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How to Grow Cosmos Seeds Cosmos are summer flowers grown for their cheery colors and ease of care They grow 2 to 4 feet tall, depending on the variety. The seeds generally are sown in early to late spring, but some varieties differ. 1.Rake a full-sun planting area to break up the soil. Cosmos will grow in partial shade, but the blooms will not be as abundant. There is no need to add compost or other nutritious mediums. A rich soil typically will produce lanky cosmos plants. 2.Sprinkle the cosmos seeds over the area and rake them into the soil to a depth of 1/16th-inch. 3.Lightly spray water over the planted area, using a garden hose with an attached sprinkler nozzle. Be careful not to dislodge the seeds. Spray a light layer of water over the seeds daily to keep the soil moist. The seeds will germinate in seven to 21 days. 4.Water the cosmos plants only if there is no rainfall during the month after seedlings appear. This is a drought-tolerant plant. When watering is performed, turn the hose on very low and let it stay in the planting location for a long, slow soaking.