Joel Rosen has operated Park Lake Farm near Mahotwa for many years and offers this advice for local gardeners:
In the past few weeks I've heard quite a few on-air conversations about when people are planting and/or transplanting vegetables in their gardens. Below are some guidelines that some listeners might find helpful. Feel free to forward to anyone who expresses an interest.
Generally, planting and transplanting dates depend on soil temperature, hardiness of species, vigor of seed, and in some cases average last frost date. First class of vegetables includes greens that are quite frost hardy--spinach, mustard, arugula, lettuce, endive, etc. These can be safely seeded anytime in spring the ground is dry enough to work. However, even these cool weather greens will be very slow to germinate in soil temperatures below 40F. For most areas of northern Minnesota, soil temps will be above 40F sometime in early May (but of course, not every year--we are still looking at 9am soil temperatures at the 4" depth in the upper 30's this week) By the time these greens come up as seedlings, they should be hardy enough to withstand the low temperatures likely to occur--for this class down to about 18F.
Another class of small seeds is just as frost-hardy, but requires a higher soil temperature to germinate: peas, onions (from seed) and their relatives, beets, carrots, and parsnips. You can safely plant all of these except peas as early as the greens, but the additional time they take to come up will allow cool weather weeds to create a green carpet that can smother your emerging seedlings. In the case of peas, cold, wet soil may cause seed rot, especially in less vigorous varieties like Sugar Snap. Generally, this group is most successfully seeded about a week to 10 days after the early greens (May 10-15 on average). Beets will be first to come up (6-7 days in favorable weather), followed by peas and onion family (8-10), carrots (12-16), and parsnips (16-20).
About the same time as this group, a small planting of seed potatoes for early new potatoes can be safely set in the ground. These should not be planted as deeply as later main-crop potatoes as the cooler soil at lower depths will delay emergence--1-2" is probably best. Once potatoes come up, (2-3 weeks at this time of year) they are somewhat frost sensitive, but they will usually tolerate an eye level temperature of about 28F if they are still only a couple of inches above the ground which will retain some warmth on a cold night. If potatoes do get burned by hard frost in June, they will usually just grow back with little ill effect. I wouldn't advise putting the main crop of potatoes in this early, as there is some risk and little advantage to be gained. Potatoes that mature at the end of the growing season will usually store better over the winter.
Following these first two groups, there is a significant time lag before other things can be safely planted from seed. The main consideration is average or last expected frost date, but soil temperature and moisture are also critical in some cases. Sweet corn is a good example. While sweet corn seedlings will withstand a light frost, they should not be planted until the soil is warm and dry. If you're anxious to plant sweet corn while the calendar still says May, be certain that weather anticipated for the next seven days is warm and relatively dry. Most sweet corn seed, especially the sweeter varieties, will not germinate until the soil temps approach 60F. In cool, wet conditions, untreated seed will rot. Over the years I have lost lots more sweet corn seed due to cold soil than due to June frost. In an average year, I plant sweet corn on June 6th, but some years the soil is still too cool and damp on this date.
Bean seed is less susceptible to seed rot, so if you're anxious to eat beans early, they can be safely planted about a week before the last average frost date.
The vines, including summer squash, winter squash, pumpkins, cucumbers, and melons, are most successfully grown from transplants, but they can be direct seeded a few days before the last average frost date. Squash and pumpkins will usually survive a brief, light frost, but cucumbers and melons are among the most frost sensitive crops in the garden, so it's advisable to separate planting dates by 4-7 days.
Transplants are a somewhat different matter, as these are going to be exposed to wind and cold temperatures in the first 24 hours. Generally, these will fall into three categories. The Brassica family (broccoli, cauliflower, cabbage, Brussels sprouts, etc) are quite frost hardy and can usually be safely transplanted after the last expected date for temperatures falling below 20F. Of course, seedlings should be healthy and should not be set out on a day that is excessively windy or has an anticipated nighttime temperature approaching the 20F mark as they are most vulnerable before they establish root system.
A group of transplants that is reasonably frost hardy but may suffer premature bolting if set out in cool spring weather includes celery, celeriac, and Swiss chard (also some varieties of broccoli may sometimes "button up" if temperatures are too cool for an extended period of time after setting out). Best to wait until late May-early June to avoid these problems.
Obviously frost-sensitive transplants should not be set out until at least close to the average last frost date and a forecast of warmer than average weather for the first few days. Some transplants like tomatoes, peppers, pumpkins, and squash will tolerate a brief, light frost if healthy and rooted into the ground, but cucumbers, melons, and eggplants will probably be a total loss.
Most sensitive of all is basil--a hint of frost will kill a basil transplant, and even an extended period of chilly weather without frost will damage basil enough to reduce production later in the summer. In our Lake Superior zone, I wouldn't set basil transplants outdoors until late June and only with a good 3-day forecast. For those who are unsure of their average last frost date, the MN DNR has a nice map which can be accessed through the climatology link on their website under agricultural climate information---agricultural data tables and maps (http://climate.umn.edu) . This map shows by color the range of dates for which the included area will have only a 10% chance for late spring frost.
Counting back about a week to 10 days will generally give the average last frost date. For example, we are in the zone dated June12-18, meaning we have only a 10% chance of frost this late. Over the past 21 years we have experienced frost 3 years after the 12th, so this seems to be a pretty good guide. Our average last frost date is about June 5, so counting back 7-10 days matches up pretty well.
Of course, none of these maps show microclimates or account for soil types, ranging from sandy to heavy clay, nd exposure (full versus partial sun, southern versus northern slope). These factors can affect safe and/or smart planting dates by as much as 3 weeks. A good soil thermometer and some accurate recordkeeping is really the best approach.