One of the conundrums new gardeners face each year is when to plant each type of seed. The seed packs always say 4-6 weeks before/after last frost date or something similar, but that's not all that helpful most of the time. They also sometimes say what temperature the soil should be for germination, which is nice for outdoor planting, but doesn't really matter for things you start inside. More important is the question of how fast the seeds will sprout and grow before you run out of room for them and are forced to put them outside? And how slow will they grow? Will you run out of summer before they are big enough to go outside?
So here's a quick list of the crops I've grown and their growing speed to give you an idea what should be planted super early, what can wait a bit and what will outgrow your indoor space before the weather warms.
Fastest to slowest:
-squashes (summer and winter)
-onion family (from seed, not set)
I haven't grown corn yet, so have no idea how quickly it grows, but this should give an idea that you would not want to start celery and lettuce together in early Spring for a harvest at the same time. In fact, celery is the slowest growing vegetable I've planted, which is why I usually buy it as a start. I could plant celery in January and it still might be too small to go outside in April. At the same time it would be pointless to plant zucchini seeds (summer squash) inside in February when it will outgrow your space by April and (here, in Oregon) can't be planted outside until the rains slow down in May. I've lost many cucurbits to powdery mildew and no longer take any chances, plus since they grow so quickly I have plenty of time for a good harvest even after waiting out the rains.
Tomatoes are much faster growers than peppers or eggplants, if you plan to transplant all three together once the weather is warm enough you'll have to plant your eggplants, wait a week, plant the peppers, wait two more weeks and then start the tomatoes. Otherwise your tomatoes will be far too large to keep under grow lights before the rest have a chance to catch up, especially if you have failed germination and have to replant something.
Point in case: One row of my garden bed right now has lettuces and swiss chard that were planted inside the same day, cared for the same, and transplanted the same day as well; it also has peas that were planted outside the day the others were transplanted. Today this row has lettuce that's nearly ready to harvest, peas that are about 6" tall, and Swiss chard that is still little more than a sprout with it's first true leaves. I transplanted all of this on March 26th, then started some tomatoes on April 10th. The tomatoes are already putting on their second set of true leaves, and are far ahead of the chard despite being planted 2 weeks after the chard went outside.
I hope this post is helpful in determining planting dates, and good luck with the gardens everyone!