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I seem to have started mine here in Durham too early. Nearly ready for planting.
That should have been 7 inch containers ,they hold a litre. If they were mine I would transplant them into 1 gall containers and hope you can fool them. it can work I have done it, worth a chance, just don't keep them in small ones Good Luck..what variety did you use I may be able to give you some info which was always sadly lacking in UK packets when it came to corn.
Cold conditions will tend to restrict growth but if you can provide a bit of protection you have the advantage of amazing day length. With up to 22 hours daylight and then just a couple of hours twilight I'd be astonished if you could not produce a good crop. Obviously you need to be very careful about late frosts but otherwise try to create a bit of a helpful microclimate. Some carefully placed wind breaks could up the average season temperature by a couple of degrees.