Every spring I face the same dilemma; I'm anxious to get started in the vegetable garden, and want to start all the seeds. But I have limited space indoors for seedlings. If I start too many, I will run out of space under the lights long before it's warm enough to plant them outside. On the other hand, if I wait until it's warm and buy starts at the spring plant sale, I will spend way too much money and I will be limited to the varieties somebody else decided to plant. To quickly separate the seeds from the starts without too much mental anguish I use two factors: how each plant grows best, and what's easiest for me.
These guidelines are meant to simplify a sometimes-overwhelming decision process for casual gardeners, and assume that you are planting in the spring for summer harvest. Gardeners who plan for year-round productivity will use different strategies that may be more labor intensive. For example, if you want to grow leeks for summer harvest, you would treat them like bulb onions and start fast-growing varieties indoors. But to extend (or delay) the harvest into fall and winter, you would direct seed outdoors and use the slower, hardier varieties.
Some plants don't handle transplanting as well as others. Others grow from seed outdoors so easily that there's no point going to the extra effort to start them early. For these plants, it makes the most sense to follow the frost dates and plant these seeds outside at the right time. Don't waste your money on starts or your valuable indoor space on these plants.
Root crops: Root crops like beets, carrots, and radishes generally dislike being transplanted. An exception is bulb onions; a typical growing season isn't long enough to form good-sized bulbs, so starting early indoors and raising them as transplants will improve the harvest.
Peas and Beans: The crows in my neighborhood keep closer track of my planting days than I do, so I have to cover my pea and bean seeds if I ever want to see a sprout outdoors. But barring birds as cagey as the ones I have to deal with, these plants are effortless garden starters, especially if the seeds are pre-soaked and dusted with inoculant.
Greens: Lettuce, kale, and even cabbage are just so easy to grow from seed outdoors; starting them indoors might extend your harvest a week or two, but unless you're competitive about being the first in your neighborhood to harvest, it just doesn't seem worth the effort.
Parsley: This herb is slow to germinate wherever it is planted, so it takes up space on a seed rack for longer than its fair share of time. It also won't germinate if the soil dries out. Since I accidentally let my seed trays dry out at least once every season despite my good intentions to check the trays daily, it is actually easier to grow direct sown outdoors. It's also satisfying to put seeds in the ground early in the spring while temperatures are low and soil is still damp.
Corn: You'll find plenty of people who disagree with me, and no plant sale is complete without six packs of corn seedlings. But I've never had any luck with starts. Even growing corn in a climate that limits me to the earliest varieties available, direct sown is the way to go in my experience.
In most parts of the country, almost everything that's not on the list above will need to be started indoors before transplanting out.
Celery: It's just such a slow grower, hardly anyone will have enough growing days in a season to harvest celery from seed grown outside. In cooler climates like mine, you might actually have better luck overwintering your celery than trying to go from seed to harvest in a single season.
Tomatoes: No matter how friendly your climate, there is never enough time or space for all the tomatoes you want to grow. Go ahead, get a jump on the season by starting them indoors, and be sure to pick fun and interesting varieties you won't find at the garden store to justify your decision.
Hot Season Crops: Plants like squash, cucumbers, melons, peppers, and eggplant are not necessarily slow growers like the celery, but they do need hot days and warm nights to grow outside. In most climates in the U.S., the period during which temperatures stay above 50 degrees isn't long enough to grow these plants from seed to harvest outdoors. These are the plants that all the traditional gardening advice about adding up germination times and counting backwards from last frost dates is written for.
Now you just have to decide which transplants to start indoors yourself and which ones to buy at the local spring plant sale.