When to Plant Cosmos in California

Hardiness Zones

• California plant hardiness zones range from USDA zones 5 to 11. The natural range of cosmos is zones 9 and 10 but they will grow in zones 5 to 8 as well. Chris A. Martin, professor of applied biological sciences at Arizona State University with specialty in landscape horticulture, states that cosmos will grow in all climate zones, making it a choice for gardeners anywhere in California.

Last Frost

• Last frost dates for California range from late January to mid-April, depending on the area. Cosmos should be planted outdoors after all danger of frost is gone for the spring, so check the average frost dates for your area when planning a planting schedule. A local nursery, cooperative extension service or Master Gardener unit will be able to provide the expected frost times for the area; or look up a frost chart online.

Inside

• If last frost comes in late spring in your area, give your flowers a head start by sowing the seeds indoors four to five weeks before last expected frost. Transplant outdoors when soil temperatures reach 65 degrees Fahrenheit or more to ensure that frost danger has passed and give your cosmos the best chance to thrive.

Outside

• Cosmos grow easily from seed, tolerating even poor soil, and last until the ground freezes in fall. As when transplanting seedlings, wait to plant the seeds until the ground temperature warms to at least 65 degrees to avoid frost danger. University of Florida Cooperative Extension Service suggests planting seeds in zone 7 in June, in zone 8 in May, in zone 9 in March and in zones 10 to 11 in February. In zones 9 to 11, gardeners can sow a second time in the fall for year-round blooms. Marin County Master Gardeners advises that a mid-summer planting in July is another option for California gardeners.