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Growing cleome from seed

Question:  I read your article a few weeks ago about starting hibiscus from seed so I thought I’d ask you about my seed starting problem. I saved seed from my cleome flowers. I thought it would be easy to start the seeds indoors because they come up by themselves in my garden.  I’ve started lots of plants from seed, indoors and out, but the cleome won’t even come up. Any ideas?

Answer:  Cleome, also called spider flower, readily self-seeds (existing plants drop seeds that produce new plants) in the garden. I wondered, as you did, why it wouldn’t germinate indoors. Quick research and a little knowledge of botany led me to the answer.

I know I always tell you to go to university resources, but for annual flower and vegetable seed starting information I go to a seed company like Thompson & Morgan. (This is not an endorsement of this company’s products.) In this case, I found better information on the Thompson & Morgan United Kingdom website than on their United States site.

Cleome needs warm temperatures of 75 to 80 degrees Fahrenheit to germinate. Use a water-proof heat mat designed for plant propagation and/or a warm room to achieve the right soil temperature for germination and seedling growth.

Cleome are one of those plants that do best with day to night temperature fluctuations. If possible, drop the room temperature approximately ten degrees at night.

The small seeds are light sensitive. They don’t require light but will germinate better when exposed to light. Barely cover seeds with potting soil and use supplemental lighting.

Now, all you botany/horticulture geeks  (yes, I’m one of those) think about cleome’s germination requirements and our typical late summer weather. No wonder the plant self-seeds in our gardens. Unless dead flowers are removed, seed pods develop that open and drop seeds. The seeds sit on top or barely underneath the soil so they get the light they prefer.  August and early September air temperatures are warm to hot and the soil is nice and warm especially in full sun gardens. Nighttime temperatures are usually ten to twenty degrees cooler than during the day. All together these are perfect  conditions for cleome to germinate and grow without any help from us. Thanks for the perfect example of how knowing the biology of a plant will tell you how to grow it.