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how to grow cosmos

How to Grow Cosmos

When and Where to Plant Cosmos

Light: Cosmos prefer full sun conditions, except in extreme heat where they can tolerate part shade.

Soil: Prepare the garden with loose, weed-free soil. Cosmos prefer dry, arid soil over wet conditions. Soil that is too moist may lead to disease. Add compost, yet hold back on fertilizers that contain high nitrogen, which can encourage foliage to develop more rapidly than flower blooms .

Spacing: Plant the shorter types (C. sulphureus) 10-12 inches apart, while the taller varieties (C. bipinatus) prefer 1- 2 feet in between. Plan to stake your Cosmos if plants get too tall, and gangly.

Planting: Cosmos is one of the easiest flowers to grow from seed that is directly sown in the garden bed.

Spring Planting: Because sprouts will develop quickly, be sure to scatter seeds after all danger of frost has passed in the spring. Alternately, if you have a very short growing season, you may sow seeds indoors 4-5 weeks before your frost-free date in plug trays, to get a jump-start on growth. Transplant into the garden after danger of frost has passed, and keep protected from wind and cold temperatures until established. Cosmos are tender annuals, and will quickly turn brown when temperatures drop below freezing.

Fall Planting: Cosmos may also be direct sown in the fall. The trick is to follow the exact opposite course of action required for spring planting, and scatter your seed after a few hard frosts have passed. This ensures that your seed won't germinate until it's ready next spring.

How to Grow Cosmos Throughout the Season

Growth Habit: Cosmos are multi-branching plants, with hollow tubular stems. Keep the flowers cut back after first bloom, to prompt new and continuous growth. After your Cosmos is well established, instead of just removing the blossoms, trim a third of the way down. Water and wait for a new crop.

Staking: When planted as a group, Cosmos can hold each other upright. Provide the taller varieties protection from the wind and be prepared to stake to keep the tall, spindly stems from breaking in heavy rain or wind.

Watering: Young plants need moisture to get started, yet mature plants will easily adapt to dryer conditions, and prefer less watering. When watering, focus on watering the roots, and avoid splashing the foliage.

Fertilizing: Soil that is too rich with fertilizer will yield fast plant growth, yet weaken stems and create a scarcity of flowers. Prepare the beds properly, yet hold back on too much compost and fertilizer. Add a side dressing of compost or fertilizer mid season, as needed.

Mulching: Mulching cosmos around the base of the plant in early spring will help to keep weeds in check, and to retain moisture.

Trimming & Pruning: Keep Cosmos trimmed of spent blossoms, to encourage new growth and continuous buds. During mid-summer, instead of just removing the blossoms when deadheading, trim the branches back to a third of the way down the plant; water and wait for a new crop for a late season display.

pink cosmos blooms

Cosmos: End of Season Care

Annual Cosmos are highly sensitive to frost and freezing conditions, and will die back naturally once temperature drops. Pull the whole plant from the ground with the stem, shaking soil the from the roots back in to the garden, and toss plant onto a compost pile. Or, leave the plant in place to provide habitat for insects (bird food). Save seeds from seed pods for growing the following year; however, know that blossoms may not match the original since most are hybrids.

Dividing & Transplanting: Cosmos are easily started from seed and transplanted into the garden. Their root system is rarely large enough to support dividing, so it's best to start new plants. Perennial forms of Cosmos, such as Chocolate ( C. Altrosanguineus) are propagated by tuberous roots since their seed is sterile. These varieties can be safely divided as needed.

Pests and Diseases: Cosmos are relatively easy to grow, and adapt to most conditions, yet gray mold or Botrytis cinerea can become a problem, especially when plants are planted too closely or there is an extended amount of rain. Gray mold is commonly found in bedding plants, and can easily infect plants that are already damaged or beginning to die, and will spread quickly to other plants. It is, therefore, important to allow for good air flow between plants when possible to prevent this aggressive mold.

Aphids can also be a problem when plants are stressed due to over crowded conditions, and severe lack of water.

Additional Concerns: Many Cosmos varieties are short-lived annuals, and will no longer bloom once they produce seed. Keep the spent blossoms picked in order to encourage new growth.

Cosmos: Favorite Companion Plants and Garden Design Advice

Cosmos sulphureus varieties, which include plants in the “Ladybird Dwarf”, and the “Klondyke Mix”, range in shades of yellow to orange to scarlet, with a bushy, low-growing habit. Plant alongside zinnia, pansies or Johnny Jump –ups for a burst of bright color in the front of a border or rock garden.

The Sonata series is bred to be shorter and sturdier, and is therefore ideal for pots or smaller gardens, and is especially decorative. Taller Cosmos bipinatus, varieties that include Sensation, Picotee, Sea Shells, and Purity are showy, and are ideally planted in the back of a flower border along with lilies, ornamental grasses, cleome and dahlias.

Cosmos: Additional Uses

Many of the plants in the Asteraceae family, of which Cosmos are included, are used in the production of cooking oils (sunflower), sweetening agents (Stevia) and coffee substitutes (chicory) and in herbal medicine (Calendula and Echinachea). Cosmos, however, appear to be purely ornamental, and are an excellent cut flower for decorative bouquets.

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