optimal temp for tomatoes is 70-75 degrees, with 65-85 being the good range (from jeavons "how to grow more veg") I wouldn't want to put young plants at 50 degrees, although at the end of the season, old timers can take it, and the soil is probably still warmer than that. In zone 4, you'll be waiting a long time to put them out. Here in zone 5 april is a no no for tomatoes without protections. If you can provide a little heat to them in the garage till whatever your plant out date is, that might work if they have enough light. I'd try to keep them at a minimum of 60, but I'd aim for about 65-68, unless you want them to grow a little more, then 70 would likely have them growing at the expected rate, a bit cooler would slow them down.

Regarding the intensity of the light---the usual planting out date here is Memorial Weekend (basically June 1) when the sun is very high in the sky. There is no way I would ever think about exposing plants to that sun without some sort of transitioning period. But---if one were to put transplants out---here---on April 1 (when the sun is less intense)---inside of a green tinted Wall O'Water---wouldn't that approximate the same sunlight intensity (or less?) as compared to what plants being hardened off in late may would be exposed to? I'm trying to listen the practical little voice in my head that says try this with a couple of transplants but harden off the rest. I was hoping someone may have experimented with this already and might have something to share

Sorry but I have to disagree. While it is true that "Tomato's are a warm/hot weather plant. they suffer in any temperature below 70F day & night.." is somewhat true for full grown plants in the garden, it is not true for seedlings. Most tomato seedlings are grown in greenhouses where the air temp is maintained between 55 and 65 to avoid leggy growth and associated problems. Your garage will work and is used by many for starting seeds. I have linked a recent post on this question here below. It also links to a long discussion on the same issue over on the growing tomatoes forum here with even more details. You only need to make a couple of modifications to be successful. 1 - place some sort of insulation on the shelf above. Anything to keep the warmth from escaping through the self above - a folded blanket, piece of foam, old newspapers, etc. and a another "chill block" on the shelf you will be using to keep the cold air from the floor from getting to the plants. You will just set the trays of plants on top of it. 2 - buy some cheap clear plastic to create a tent around the set of shelves. It needs to be cut at the corners or down the center to allow you access and so you can open it for air when the temp inside gets above 70-75 for the heat the lights put out. 3 - include inside the tent a small thermometer to monitor temps and a small fan to help circulate the air around the plants. 4 - if the temps in the garage drop toward freezing you may need a space heater for those really really cold nights. Good luck with your plants. Be sure to harden them off well before putting them in the garden. ;) Dave PS: plants in the garden will easily tolerate temps down into the upper 40's if there is no wind. They won't set fruit at those temps but the plants will survive. It is the soil temps in the garden that control plant safety in the spring, not the air temps. ;) Here is a link that might be useful: Growing Seeds in an Unheated garage?

I dont know how much space you have, but with lupine and poppies, I probable would just winter sow them. My space is limited however, my indoor set up is for veggies and annuals and other tender plants. Any temperate plant I will try to wintersow, besides its fun to throw seeds and dishes into the snow and have all your relatives and friends think your nuts if you dont know what im talking about you can check out www.wintersown.org or just check out the forum here on gardenweb.