Cosmos flowers, known for their cheerful, daisy-like blooms and feathery foliage, are rewarding annuals for home gardens. These plants provide continuous color from mid-summer until the first frost. Starting cosmos seeds indoors allows for greater control over the initial growth phase, resulting in earlier blooms once the seedlings are moved outside. This method effectively extends the growing season, giving the plants a significant head start.
Essential Supplies and Timing
Successful indoor seed starting depends on precise timing and the correct materials. Sow cosmos seeds indoors approximately four to six weeks before the last expected spring frost date. Starting seeds too early can result in overgrown, leggy plants that suffer transplant shock when moved outside.
Supplies include a sterile, fine-textured seed-starting mix. Plant the seeds in trays or small containers that have drainage holes to prevent waterlogging. Since natural light is often insufficient, a supplemental light source is required to encourage healthy, compact growth. A simple fluorescent shop light or dedicated LED grow light fixture placed close to the seedlings works well.
Sowing the Seeds
Fill the trays with the prepared seed-starting mix, then gently moisten the medium thoroughly with water until it is uniformly damp but not saturated. This ensures a consistent moisture level for the emerging roots.
Cosmos seeds are relatively large and should be planted shallowly, about one-eighth to one-quarter inch deep, and covered lightly with soil. Planting them deeper can inhibit germination because these seeds benefit from some light exposure to sprout. After sowing, cover the trays with a clear plastic dome or plastic wrap to create a humid microclimate, which helps maintain soil moisture. The optimal soil temperature for germination is between 70 and 75 degrees Fahrenheit, often achieved using a specialized heat mat placed beneath the trays.
Caring for Seedlings Indoors
Once the seeds sprout, typically within five to 14 days, the plastic dome should be removed immediately to prevent fungal diseases like damping off. Seedlings require intense light, ideally for 14 to 16 hours daily, to grow strong and stocky.
The supplemental light source needs to be positioned very close to the young plants, usually about four inches above the tops of the seedlings, to prevent them from stretching toward the light. This stretching, known as etiolation, results in weak, spindly stems. Maintaining a slightly cooler ambient temperature, ideally in the low 60s Fahrenheit, can also help promote compact growth.
Proper watering is fundamental for the growing seedlings. Allow the top surface of the soil to dry slightly between waterings, which helps prevent root rot. Watering from the bottom allows the soil to wick up moisture evenly without disturbing the delicate seedlings. Once the seedlings develop their first set of true leaves, they should be thinned out, leaving only the strongest plant in each cell. Thinning involves clipping the weaker seedlings at the soil line to ensure the remaining plants have adequate space and resources.
Hardening Off and Transplanting
The transition from a controlled indoor environment to the variable conditions outside requires a gradual process called hardening off. This step is necessary because the intense sun, wind, and fluctuating outdoor temperatures can severely shock plants. The hardening off process should take place over a period of seven to ten days, beginning after the danger of frost has completely passed.
Start by placing the seedlings in a sheltered, shady outdoor location for only two to three hours on the first day, then bring them back inside. Over the subsequent days, progressively increase the amount of time the plants spend outside and the level of direct sunlight they receive.
After the hardening off period is complete, the cosmos seedlings are ready to be planted into their permanent spots in the garden. Choose a location that receives full sun and has well-draining soil, as cosmos thrive in conditions that are not overly rich in nutrients. Gently remove the seedlings from their containers, taking care not to damage the root ball, and plant them at the same depth they were growing indoors, spacing them about 12 to 18 inches apart.