Like many native plants they need fluctuating temperatures. Direct sow in the fall or direct sow when temperatures day times are warm and cool off at night. Germination is from 10 to 20 days. Al

I read that they need some cold first. The thing I was reading said to put in the fridge for about a week or 2. I winter sowed mine (and didn't put in the fridge) and I think every seed germinated. Hope that helps.

I did mine the wintersow method on Mar. 24th and by Apr. 17th I had about 60 percent germination...a few more popped up afterwards :D

Vera

I never could get cleome to germinate. I finally just bought some plants and they reseeded aggressively.

I have never been able to get Cleome to germinate indoors (Like Al, I think my temperatures were too constant.) I finally just threw the seeds into the garden last March, and they came up all over the place. Go figure. Guess they thrive on neglect. MacThayer

They definitely seem to need cold to germinate. I had great luck with them when I sowed early indoors in late winter--and bad luck indoors in mid-spring. Cold is the secret to germination, I think.

If you were close, you could have mine! They are everywhere in my garden now! Made the mistake of planting them last year. Right up there with forget-me-nots on the list of self sown weeds as far as I am concerned.

The secret is constant moisture.
I threw some seed last summer (in the middle of June) into a self watering pot, small, already had basil growing in it just to see what would happen.
Forgot about them, and about a week later, I had huge spider flowers getting bigger than the basil.
My friend took them home and planted them in her sunny bed under her kitchen window, they grew 5 feet tall.
Just keep them constantly wet, or get a self watering container from Walmart for about 8.00 and plant the seed in that.
I don't have any more self watering containers left, I got rid of mine because I didn't like them.
Great for starting seed in though.
Good Luck!