I've never tried growing them from seed. Once you have one you'll have more whether you like it or not! I know that doesn't help but just thought I'd tell you my experience. :-)

take the flat or pot outside ..north side of structure ... see if they might pop outside ...

I did cleome from seed this year. Only a 9 cell. They germinated fine for me in a few days.

How long ago did you plant your seeds?

^^ Over a week ago. I sowed another batch last night.

How warm are your seeds?

Same temp as other majority of other germinating seeds in my set-up -- about 80-ish ambient temp (very close to the lights with ballast of light on shelf underneath generating heat, too). If I know a particular seeds needs different conditions, I will provide them (e.g. cooler temp, cold stratify) -- but not aware that cleome needs anything different - ?

Hmmm, sounds like you are warm and normal. Not sure why your seeds aren't popping. I didn't do anything to my seeds before planting them.

Just press them into the soil. Do not cover them. I get the best results that way.

Not helpful, but I never could germinate the native Rocky Mtn beeplant (Cleome serrulata). I don't know if it just doesn't like me, or if it requires some stratitfication.
I do agree that the C. hassleriana (SA spider flower, the one you're probably trying) establishes and reseeds well, though a trial or two trying to establish from seed gave me the same results (none). The best way MIGHT be to buy young plants and let them reseed, unless you want to try to stratify them.

I too am trying to grow The Violet Queen from seed planted in the fall. I pressed them into the top of the soil and none have come up. I planted some more using winter sowing in a gallon milk jug in early March and they are up one inch tall. You might try that method next.

Cleome needs light and oscillating temperatures in order to germinate. Try putting them outside if it's above freezing (at night) and taking them in during the day. They should germinate in a few days.

If it's still too cold out put them in the fridge for the night.

I live in Seattle area. Unfortunately, I haven’t noticed any of my cleomes setting seed. I started seeds of Violet, Rose and White Queen on April 1 with heating pad. Seemed slow to germinate and grow. Violet Queen was first to show a week later, then Rose Queen and only this week White Queen. I will start them earlier next year as these will take awhile to get to transplanting size. I used commercial seed starting mix and by the look of all my seedlings, I think I will mix my own next year. Anyway, be patient, mine were slow and uneven in germinating.

I once saw a gorgeous combination of multi colored cleomes paired with Perovskia atriplicifolia in Vancouver BC one summer, I will look for the photo. Of course I never seem to grow either at their best....

Thanks for the tip on oscillating temps - I think it worked! I've been putting the new batch in the fridge overnight, and this morning I saw green with the seed capsules opening. I guess that is the trick. YAY! (now I just have to get them to fully germinate -- every twelfth blue moon or so something goes awry mid-germination and the seeds abort their effort...)

I haven't tried germinating them under lights - I direct sowed them one year and as others have said, every season I now seem to have a good amount of self sown seedlings. I love that. No effort required. Just editing where you want to keep them. I really enjoy Violet Queen, the color is so pretty.

I got 3 out of 5 to come up - woohoo! I wrote a note about the oscillating temps on the front of the packet so I won't forget next year. :0)