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Behold...the Cosmos! How to Grow C. Pinnatus and C. Sulphureus from Seed

When we hear the word "cosmos," we can't help but think of the television series about astrophysics first hosted by the late, great astrophysicist Carl Sagan, and then passed on to Neil deGrasse Tyson. In modern culture and in Latin, "cosmos" is associated with the universe, beauty, and orderliness; according to Texas A&M University's website Aggie Horticulture, Spanish priests in Mexico named the genus and grew tons of cosmos in their mission gardens.

Cosmos are symbolic of deep love, a lifetime of companionship, and the belief that life is wondrously beautiful.

Cosmos plants, as heavenly as they may be, aren't mysterious at all. They thrive without much attention—in fact, they do best when, aside from harvesting blooms for floral arrangements, you don't mess with them—and they're extremely easy to grow from seed.

Slightly off-topic: Carl Sagan called lettuce sandwiches as his "pet peeve." We'll forgive him; not everyone's bright enough to add sliced tomato and bacon.

Seed Needs' Favorite Cosmos Species

There are dozens of cosmos species, including both annuals and perennials. The annuals we're highlighting today share the same basic environmental needs:

  • Long, deep, intermittent watering
  • 8 to 10 hours of full sun
  • Medium to compost-rich well-draining soil
  • pH 6 to 7
  • Somewhat drought tolerant but not a good prospect for xeriscapes

Additional fertilization will make them spindly, so don't coddle your cosmos. In fact, we pretty much advise our gardeners growing cosmos from seed to plant them, keep the seeds moist until they germinate and get a few inches of growth, then slowly back away.

Of course, eye contact is encouraged. Their bright yellow centers and towering blooms make that part easy; just plant them at the far end of the garden where they won't block smaller plants. Cosmos make excellent background ornamentals.

Cosmos are in the same family as daisies and asters (interchangeably called Asteraceae or Compositae) and have what we call composite flowers: Their central discs are a mashup of many tiny blooms, and each petal (or "ray") is an individual flower. Cosmos have an upright, bushy growth habit, and can take up quite a bit of room.

Native to Mexico, they've naturalized in temperate to tropical regions of North, South, and Central America where they're considered to be wildflowers.

We offer C. bipinnatus and C. sulphureus, for their popularity, diverse varieties, and vigor. You can select individual varieties or mixes; both species grow well together and their colors are complementary.

Cosmos bipinnatus

Sometimes (but not often) called "cut leaf cosmos" or "garden cosmos," C. bipinnatus's flowers may be white, pink, lavender, deep red or a rich purple. They may be bicolored or bloom in different shades and patterns.

  • Annual in zones 2 to 11
  • 1 to 4 feet tall
  • 1/2 to 2-foot spread
  • 2 to 4-inch wide heads
  • Petals are sometimes fluted; also may be double-layered
  • Blooms June through September

Cosmos sulphureus

C. sulphureus, "sulfur cosmos" or "yellow cosmos," didn't earn the name by wafting air biscuits. The specific name stems from the Latin word for yellow, and the flowers are shades from the same side of the color wheel: Yellow, orange, salmon, or red. As with both species, some cultivars may be bicolored or have varying shades and patterns. While it's unknown exactly when C. sulphureus were introduced to Europe, in 1791 the botanist Cavanilles recorded his observations on plants cultivated from Mexico-sourced seeds grown in the Real Jardín Botánico de Madrid or, for those who took French or just cut Spanish class, the Royal Botanical Garden of Madrid.
  • Annual in zones 2 to 11
  • 2 to 6 feet tall
  • 1 to 3-foot spread
  • 2 to 4-inch wide flower heads
  • Oblong petals may be multi-layered
  • Blooms June through September

Both species are sometimes called "Mexican aster."

You can most easily tell the difference between out-of-bloom C. pinnatus and C. sulphureus plants by taking a look at their medium leaf shapes. "Pinnatus" is Latin for "feathery" or "winged," and C. pinnatus cosmos indeed have delicate, fern-like leaves resembling those of dill or fennel. C. Sulphureus foliage is similar to that of the daisy, with narrow, elongated, and lobed leaves.

Growing Annual Cosmos from Seed

Cosmos, like many wildflowers, are perfect for black-thumb gardeners. With minimal soil preparation—a little compost, a little raking to smooth things out—you can simply stick a handful of seeds on the ground and get on with your day.

Even still, you'll want to maintain consistent moisture in your seeded area until the plants have germinated and begun to grow in earnest.

