Cosmos

Cosmos (Cosmos bipinnatus) is a tall annual from Mexico with daisy-like flowers in maroon, pink, lavender and white on stalks that can reach 4 feet (1.2 m). Its leaves are deeply cut and lacy.

Annual cosmos is easy to grow from seed, looks best planted in groups, and gives you flowers for cutting over several months. It needs only basic care to bloom all summer, and it does better without rich soil or fertilizer.

Sow outdoors after frost. Sow seeds outdoors once the danger of frost has passed and the soil has warmed to at least 65 °F (18 °C). Scatter them on the surface and rake them in lightly, about ⅛ inch (3 mm) deep, because seeds planted deeper germinate poorly. Keep the soil moist after sowing. Seedlings appear in 7 to 21 days, and germination is best at soil temperatures of 70–75 °F (21–24 °C).

Or start indoors. You can also sow in plug trays 5 to 7 weeks before planting out, covering the seeds lightly. At 68–72 °F (20–22 °C) they come up in 7 to 10 days. Harden the seedlings off and plant them out after the danger of frost has passed.

Sun. Cosmos likes full sun, at least 8 hours a day, and won’t flower in shade.

Soil. Cosmos grows in most ordinary garden soils as long as they aren’t soggy in summer, though very dry, poor soils and heavy, wet soils don’t suit it. Gardening experts recommend well-drained soil of average to poor fertility and warns that nutrient-rich soil gives floppy stems, late flowers and fewer blooms.

Spacing. Gardening experts space plants 9–12 inches (23–30 cm) apart and notes that wider spacing gives stronger stems. Gardening experts recommend 1–2 feet (30–60 cm), adding that plants set closer together hold each other up.

Pots. Cosmos can be grown in containers as well as beds.

Watering. Once seedlings are up, water deeply about once a week. Cosmos does well in the drier parts of the garden, and overwatering reduces flowering.

Feeding. Don’t fertilize cosmos.

Pinching and deadheading. Pinching young plants encourages branching. Deadheading keeps the flowers coming all summer and reduces self-sown seedlings. If plants start to flag, trim them back to 12–18 inches (30–46 cm) and they’ll be flowering again in about a month.

Support. Tall varieties may need staking in places with strong winds and frequent storms.

Day length. Cosmos responds to day length and flowers best in late summer, as the days get shorter.

Few insects and diseases affect cosmos. Pests to watch for include aphids, aster leafhoppers and the gray looper moth, and the most common diseases are stem canker, powdery mildew and gray mold. Slugs can also be a problem.

The biggest issue is often self-seeding: cosmos produces so much seed that it’s sometimes called a weed. Plant it where you’re happy for it to come back, and deadhead to reduce seeding. If you do want it to self-seed, leave a few seedheads to ripen.

For cut flowers, gardening experts suggest cutting when the petals are just opening but not yet flat, while gardening experts suggest cutting stems of flowers in full bloom. You can keep harvesting all season, and the flowers can be used fresh or dried.

Seeds. Dry the flowers completely, remove the seeds from the heads, and keep them in a paper bag somewhere cool and dry. Because cosmos reseeds itself so readily, you may not need to save seed at all.

Dried flowers. Dry blooms in a well-ventilated spot out of the sun, or in a food dryer; they should be dry in 1 to 5 days. Store them in a moisture-tight container somewhere cool and dark.