I'm a huge advocate of starting some of my frost-tolerant crops very early. This isn't necessarily the best practice for all gardeners or farmers, but there are some practical reasons for group gardens to get a jump on the calendar for early crops, some of which will be harvested early to be replaced with another crop for succession planting. These are all the frost-tolerant crops mentioned in other garden planning postings, such as peas, fava beans, radishes, arugula, turnips, spinach, etc. As long as the garden is being planted, I would also start the frost-tolerant, full season crops like kale, collards, chard, beets, etc.
Here, I'm only referring to planting in the open with no row covers or hoop houses. Using those for season extension is another subject that I'll discus in the future.
My opinion on early sowing is based more on human scheduling complications, rather than science, climate or gardening. While many gardeners or farmers in NYC will start their frost-tolerant early direct sowing around April 10 or 15, I like to start during the last week of March and have even started as early as March 15. I just see more upside and little downside in doing so.
When working with groups of volunteers or classes at a school garden, sometimes the scheduling is based on peoples' availability, not the outdoor climate. If we scheduled the initial planting for April 15, and any given delay was to occur, the planting may not happen for another week or two. Some of my gardens are only worked on during the weekends, and some of the school gardens have classes that follow a strict day-of-the week schedule. There could be rain outs, Spring Break, field trips, school assemblies, etc., so rather than risk a late start, I would err towards early. New York City's public schools schedule their Spring Break around Easter, so I always try to schedule some of the planting before, and then some after the break. If a few classes don't get outside until mid-April, most of the garden is already planted, so we're off to a good start. If a school garden is ran by teachers that can just plant when the weather is fine, I would recommend a later planting date, but the gardens that I run rely on an outside coordinator (me) with specific times scheduled. I hope these non-gardening complexities are clear(?).
Another upside to early sowing is that if we happen to have an unseasonably warm early-April, our crops will get a great head start. If we have a cold April, they'll probably linger and then not grow any quicker than the crops sown in mid-April. But, so what...? We shouldn't be any worse off than the later starters. I personally have had no crop losses due to early sowing. The only danger that I could foresee is that if it's unseasonably warm at germination time, and then after most seedlings have germinated, there's a small snow storm that crushes everything. It's not the frost, but the snow that I worry about. Or, very, very cold frosts, not merely dipping a little below 32. But, the typical scenario would be very little germination during the first week or so, and then by the time the seeds are emerging, it's already a bit past the risky dates. Those same risks are why I would not sow these seeds during a warm early March. The likelihood of a crop failure greatly increases if we start earlier.
Still, my opinion here is evolving. This is just how I have been doing things during the past few years. This March, two of my schools began sowing shortly before or after a small snow storm. In years past, another one of my gardens did all the early sowing in late March when the thermometer registered 28 degrees.These seeds didn't actually germinate until it was warmer, but we suffered zero casualties and had no regrets working on a sunny but cold (and thankfully windless) day. Just not a huge head start, but good to get the work done and over with.
Just to add a contrary opinion... I was told by one organic farming guru from Western Massachusetts, that we should start a bit later because the plants do better when they get off to a quick and strong start, rather than lingering. I'm still deciding whether that's important for smaller scale, non-money making operations, or if urban gardeners (but not farmers) should follow the schedule that I advocate, just to stretch our scarce land a bit more. For now, I still believe that there's little to no downside in getting things started early.