For any seedlings planted in trays, do I transfer them into a small pot and then begin the harden process? The seeds were planted March 28 and are growing quite vigorously, should I give them at least another 2 - 3 weeks before hardening them? I will be ready to plant them in the ground in May. How long do they need to be hardened?
You can start the hardening off process now for peas, lettuces, broccoli and all cool-season crops. Wait until the soil warms to 60F before putting out warm season veggies like tomatoes, cukes, squash and peppers, among others. If you are growing pansies they can go out now but other flowers should wait until after your frost-free date. Tiny seedlings are very fragile. Waiting until your seedlings are 3-4 inches tall and have several sets of true leaves is preferable. Depending on the size of your cells, you may have to up-pot them. If you see roots poking through the cell, that's a sign that it is time to repot. https://www.gardeningknowhow.com/garden-how-to/propagation/seeds/how-to-harden-off-your-seedlings.htm