I ran this post last year, just after a late frost in the middle of May. It occurs to me that running it a bit earlier this year might prevent some over-eager gardeners out there from planting too early and risking damage to their tomatoes and other heat-loving plants.
Here I am gardening in hardiness zone 6B. Depending on whom you believe, our average last frost date is either May 5th or May 10th. It's May 19th and there's frost on the grass outside as I type this. I watched my neighbor cover up his tomato seedlings yesterday evening, as I brought my tender heat-loving seedlings indoors for the night. I'm not going to gloat. I hope his seedlings made it. He built some beautiful wooden trellises this year to support his plants. But I am going to take this opportunity to throw my own piece of two-bit advice into the marketplace of gardening ideas.
Don't plant your tomatoes as soon as your last frost date has passed. Don't even plant at a certain amount of time after that date. I'm not saying this simply because of the chance that a late frost could surprise you. The thing I watch for is the day when I can be reasonably certain that overnight temperatures will no longer fall below 50 F (10 C). I know this will come at least a week or two after our average last frost date, and possibly three weeks later or more. I watch the five-day weather forecast, and I won't plant until I see five solid days of overnight temps above 50 degrees. If the predicted overnight low a few days out is exactly 50, I wait. If I see an overnight temperature of 51 or 52, I might plant with a row cover. But mostly I wait.
Why do I use this temperature as a guideline? Because tomatoes are essentially tropical plants. Yes, they've been bred to survive in our northern climes. They'll live through 40 degree nights. But they won't thrive. In my experience, any tomato fruit which has ever been exposed to temperatures below 50 degrees will never develop a good tomato flavor. Even if that fruit is a tiny green bud that has just shed its blossom. And what are we growing our own tomatoes for if not for superb, better-than-candy flavor? Tomatoes need heat and are completely allergic to cold, especially at the beginning of the season. Other gardeners get earlier tomatoes than I do. But the fruits aren't worth a damn, in my opinion.
Even my earliest fruits don't compare to those that mature during the three golden weeks of August. Those fruits probably never know temperatures below 60 or even 65 F. Now those are tomatoes worth eating. They're also worth waiting for. That flavor is the reason why I gave up buying fresh tomatoes at the grocery store, no matter what it says on the sign. "Vine-ripened," "hothouse," - whatever; I don't care. Once you've tasted a real tomato, you won't see the point of eating those red globs of cardboard that are offered for sale 52 weeks out of the year.
All of this goes doubly for pepper plants. They are even more heat-loving than tomatoes. As a rule of thumb, I aim to have my tomatoes in the ground on June 1st, and my peppers in the ground in mid- to late June. That's what works for me in my area anyway. Just don't let your tomato or pepper seedlings get rootbound as you are waiting for the temperature to cooperate. Repot them in larger containers if you need to. Rootbound plants seldom recover to do well in the ground.
I'm sure that seasoned gardeners have their own habits and preferences based on long experience, and I don't intend to start any argument. Direct experience is the best teacher, and it should not be lightly set aside on any authority. My recommendation is offered however to novice gardeners, and I understand that there are many people taking up gardening for the first time this year, due to our lousy economy. It's very easy to get discouraged when our first attempts at any new enterprise fail. So if you're new to gardening, don't necessarily follow what your neighbors are doing. Gardeners can be notoriously optimistic about frosts and planting dates. Everyone is dieing to get that first homegrown tomato. You may do better to wait it out with your heat-loving plants.
Gardening can be an extremely frugal hobby. But it's only frugal if you manage to keep your plants reasonably happy and productive. Losing plants you've grown from seed is absolutely heartbreaking. So take rules of thumb about planting dates with a grain of salt, and beware those late frosts!