I've had trouble with anything but salad greens this early. A cold freeze can still bring temps down into the 20s or lower in the GH, and it can get into the 90s on a sunny day. Late March is safer for me. But it depends on the size of your gh, whether you use the little plastic covers over the trays to retain heat not just in the soil but in the air around the seedlings, etc.

The answer will depend on several things. How cold does your greenhouse get at night (a function of solar heated mass inside the greenhouse and type of glazing), what type of veggies are you starting, and when is your last average frost date? Do you have a spunbond row cover you can put over them to reduce temperature swings?

I think you will want to actually start them inside your house on the heating pad, and then move them to the greenhouse after they have gotten their first true leaves or so since many seeds have an optimal sprouting temperature, and an unheated greenhouse, even with a heating pad, may have too much variation of temperature for good germination. We still have nighttime temperatures predicted below 10F in the coming week, so that will be far too cold for most (all?) veggie seeds to germinate even on a heating pad. To get a good idea of best temperatures for germination, Johnny's has a little graph on every veggie page to show temperatures of best germination for that type of seed. So for instance, I just went to a variety of tomato, and the graph shows that in general tomato seeds germinate best in temperatures around 85F. Broccoli is optimal around 75F, and spinach is optimal around 70, though it will continue to sprout in cooler temperatures.

The other consideration is when you will be able to plant them out in the garden. Something like frost tolerant greens will be both happier in a cool greenhouse and happier to be out in the garden earlier than warm weather veggies like tomatoes. Also, different crops have differing times between sowing and planting out. Leeks and onions take the longest, and we used to plant them indoors under a grow light around now. Peppers and eggplant were the next longest, and tomatoes only took about 6 weeks to raise good-sized transplants.

However, peppers, eggplant and tomatoes and other warm-weather crops all need to stay above freezing, so I would get a high-low thermometer and put in out in your greenhouse to see what the current temperature range is. If you have a small, plastic covered greenhouse, it will only be useful for hardening off transplants that have been grown, but if your greenhouse is larger and has double glazing, you can use it for your plants earlier.

Finally, if you haven't done this before, be aware that as days get longer, it is possible to cook your plants if the greenhouse gets too warm. I have done this when the open ends of my low tunnel blew closed and it overheated in early May.

Great advice, my small hobby GH doesn't hold the heat like some larger ones with glazed glass do, so starting indoors is probably my best bet. I just hate the mess lol. Thanks!