.
It is included in an evolving list of plants carefully researched and chosen by RHS experts. Divided into 3 groups these lists, linked below, are maintained by a team of RHS staff and are reviewed annually.
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Other common names cosmea Mexican aster
purple Mexican aster Spanish needles
Synonyms Bidens formosa
Family Asteraceae
Genus Cosmos can be annuals or perennials with simple or pinnately divided leaves and large, long-stalked daisy-like flowers in summer
Details C. bipinnatus is a tall, bushy annual to 2.5m, with very finely divided, mid-green leaves, and large flowers to 8cm across, usually white or pink the wide outer rays surround a central disc of tightly clustered, usually yellow, inner disc florets. Flowering late summer and into autumn, until frost occurs. Many named cultivars and selections are available, including doubles, forms with tubular rays, and picotee colours
Plant range Mexico, S US
Foliage Deciduous
Habit Columnar/Upright
Hardiness
All ratings refer to the UK growing conditions unless otherwise stated. Minimum temperature ranges (in degrees C) are shown in brackets
H3
Full Sun
Aspect West-facing or South-facing
Exposure Exposed or Sheltered
Chalk
Loam
Sand
MoistureMoist but well-drained
SoilChalk, Loam, Sand
pHAcid, Alkaline, Neutral
Ultimate height 1.5-2.5 metres
Ultimate spread 0.1-0.5 metres
Time to ultimate height 1-2 years
Cultivation Grow in moderately fertile, moist, but well-drained soil in full sun. Deadhead flowers to prologue flowering season. Leave a few seedheads to ripen to selfseed if required
Propagation Propagate by seed at 16°C in mid-spring under glass, or in situ in late spring
Suggested planting locations and garden types Cottage & Informal Garden Wildflower meadow City & Courtyard Gardens Patio & Container Plants Prairie Planting Wildlife Gardens
Pruning none required
Pests Aphids and slugs may be troublesome
Diseases Susceptible to grey moulds
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The Royal Horticultural Society is the UK’s leading gardening charity. We aim to enrich everyone’s life through plants, and make the UK a greener and more beautiful place.