Cosmos seeds look like tiny brown bananas, which might make you think of space monkeys. Since the seeds are fairly easy to handle, you can be more intentional (or maybe strategic) in planting them by hand. This is the most economical approach (benefiting you, not us). But then again, scattering is perfectly acceptable when growing cosmos from seed. Since cosmos are prolific re-seeders, regenerating as "colonies" in subsequent years, go ahead and sow your seeds willy-nilly the first season. We've got college-bound kids, after all.

Pro Tip: Golfers usually carry shakers of garden sand and grass seeds—which look similar to cosmos seeds—to rejuvenate patches of turf. These "divots" are typically the result of the golfer having enjoyed too many cosmopolitans at the clubhouse the night before. Anyway...us being slightly less fancy, we like to repurpose those plastic shaker cans from cheap Parmesan cheese or, for smaller seeds, the bottle-type shakers for culinary herbs. Experiment with the seed-to-sand ratio when you're using the seed-scattering method, and don't go dumping out your spouse's herb rack. Trust us.

Some gardeners will go so far as to add a little bit of colored masonry chalk to the sand to help ensure even seed distribution.

If you do broadcast cosmos, remember that these plants have a wide spread. You'll want to go back and thin out or relocate surplus seedlings if you surface seed them too densely.

Seed Treatment: None required or even recommended, but for kicks (or practice) you can subject your cosmos seeds to cold stratification or striation if you're starting them indoors.

When to Plant Outdoors: Scatter or sow seeds in the fall (recommended) after consistent frost patterns have kicked in, or in spring about 2 to 4 weeks prior to your last frost. Frost seeding improves seed-to-soil contact as the ice crystals manipulate the soil's surface, and when you sow cosmos seeds in the fall, you'll likely have earlier growth when your garden wakes up in spring. This is a popular method for graziers when they're prepping their pastures, and why turf seed dealers recommend fall lawn seeding.

When to Plant Indoors: We highly recommend direct-sowing cosmos outdoors in zones 5b and above, since they take hold and thrive so readily once the soil's warmed up. If you live in a short-season climate and want to get a jump on things, plant them indoors about 4 to 6 weeks before your last spring frost.

Seed Depth: Sow seeds 1/16 to 1/4" deep. Don't go deeper!

Seed Spacing: Sow, transplant, or thin 12" to 18" apart.

Days to Germination: 7 to 14 days at 70°F to 75°F.

Days to Maturity (Flowering): 60 to 90 days after germination.

Transplanting Tips: As with all transplants, take care not to disturb the root systems. We recommend starting your cosmos seeds in peat pots or the more ecologically sustainable CowPots. Once your seedlings have hardened off for a few days, you can place them—container and all—into their planting hole. It's a good idea to thoroughly soak the pots, and moisten the garden soil before you do.

Maintenance: Taller cosmos plants might require staking, especially if they're planted where they're not protected from wind or strong breezes. Deadhead spent flowers to extend the bloom period.

Pests and Diseases: Watch out for the occasional aphid, snail, and slug flash mob. Otherwise, cosmos are disease and pest-resistant.

Choosing Plants to Share Space with Cosmos

Cosmos attract beneficial wasps, bees, and butterflies, so if you want to boost the pollination rate in your veggie garden, plant some at the back. They can draw aphids away from more delicate or valuable plants, including roses.

For aesthetics, try planting cosmos with cleome, echinacea, sunflowers, asters, or daisies. We also love to plant cosmos with carnations or the dianthus species most commonly referred to as "pinks."

Since cosmos flowers typically bloom on the upper half of the plant, foreground plants that complement their colors and textures create stunning, eye-pleasing displays. Be sure to note the height differences between cosmos species before you map out your ornamental garden.

In areas where you can expect early blooms, try planting poppies and crocus in front of your cosmos.

Stellar Seeds from Seed Needs

Wanna be a cosmonaut? Looking through our catalog, it occurs to us that we've got a lot of space-themed plants. From asters to flying saucers morning glory to alien-looking passion flowers and Romanesco broccoli, we can dig our hands into the dirt while we're dreaming of what's above us in the sky.

Let us help you launch a successful gardening adventure with our collection of ethically-sourced, high-quality non-GMO seeds. Our seed producers harvest seeds from healthy, disease-resistant plants, and we purchase only what we expect to sell within a single season. This practice, as well as our climate-controlled storage facility here in Michigan, ensures the highest possible germination rates and your best shot at a gorgeous garden.

Want to learn more? Contact us! We're always happy to talk shop and help answer your gardening questions. Just don't ask us where space finally ends, because that will make our heads explode.
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