Cosmos Guide
WHEN: Start cosmos seeds indoors 4-5 weeks before your last frost date. Alternatively, you can direct sow cosmos into your garden after your last frost date.
PREP: Fill 4” pots, or 72 cell seed starting trays with high quality potting mix that has been hydrated. It should feel moist but not wet or drippy.
SOW: Poke one hole in each cell with your finger, 1/4” deep. Place one seed per hole and cover lightly with soil. Arrange cell packs or pots in a closed bottom seed starting tray.
GROW: Cover for a few days with clear plastic dome lid, or plastic bag to speed germination. If you have seed starting heat mats, cosmos will appreciate them. Remove cover once first set of leaves emerge. Place trays under bright lights, keeping seedlings about 4” from light. If you don’t have lights, put seeds in your brightest window sill. Monitor and water as needed, allowing top layer of soil to dry between watering. If growing in seed starting trays and seedlings stop growing before all risk of frost has passed, transplant seedlings into 4” pots and continue growing.
PLANT: After all risk of frost has passed, plant out seedlings into your garden. Cosmos need full sun, but are not as picky about soil and water. Space your cosmos 12-18” apart. Place a tall stake next to (or tomato cage over) your cosmos seedling at planting.
PINCH: When plants are 8-12” tall, cut the plant down to 6-8” tall, cutting just above a set of leaves. This will force the plant to branch from the base and produce more flowers on longer stems.
TEND: Tie cosmos to stake as it grows to keep it upright. Cosmos are more drought tolerant than most cut flowers, allowing them to be grown without irrigation. Always monitor for drought stress and hand water as needed. For best results, water deeply 1-2 times a week during the dry season.
HARVEST: For best vase life, harvest when the petals are just unfurling, and the yellow center of the flower is flat. If you want to keep your cosmos blooming all summer, keep harvesting and don’t let the plants develop seed until fall. Harvest seed before fall rains set in.