I found out that I am going to be raising seedlings for two different fund-raiser plant sales in May. I need to start more plants than I originally intended. Does any one have suggestions for plants that are cold-hardy or need cold stratification that can be started outdoors about now (Colorado Zone 5 Frost Date April 28 - May 10)? I could double my seedlings if they didn't all need to be indoors under lights.
Although they don't need the stratification and are not cold hardy, you might consider tomatoes. Whereas customers like to see their annual bedding plants in flower when they buy in May (and WS won't achieve that), tomato transplants are specifically NOT supposed to be in flower. I WS'd tomatoes last year in the beg of March, they germinated beg of April, and I and I had lovely, stocky, green seedlings in May that I did not have to harden off. I set up a table outdoors and transplanted them into 4" pots at the last frost date, about a week before transplant time. They looked better than the leggy ones I had started indoors.
Now, all that is a moot point if your sale is before your frost-free date. (Maybe that's why you said they had to be cold hardy.)
Yes, I'm planning on doing a flat of tomatoes. I sold a few extras I had on hand last year, and they sold out right away.
The problem is I'm doing two fund-raiser sales. One for the library where I can use the easy annuals and veggies started indoors under lights. The other is for my Xeriscape garden group - those need to be tough plants that really should have been started last fall. I'm probably going to be digging up a lot of small volunteers out of my own yard to make up for the fact I'm starting on this so late.