How to Grow Cosmos in Your Flower Garden

There are three varieties of cosmos flowers popular among home gardeners:

1. 1. Cosmos bipinnatus: Commonly known as Mexican aster or garden cosmos, Cosmos bipinnatus is an annual flower capable of growing eight feet tall but typically remaining in the two- to five-foot range. Its flowers have yellow centers surrounded by single, semi-double, or double florets that come in various shades of pink, purple, and white.
2. 2. Cosmos sulphureus: Known as yellow cosmos or sulfur cosmos, Cosmos sulphureus is an annual flower capable of growing six feet tall, but it usually only reaches a height of one to three feet. Like Cosmos bipinnatus, its flowers have yellow centers, but its surrounding florets instead bloom in vibrant shades of yellow, orange, and red.
3. 3. Cosmos atrosanguineus: Commonly called chocolate cosmos, the Cosmos atrosanguineus is an herbaceous perennial plant with a tuberous root. This rarer type of cosmos grows one to two feet tall, has dark maroon to brownish-red flowers, and is known for its fragrant blooms that produce a chocolatey aroma.

Cosmos flower seeds are easy-to-grow and produce a low-maintenance plant ideal for any cutting garden. Cosmos seeds typically germinate in one to three weeks, bloom in two months, and continue to flower up until the first frost in the fall.

• Decide whether to start seeds indoors or outdoors. To get a head start on planting cosmos, sow your seeds indoors in small pots or a seed starting tray five weeks before the last spring frost, then transplant seedlings outdoors once the danger of frost has passed. To sow cosmos seeds directly into your garden outdoors, wait until after the last spring frost. Make sure you don't plant outside too early since cosmos plants flourish in hot weather similar to their native climate in Mexico, Central America, and South America.
• Choose a sunny location. Cosmos grow best in full sun areas but also tolerate partial shade in extremely hot climates. When grown in a shadier environment, your cosmos may produce fewer flowers than it would in full sun.
• Plant cosmos in well-drained alkaline soil. If you plant cosmos in a soil type that is too fertile and rich, the plant may grow more greenery than flowers. Sandy soil is ideal. Cosmos are drought-tolerant plants that grow best in relatively dry soil.
• Leave one to two feet of space between cosmos plants. Sow seeds a quarter-inch deep into the soil. Whether planting seeds or transplanting seedlings, space smaller varieties one foot apart and taller varieties 18 to 24 inches apart.

Follow these growing tips to produce stunning cosmos blooms.

1. 1. Water less as the growing season progresses. Water regularly early in your plants’ life cycle, making sure not to oversaturate the soil. As your cosmos plants mature, they will adapt to the dry soil conditions and need less watering. When you do water, do so at ground level to avoid dampening the leaves.
2. 2. Use stakes to support taller varieties. Staking taller cosmos plants provides protection from strong winds and heavy rain, making sure the plants’ stems don't snap. A fence or trellis can also serve as an alternative support structure.
3. 3. Use minimal fertilizer. Cosmos flowers are capable of thriving in poor soil. Fertilizer often increases green foliage and decreases cosmos flower blooms. If your cosmos are struggling in the middle of their growing season, it's okay to use a dose of fertilizer, but avoid fertilizing regularly.
4. 4. Check for weeds. Regularly look for weeds and remove them if necessary. You can also spread a light layer of mulch around the base of your cosmos in early spring to help suppress weed growth.
5. 5. Prolong your blooming season by deadheading. Deadheading is a technique involving the removal of old flowers during a plant's growing season to encourage the development of new blooms. Pinch flower heads off the stem, or cut the stem below the first leaf closest to the flower head.
6. 6. Monitor for pests and disease. Aphids commonly plague cosmos flowers, but you can deter them with insecticidal soap. Powdery mildew and gray mold are harmful diseases, but they shouldn’t affect your plants if you water your cosmos at ground level and allow for good air circulation between plants. If you spot any diseased plants, remove them immediately to stop the infection from spreading.

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