Cleome is easy, fast-growing annual that reaches 3-4 feet in height. They produce rounded, spiky flower heads from late spring until the first frost. Often used at the back of the border or in mass plantings, this native of South America sometimes goes by the name Spider Flower. Classified as a half hardy annual, cleome blossoms are white, pink or purple. The seeds require a short stratification period and are easy to start indoors and grow outside all summer long.
Moisten some seed starting medium and place it in a plastic zipper bag. Add the cleome seeds and place in a refrigerator for 14 days. This is called stratification and should begin about 8 weeks prior to the last frost for your area.
Plant the stratified seeds 1/4 inch deep in seed starting cells or small pots. Seedlings emerge in about 10 days. Keep the germinating seeds and young plants at 70 to 75 degrees in a sunny window or under fluorescent lights until 2 weeks after the last frost.
Harden the seedlings outdoors for three days before planting. Place the seedlings in a protected area out of the full sun and wind. Bring in after a few hours on day one, half a day on day two, and at the end of day three.
Plant the seedlings outdoors in full sun to very light shade 2-3 weeks after the last frost date. Outdoor temperatures should remain above 40 degrees.
Feed and water the plants throughout the growing season. Prevent self-seeding by removing the seed capsules before they open.