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How to Grow Cosmos from Seed, Outdoors

Cosmos are bright little flowers that will perk up any part of your yard or garden. There are over 20 species of cosmos and depending on the variety, they grow from 12 inches to 7 feet tall with a spread of 12 to 15 inches. Cosmos are hardy in zones 9 and 10. In zones 5 through 10, the cosmos will often selfseed themselves back.The flowers range in a variety of colors like white, reddish-orange, yellow and bicolors. The 1 to 5-inch blooms can be single or double. The bloom time starts in late spring and continues to mid-fall. Cosmos sow easily into the soil and is the preferred method of many gardeners. Plant cosmos as a border plant, in containers, or in the background.Actually, you can plant them almost anywhere to give the area a splash of color.

Prepare the Garden

In May, or when all danger of frost is past, prepare the flowerbed to sow the seeds. Find a location that has the full sun exposure. Dig or till the soil, Cosmos grow well in almost any soil as long as it is well-drained. If your soil needs help in drainage, amend it with 2 to 3-inches of well-rotted organic compost. For this flower, you don’t want a too rich or fertile soil or the plants will grow tall and spindly. With a rake, work the soil so it is smooth. Take the back of your rake to level the soil. removing any rocks, sticks, hard clods or other debris the the rake brings to the surface. Let the soil settle for several days, otherwise the seeds may fall too deep and not germinate.

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Planting the Seeds

Make a shallow trench with your hoe. If you want straight rows, insert a stake at each end of the row. Connect the stakes with a piece of string or twine. This gives you a visual line to follow. I usually don’t care if my rows are straight or slanting, but my mother was just the opposite. Her rows had to be as straight as possible.

Sow the seeds over the soil at the bottom of the trench. The spacing between each seed is determined on the variety you’ve chosen. Look for the spacing requirements on the back of the package, in gardening catalogs or online. You can also contact the professional at the nursery for information.

Cover the seeds with soil. To determine the depth you should plant the seeds, plant them at four times the width of the seeds. Lightly firm the soil over the seeds with your hands, or the flat part of your hoe bottom.

Water

Water the seedbed with a sprinkler, light sprayer nozzle or sprinkle head watering can. You don’t want the water to pound into the soil, because that will wash the seeds too deep to germinate.. Keep the soil moist until the seeds germinate. It usually takes 5 to 10 days for the seeds to sprout. If you buy the taller variety of cosmos, you may want to need to stake them to keep them standing upright.

Through the growing season, keep the soil evenly moist. The best time to water is early in the morning or early evening. When you water, do it at ground level instead of overhead. This prevents disease from foliage that stays wet.

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Deadheading

Deadhead the cosmos when the flowers are spent. If you have a large area of cosmos or if you’re like me and don’t like to deadhead each spent flower, wait until there are many spent flowers. Take your scissors, pruning shears, or nippers and cut the plant back by a third. This encourages your plant to set on an abundance of new blooms. You can also cut the flowers to use in arrangements.