Echinops

Does anyone know if Echinops species cross easily.I have five different species in my garden, and I wonder if you can collect seeds of them and be reasonably sure that you will have true-to-type seeds ?
Last year I got some seeds of Echinops tournefortii, and I stored them in the refridgerator until March this year when I sowed them. They were very slow and actually quite reluctant to germinate. Of 18 seeds 6 germinated ( 33 % ) in a time laps of 18 days - 2months and 5 days, while the seeds of E. sphaerocephalus, which I had stored for the same time but at room temperature germinated in 3 - 10 days, and 25out of 30 seeds germinated ( 83 % ). Could storage be a factor in the germination rate, or is it more likely that there are just natural variations between the two species in germination ?
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