Cosmos bipinnatus is a tall, deciduous, bushy annual to 1.5m.
Flowers and foliage
Very finely divided, mid-green leaves and white or pink daisy-like flowers up to 8cm across. Flowers from summer to autumn, until frost occurs. Many cultivars and selections are available including double forms with tubular petals and picotee colours.
Best grown in full sun with moderately fertile, moist, well-drained soil. Can tolerate dry soils but avoid rich, fertile soils. Shelter plantings from strong winds, as tall plants are damaged easily.
Preparation for planting
Cosmos can be bought as seedlings or propagated from seed. Generally, annuals are sown 4 to 6 weeks before you intend to plant out. Plant seedlings after the danger of frosts has passed.
If propagating from seed, cosmos can be sown in trays or directly into the garden. If using trays, use a seed sowing mix, as normal potting mix has higher nutrient levels that can harm young seedlings. Before you sow the seeds, water so that the mix is damp. Plant seeds about a fingertip's depth in the seed mix and cover the tray with a small sheet of plywood, cardboard or other opaque material to keep moist in and light out. Keep the seed mix moist but not too wet, and always water gently. As they grow, thin seedlings if they get crowded. As soon as young seedlings are large enough to be handled, they can be transplanted into punnets. Grow the seedlings in a warm, sheltered place until ready to be planted in the garden.
The general rule when sowing seeds directly into the garden is to sow at a depth of about twice the seed’s size. Sow seed in spring after the soil has dried a little. Work the soil to a fine texture and rake the surface evenly. Plant cosmos seeds in groups of two around 30cm apart. Once seeds are sown, keep the soil moist but not saturated. Thin seedlings as they grow so that they are a hands-width apart.
Prior to planting seedlings in the garden, rake general-purpose fertiliser into the soil at about 100 grams (about 2 handfuls) per square metre. Keep seedlings outdoors for a few days to become hardened before planting. Plant at the same depth the plants were growing in their container, handling carefully to minimise root disturbance. If bushy growth habit is desired, pinch out the growing tips from the seedlings when planting. Water immediately after planting. Shelter plants from strong winds and plant taller plants close together for support.
Deadhead flowers to prolong the flowering season. Leave a few seed heads to self-seed if required. It is advisable to annually apply lime to the soil and organic matter, such as compost, every second year.
Ecological and biodiversity benefits
Attract beneficial insects and pollinators. Birds may eat seeds.
Pests and diseases
Aphids and slugs may be troublesome.
Location at Auckland Botanic Gardens