Cosmos are among my favorite summer cut flowers. The plants’ light, wispy, foliage, reminiscent of dill, is topped by colorful, daisy-like flowers that sway in the breeze. Popular cottage garden picks, I tend to plant cosmos in my raised beds because they attract bees and butterflies. Growing these semi-hardy annuals from seed is super easy. In this article, I’m going to share some tips on seeding cosmos indoors so you’ll have seedlings for planting season, as well as how to direct-sow seeds right in the garden.
I find cosmos to be one of those plants that don’t look all that great at the garden center. You don’t usually find them in bloom, so unless you recognize that feathery foliage, you may walk right on by. It’s easy to start plants from seed and you’re in control of which varieties you choose.
Types of cosmos
Cosmos flowers are native to Mexico, with the range extending into some of the States and into South America. There are about 20 known species to choose from, with a range of varieties. “Cosmos” is the common name and the genus, which makes it easy when you’re looking at seed packets and plant tags.
Cosmos bipinnatus is probably the most common species you’ll find growing in the annuals section at garden centers. ‘Picotee’ is a popular C. bipinnatus variety. My favorite seed blend is ‘Dancing Petticoats’ from Renee’s Garden, which includes ‘Sea Shells’, ‘Psyche’, and ‘Versailles’. There is also a yellow and orange species called Cosmos sulphureus, and Chocolate cosmos (Cosmos atrosanguineus), which is a tuberous perennial.
There are also different petal types to choose from. There are tubular, frilly, and flat petals with various shapes.
Seeding cosmos indoors
Order your cosmos seeds when you place your veggie garden seed orders. Cosmos plants aren’t particularly fussy, so if you start them indoors, seedlings can easily be transplanted to the garden. Don’t plant seeds too early, you’ll develop very long, leggy plants. Instead, wait four to five weeks before your last frost date. For me that’s about early April.
In seed trays filled with soilless mix, plant seeds about a quarter of an inch (about a half a centimeter) deep.
Or, you can wait to direct-sow cosmos seeds in the garden, which I explain below.
Planting cosmos seedlings outside
Even though they are hardy annuals, cosmos still need to be hardened off before planting them in the garden. Wait until all danger of frost has passed, then choose a well-draining spot in the garden that gets full sun (a little partial shade is okay, too). It’s worth noting that you don’t have to heavily amend your soil with compost like you do with other flowers and veggies. This may help encourage more blooms. And you don’t really need fertilizers either. Too much nitrogen in the soil will just result in more leaves.
Also, be mindful of the heights that cosmos plants reach. Cosmos bipinnatus can grow to be about three feet (roughly one meter). That means you don’t want them shading out other plants in your garden. And because of the towering heights of cosmos, compared to other plants, they don’t do that well in pots either.
Seeding cosmos in the garden
For direct-sowing cosmos seeds, follow the advice above for choosing the right location in the garden. Your seed packet is also a wealth of information, explaining the right conditions, depth, mature size, etc. Wait until after your last frost-free date to plant seeds.
Sow seeds a quarter of an inch (about a half a centimeter) deep. You can stagger your planting to play with plant heights and bloom times. Water well until plants are established.
Caring for cosmos plants
Cosmos are pretty low-maintenance plants. Once they get going, they’re quite drought tolerant. If you have a variety that grows too tall, you may find they flop, so staking may be something to consider. Deadhead spent blooms throughout the growing season to encourage more growth. This will also keep plants a bit shorter, encouraging new “branches” to grow outwards. You may even want to prune back some of the stems (up to one third) to keep it more contained.
Even though you need to wait until the soil warms up to grow cosmos from seed, once established the plants may bloom right through the fall. I’ve found some growing among the last tenacious blooms of the growing season. Also, if you allow seed heads to form, cosmos will self-sow in the garden. Keep an eye out for them in the spring!
More annuals to grow from seed
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