The three main reasons to start some seeds indoors ahead of time in February and March is to get a head start on growing 1) heat loving plants, 2) cool season plants (see above) and 3) root and bulb plants that take FOREVER to grow.
HEAT LOVING PLANTS: This includes veggies such as artichokes, celery, peppers, tomatillos and tomatoes, certain flowers like zinnias, calendulas and perennials and certain herbs like oregano, thyme, parsley and rosemary, I usually transplant all these heat loving plants outdoors in late spring in Pittsburgh.
COOL SEASON PLANTS: This includes veggies such as broccoli, brussels sprouts, cauliflower and kohlrabi, certain flowers such as pansies and voilas and certain herbs such as chervil, dill and cilantro. Greens such as arugula, bok choy, cabbage, lettuce, chard, spinach, tatsoi, etc. can be started indoors or outdoors. I usually transplant the cool season seedlings outdoors in early to mid spring under cover.
ROOT and BULB PLANTS: The ones that need a head start include leeks, onions, shallots and celeriac / celery root. I usually transplant these seedlings outdoors in late spring under cover.
