The only thing to worry about, I guess, is whether the cardboard will stand up to the water. Eventually you may want to individually plant them anyway.

I noticed that you're in zone 5. I am too. When are you planning on transplanting them outside? I'm dying to start something seed wise, but don't want huge plants when I set them out.

This is my first year starting onion seeds. Most everywhere I looked said start inside 10-12 weeks before transplanting. April-May is the date set by Purdue University, That is what guide I use. Last Year I planted the plants Mothers Day and they done great. I just looked at the directions on the package of seeds and it said start 8 weeks before transplanting. If thats the case then I may be a little early.

For some strange reason I can not get onion seeds to sprout this year. Walla Walla or Spanish sweet varieties. Indoors OR out. I finally bought some starter plants today locally. Not sure why they won't pop up for me! Normal under florescent light method.