I'm in a little cooler zone and still started lettuce, chard, and peas in late Feb and early March. True, I'm using a minature hoop house (five bent-over 10' high and 5' wide letter 'H's made using 3/4 PVC with 'T' in the middle to allow a cross support for stabilizing against the winds), so it's easier on the plants, yet it's only 4 mil plastic so there is very little resistance to heat transfer. I'm surprised at and pleased with how well the spinach and chard did early (the peas have grudgingly appeared).

Frost tolerant plants are basically those that like to grow in cool weather and can survive freezing temperatures, like the greens you've mentioned. While they can survive frost without being covered, they will experience less stress and be happier if you provide some covering that will protect them from harsh changes in temperature.

All the greens you started can make it through extremely cold temperatures. They winter-over in my hoop-house, surviving sub-zero temps and frozen ground. The key is to protect them from cold, dessicating winds and destructive freeze-thaw action using some kind of covering.

In Zone 6, you should be growing cold hardy greens outside at this point. They would do well, covered with a row cover or other protective covering that can release heat on sunny days. Since you've planted your starts indoors, be sure to harden them off for a week or so before you plant them out -- they certainly won't be frost tolerant until they've had a chance to get used to outdoor conditions.

Unlike the greens you're growing, many of the vegetables we associate with summer -- tomatoes, peppers, cucs, melons, beans -- are not frost tolerant and will sulk in cool temperatures. That's why it makes sense to take a 4-season gardening approach -- grow each vegetable in the seasons they like best. In other words, don't wait until Memorial Day to start your spinach!

i highly recommend you come over and check out the winter sowing forum. we plant seeds outside in recycled plastic containers like 2 liter bottles, etc, and get sprouts when spring breaks. i get sprouts starting in february here in z6b virginia mountains, lots more in march. since these plants are born outside they are already hardened off, no wasteful light setups, no guessing about when it's time to plant (you plant when they get true leaves), no greenhouse, no damp off, no concern about frost dates, nothing. i've had lettuce, land cress, swiss chard, brocolli, brussel sprouts, india mustard, etc, growing in the garden for a month now, some of it is really taking off now that april is half past. i highly recommend it. check out the FAQ in the winter sow forum for instructions.

Here is a link that might be useful: winter sow forum