Specifically in my garden it is beans and tomatoes for sure. They absolutely need to go into the ground at the end of March in my garden in order for both of them to grow, bloom and produce fruit before the temperatures get too hot for the blooms to form tomatoes or beans. That's a huge problem since we often get those late killing freezes through early May. That's why I use floating row covers. (I barely used them at all this year, but our last freeze was earlier than average and we didn't get those late cold nights.) The later I plant them after the end of March, the less likely I am to get a good harvest. When I don't get beans or tomato pants in the ground until late April or earliest May, my harvest can be 25% or less than what it is when I plant early. Since I am always trying to grow as much as possible, harvest-wise, it kills me to plant late because I know that, unless we get an abnormally cool spring, we won't have a good harvest. As a case in point, in 2011 (remember that horrible year!), I was frantically trying to get my planting done in early April because we had a week-long fire-training course scheduled to be held at Lake Murray, and it was going to fall during my prime planting week---the week leading up to Easter. I didn't get nearly as much planted as I wanted before that week, and that week turned hot with highs in the 90s. There I was, with tiny bean seedlings newly emerged and roasting in 90+ degree heat, and not even half my tomato plants in the ground. We then started hitting the mid- to upper 90s in May. The only tomatoes and beans I got were from blooms that were on the plants before or shortly after Easter. Not every year is that bad, but it is amazing to me that we often have days in the 90s when nights are still continuing to drop down occasionally to the 30s. Nothing is harder on a garden than having temperatures get too hot too quickly. That did not happen this year, but we had over 2' of rain in May and another 15" or so in June, so heat wasn't our issue but standing water (for over two months) in the garden was. Still, most years heat/drought are a much bigger challenge than rainfall. Usually our rainfall challenge is that not enough of it falls in summer.
Sweet corn is almost as challenging. My corn has to be planted (I do three succession plantings, each about 2 weeks apart) beginning in late March and continuing through late April, though mid-March through mid-April is even better in the years when soil temperatures allow Otherwise, if temperatures start hitting 100 in June, as they do some years, then I get pull kernel formation and poor tip fill.
Some plants aren't really bothered by the temperatures and I can plant them later with little fear that it will harm them. Squash is that way. Unfortunately, the later you plant the summer squash, the less time it has available to grow, bloom and produce before squash bugs and squash vine borers arrive, so I like to plant it as early as possible to get ahead of them. I always plant winter squash late on purpose because high temperatures will not interfere in the blooms setting fruit until it is incredibly hot (like in the 105-110 degree range), but I am careful to only plant C. moschata varieties because they are tolerant of squash bug damage and highly resistant to squash vine borer damage.
With peppers, I am very careful not to plant them too early, though I probably do plant them early by most people's standards. With peppers, if they are exposed to cold soil and cold air temperatures early in their lives, they often can remain stunted and produce poorly for months, so if I plant early, they each get a 2-liter bottle of water placed on their north side. The bottle absorbs sunlight and heat during the day, and releases heat as it cools at night, keeping the young plants warm. Sweet bell peppers, which I need to have at the harvest stage in late May through early July when the tomatoes are ripe for salsa-making, drop blooms like tomatoes do when temperatures are in the 90s, so I like to plant them early. I'll plant a handful of jalapeno pepper plants early in order to have jalapenos for salsa when the tomatoes are ready too, but otherwise I hold most of my hot peppers in the greenhouse a couple more weeks and wait for the cool nights to pass.
I never plant the true hot-season crops too early (sweet potatoes, winter squash, okra, southern peas) because they don't like cool soil or cool nights and won't grow well in them.
Every year, I find myself in a frantic race to "beat the heat". Realistically speaking, maybe 2 or 3 years out of 10 we don't get too hot too early, but it is hard to guess which years that will happen, so I always push hard to plant earlier rather than later.
My friend, Fred, who is gardening strong as his 92nd birthday approaches, always pushed me to plant beans and corn in mid-March like he did in my earliest years here, but I didn't like watching crops freeze and then having to replant, so I usually waited until late March. That's worked pretty well for me. He gardens on a slightly higher elevation and doesn't always get those last couple of cold nights that my garden gets. You have to study your location year i and year out and see when it gets too hot for various crops, as well as when it gets warm enough to plant them, and modify your plants annually from there. It has taken me a long time to figure out what works for me in my specific location, and it has taken lots of trial and error.
Q