Lilli, probably you got too small bulbs when you purchased your Dutch Irises. Or they had not been heat treating them the way they should have. Mine developed much bigger bulbs in their second year.
A dark pot might help some, but that would be highly dependent on the year and where you placed that pot. I haven't tried Dutch Irises in a pot, but in ground they divide very quickly. I suspect that when grown in a pot, they would need annual division and replanting to perform their best.... and if you had to do that, you could just as easily lift them and store them warm.
Of course if every summer would be as hot as last year, heat treating them probably wouldn't be a problem. Unfortunately I removed mine before I knew we were going to have a record-breaking hot and dry summer... otherwise I would just have left them in place
...and hopefully enjoyed the blooms this spring.
