Can you plant in April in Ontario Zone 5a/b?

It's that time of the year to start planting your veggies. If you have a greenhouse or anything that helps with season extension you can start planting here in zone 5a/b. I start with cold hardy crops like lettuces, spinach and radishes in my carport greenhouse. With a greenhouse or row cover you can increase the temps and protect from frost so that you can extend your growing season.