I have been doing head to head starts in my greenhouse using a mix of coconut coir & worm castings for one, and an organic potting mix for the other. I had 100% germination rate for cucumbers with my coconut coir mix, and zero percent with the potting soil mix. I figured there would be a difference, but didn’t know it would be that dramatic!
Definitely will be using only coconut coir & worm castings in the future.
I’ll give these another week, if they don’t start, I’ll try to recover the seeds and put them in coconut coir.