How hardy are Siberian iris seeds?

Hi everyone,
in late December I planted some of my Siberian iris seeds in large pots and put the pots under my balcony. At that time the night temperatures were about 5 degrees Celsius (41 degrees Fahrenheit). Then suddenly three nights later the temperatures dropped to minus 20 degrees Celsius (minus 4 degrees Fahrenheit). I was not at home so I could do nothing to protect the pots. I suppose Siberian iris seeds are very hardy but now I wonder if they will have survived such a sudden and hard drop in temperature? Any experience?
Thank you
Anja Germany zone 6
 

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