Cosmos Growing Guide

Cosmos

Crop Rotation Group

Miscellaneous 

Soil

Any sunny site with good drainage.

Position

Full sun to partial afternoon shade.

Frost tolerant

Seedlings tolerate light frosts, but plants are killed by freezing temperatures.

Feeding

Mix a light application of a balanced organic fertiliser into the soil prior to planting.

Companions

Dill, Agastache and Echinacea. Tall varieties can form a colourful hedge that attracts bees and other pollinators, and mix well with tall herbs like dill and fennel.

Spacing

Single Plants: 20cm (7") each way (minimum)
Rows: 20cm (7") with 20cm (7") row gap (minimum)

Sow and Plant

Sow cosmos in early spring so the plants can become established before hot weather comes. Cosmos also can be started indoors in containers and set out when the seedlings have at least five leaves, or purchased as bedding plants. Plant seeds in prepared soil about 2 inches (5 cm) apart and one-half inch (1 cm) deep. Thin to 8 inches (20 cm) apart in all directions for dwarf varieties; allow 12 inches (30 cm) between very tall varieties.
Our Garden Planner can produce a personalized calendar of when to sow, plant and harvest for your area.

Notes

Varieties vary greatly in height, colour and flower form. Choose dwarf varieties for tight spaces. Sulphur cosmos (C. sulphureus) thrives in humid heat.

Harvesting

Cosmos make good cut flowers. Pull up the plants and compost them when blooms become few or freezing weather is expected.

Troubleshooting

In some climates, cosmos planted in spring may not come into full bloom until days become short in early autumn.

Planting and Harvesting Calendar

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Pests which Affect Cosmos