Cleome, often called the Spider Flower, is a popular annual garden plant known for its striking, long-lasting blooms and unique, whisker-like stamens that give it its common name. Varieties of Cleome hassleriana can grow quite tall, sometimes reaching four to six feet, providing dramatic vertical interest in the garden. These plants are valued for their extended blooming season, which typically lasts from early summer until the first hard frost. Successful growth begins with precise timing when starting the seeds.
Timing Initial Indoor Planting
Starting cleome seeds indoors is the preferred method for gardeners with shorter growing seasons, as it allows for earlier bloom times. The ideal window for this indoor start is approximately six to eight weeks before the average last expected spring frost date in your region. This head start is necessary because cleome requires a significant amount of time to develop before it is ready to flower.
The seeds germinate best when the soil temperature is consistently warm, ideally maintained between 70°F and 75°F. Providing bottom heat with a seedling heat mat can help maintain this necessary temperature range for optimal germination, which typically occurs within 10 to 14 days. Once the seeds sprout, they require supplemental light from a grow lamp to prevent them from becoming weak and “leggy” as they stretch toward a light source.
Timing Direct Sowing Outdoors
Direct sowing cleome seeds into the garden bed is often simpler and avoids the challenge of transplanting. This method requires waiting until a specific point in the spring when the outdoor conditions are reliably warm. All danger of frost must have completely passed, as these plants are not cold-tolerant.
The soil itself needs to be sufficiently warm before planting, ideally between 65°F and 80°F. This usually corresponds to about three to four weeks after the last frost date, ensuring the soil is warm enough to trigger germination. Direct sowing also takes advantage of the plant’s natural tendency to self-seed; seeds dropped by mature plants in the fall will often germinate naturally the following spring.
Essential Planting Steps and Post-Germination Care
Regardless of whether you start the seeds indoors or outdoors, the planting mechanics are relatively consistent. Cleome seeds are tiny and should only be sown shallowly or pressed directly onto the soil surface. Some varieties are negatively photoblastic, meaning they require dark conditions to germinate, so lightly covering the seeds is often recommended.
Maintaining consistent moisture in the seed-starting mix or garden bed is important until germination occurs. Once the seedlings have established two sets of true leaves, thin them out to prevent overcrowding. Thinning ensures proper air circulation and gives the remaining plants enough space to develop into robust specimens.
For seedlings started indoors, the process of hardening off is mandatory before permanent transplanting. This involves gradually acclimating the young plants to outdoor conditions, such as direct sunlight and wind, over a period of seven to ten days. When transplanting, handle the cleome seedlings carefully to minimize root disturbance, as damage can significantly slow their growth and lead to transplant shock.