When to Plant Cosmos Seeds in Zone 6

Annuals

• Cosmos are annual plants, meaning you have to plant them every year -- especially if you want a particular color. However, cosmos often reseed themselves, so if you want them in the same area every year and don't mind a bit of variety, let them go to seed and see what you get. If the original seeds were hybrids, the self-sown seed may not result in the same plant. While deadheading improves cosmos re-flowering, stop deadheading in the early fall to allow the seeds to ripen.

Planting Time

• When you should plant your cosmos seed depends on whether you're planting them inside or planting them directly outdoors where you want them to grow. Planting cosmos indoors will give you a jump on the flowering season, since cosmos bloom about seven weeks after you plant the seed and continue all season long, until the fall. Cosmos cannot handle frost, so if you are planting outside, you will need to wait until after the last frost in your region. You can plant cosmos seeds indoors six weeks before the average last frost date, but tall cosmos don't transplant as well as some other seedlings, so you may want to wait until four weeks prior to the last frost date to plant these seeds.

Zones vs. Last Frost Date

• The U.S. Department of Agriculture maintains a map that details plant hardiness zones. This map is based on the average lowest temperatures in an area. USDA zone 6, for instance, is broken down into zone 6a and 6b. Zone 6a has an average winter low of 5 to 10 degrees below zero Fahrenheit, while the average winter low temperature in zone 6b is between 5 degrees below zero and 0 degrees Fahrenheit. The average last frost dates in this zone will vary from location to location. Low-lying areas have a later last frost date than level locations, while areas near lakes often have earlier last frost dates. Soil characteristics, wind exposure, rainfall, altitude and cloud cover all affect local last frost dates. Major seed companies often list May 15 as the average last frost date, but you should also consider the current year’s weather patterns when deciding when to plant seeds, as some years are harsher than others.

Direct Seeding Times

• Cosmos seeds are small and should not be deeply planted. Instead, sow the seed on the soil surface in a full sun location and then rake it lightly to mix it into the soil. The goal is to position the seed about 1/4- to 1/6-inch beneath the soil. Cosmos is best planted outdoors once the soil temperature is above 65 degrees Fahrenheit. The seeds germinate in one to three weeks and usually flower within seven weeks of germination.