How to grow Cosmos in New Zealand
Cosmos is one of the easiest and most rewarding summer annuals, with feathery foliage and daisy-like flowers from late spring to autumn. It draws bees, hoverflies and lacewings, makes a lovely cut flower, and does best in ordinary or even poor soil, as too much feeding gives leaves rather than flowers. It is frost tender, so sow it once the soil has warmed.
📖 Our full guide: How to Grow Cosmos in New Zealand
At a glance
When to sow by region
| Region | When to sow | Regional note |
|---|---|---|
| Northland | September to December | |
| Auckland | September to December | |
| Waikato & Bay of Plenty | October to December | Sow from October, once the inland frosts are over in late September. Warm coastal Bay of Plenty gardens can start a few weeks earlier under cover. |
| Central North Island | November and December | |
| Wellington & Lower North Island | October to December | |
| Nelson & Marlborough | October to December | |
| Canterbury | November and December | |
| Otago & Southland | November and December | |
| Central Otago | December |
Where to plant
Grow cosmos in full sun or part shade in well-drained, ordinary soil. Do not enrich the soil or feed heavily, as rich conditions give tall, leafy plants with fewer flowers. Tall varieties may need a stake or a sheltered spot.
- Northland and Auckland: sow from September to December.
- Waikato and Bay of Plenty: sow from October, once inland frosts are over; warm coastal Bay of Plenty gardens can start a few weeks earlier under cover.
- Wellington and Nelson and Marlborough: sow from October to December.
- Central North Island, Canterbury, and Otago and Southland: sow in November or December, after the frosts.
- Central Otago: sow in December, or raise seedlings under cover and plant out after the last frost.
Choosing a variety
- Sensation Mix: the classic tall cosmos, about 1.2 m, with large single flowers in white, pink and crimson. Self-seeds freely.
- Double Click: double and semi-double flowers like small dahlias on tall stems, excellent for cutting.
- Sea Shells: unusual tubular, fluted petals in mixed colours, to 70 to 90 cm.
- Cosmos sulphureus types such as Brightness Mix and Cosmic Orange: compact plants, 30 to 60 cm, with gold, orange and red flowers. Heat loving and good in pots.
How to plant
- Sow seed about 0.5 cm deep directly where it is to flower, or in trays once the weather has warmed.
- Keep moist; seedlings appear in 5 to 10 days.
- Thin or plant out about 30 cm apart.
- Pinch out the growing tips of young plants for bushier growth and more flowers.
Watering and feeding
Water seedlings until established, then only in long dry spells. Cosmos needs no feeding in ordinary soil.
Pruning and care through the year
- Deadhead or pick regularly to keep the flowers coming over a long season.
- Stake tall varieties in windy gardens.
- Leave some flowers to set seed if you want plants to self-sow next season, and pull out seedlings that come up where you do not want them.
Harvesting
Cut flowers for the vase when the petals are just opening. Regular cutting keeps the plants flowering.
Growing in pots
Compact varieties such as Brightness Mix grow well in pots at least 25 cm wide, in full sun, with quality potting mix. Water when dry and deadhead often.
Companion planting
Cosmos is grown among vegetables to bring in pollinators and predators of aphids.
Good companions
- Tomato Draws bees and hoverflies to the tomato bed.
- Beans Brings pollinators and aphid predators to the bean row.
- Capsicum Attracts beneficial insects to the summer bed.
- Cucumber Cucumbers need bees to set fruit, and cosmos draws them in.
- Marigold Another easy summer flower that supports beneficial insects.
Pests and diseases
Cosmos is easy and rarely seriously troubled.
Pests to watch for
- Aphids Clusters of small insects on stems and buds.
- Thrips Silvery streaks on leaves and petals, and distorted flowers.
- Slugs Seedlings chewed off.
Diseases to watch for
- Powdery mildew White powdery coating on leaves, mostly late in the season.
- Aster yellows Yellowed, stunted growth and green, distorted flowers. Remove affected plants.
Common questions
When do I sow cosmos in New Zealand?
After the frosts, once the soil is warm: from September in the warm north, October in the Waikato, Bay of Plenty, Wellington and Nelson, and November or December further south.
Why is my cosmos all leaves and no flowers?
Usually rich soil or too much fertiliser. Cosmos flowers best in ordinary or poor soil with no feeding.
Does cosmos come back every year?
It is an annual, but it self-seeds freely, so it often returns if you leave some flowers to set seed.
How do I keep cosmos flowering?
Deadhead or cut flowers regularly, and pinch out the tips of young plants for more branches.
Can I grow cosmos in a pot?
Yes, choose a compact variety such as Brightness Mix and a pot at least 25 cm wide in full sun.
Sources we checked: Kings Seeds: Cosmos Sea Shells; Kings Seeds: Cosmos Brightness Mix; Kings Seeds: Cosmos Double Click Cranberries; Wikipedia: Cosmos bipinnatus.