How to Keep Cosmos Blooming All Summer

Few flowers earn their space in a garden like cosmos does. Beautiful and easy to grow from seed, annual cosmos provides nectar and pollen to bees and other insects, the blossoms look great in a vase, and birds eagerly gather the seeds. Long admired for their orderly, evenly spaced petals, the cosmos genus name came from the Greek word ‘kosmos’, which means order, harmony, or the world.

There are two species you can use to bring quick color and diversity to your garden. Blooming in endless shades and patterns of pink and white, willowy Cosmos bipinnatus prefers an early start in cool weather, with more seeds sown in late spring for fall bloom. Fast-growing yellow to orange Cosmos sulphureus prefers warmer growing conditions, and it’s invaluable for filling in gaps after spring flowers finish blooming. Why not try both? That’s what they do at Tokyo's annual Cosmos Flower Festival at Showa Kinen Park, which opens with yellow sulphur cosmos and then features millions of pink cosmos blanketing the hillsides.

Growing Garden Cosmos

Also called cut leaf cosmos, Mexican aster or garden cosmos, C. bipinnatus comes in a range of sizes and shades of pink from magenta to white. Dwarf varieties like Apollo and Sonata are great for containers, or you can grow taller varieties that feature splashy bicolored blossoms or petals that are curved or frilled. If you live in a rainy climate, consider carefully before stocking your garden with fancy varieties like Seashells or Cupcake, because slender cosmos stems cannot support blossoms gone heavy with rain. Single-flowered varieties like Candy Stripe and Sensation Blend grow to waist high or taller and often need stakes or cages to keep them upright, but they have no trouble holding their blossoms high and dry.

Getting Cosmos to Bloom

But there is a catch. Like morning glories, tithonia, and other plants from central America, garden cosmos are sensitive to changes in day length. They want to hold their flower power until days are getting shorter, which is the opposite of what happens in spring. Yet cosmos seedlings are easily tricked, and plants started indoors in early spring and set out as the last frost passes are exposed to enough short, cool days to trigger flowering. If you’re not sure if your home grown or purchased seedlings have seen enough dark days, simply bring them indoors for five days and keep them in a dim place from late afternoon to midmorning to simulate nine-hour days. This is all it takes to prime garden cosmos for early blooming.

You can use the same trick when growing garden cosmos from seeds sown directly in spring. After seedlings have been up and growing for a couple of weeks, perhaps after weeding and thinning, cover them with a carboard box or other heavy shade cover from late afternoon to midmorning for seven consecutive days. A brief spell of short days will always get cosmos in the mood to bloom.

Pinching back the primary growing tips will help cosmos get bushier, and deadheading can prolong blooming time. Don’t fertilize established plants, and water only if they look droopy in the morning.

Sow Sulphur Cosmos for Summer

Native to Mexico and South America, the bright blossoms of sulphur cosmos (C. sulphureus) have been used medicinally and as a fabric dye for thousands of years. The petals are rich in antioxidants, and Peruvian cloth makers still use them as a yellow dye in their beautiful tapestries. Whether you start seeds in pots or direct sow them into beds, sulphur cosmos will start popping blooms in a few short weeks. Pot-grown plants are easy to slip into small open spaces, or you can grow seedlings in a nursery bed and lift and move them to where you want them to bloom.

Sulphur cosmos are minimally sensitive to day length, and because they grow best during summer’s second half, no special measures are needed to get the plants to set buds. Pinching and deadheading will keep plants in bloom longer, or you can let them go straight to making seeds. Edible sulphur cosmos seeds can be lightly toasted and eaten like sesame seeds, you can store them for replanting, or leave them to be gathered by goldfinches and other birds. Everybody wins.

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