There may not be such a thing as foolproof planting, but knowing what your plants need can help you know when to put them in the ground.
When you're trying to figure out when to plant your vegetables, ask yourself two questions:
• Are you a cautious person?
• Do you believe in the power of the stars?
Your answers may determine when your plants will meet the soil. Read on to find out why.
Not all about frost
Vegetables can be planted as a seed or as a plug. In either case, you need to time the planting so that vegetable plants will reach their maximum potential. Plant too early and a spring frost might get them. Plant too late and a fall frost might get them. But frost isn't the only factor that determines when you should plant. You must also take soil temperature into consideration.
If the soil is too cool when seeds are planted, cold water can get into the seed and wash out the nutrients that the seed has stored to fuel its growth. Without some of those nutrients, the seed will be less likely to germinate. Of course, different seeds have different temperature requirements. For example, carrot seeds can handle soil temperatures around 50 degrees; for green beans soil should be at least 60; tomatoes need 70 degrees, and peppers prefer at least 80 degrees.
(For more information on the soil temps that seeds prefer, go to the online catalog for Johnny's Selected Seeds at www.johnnyseeds.com/default.aspx.)
Because the seeds of tomatoes and peppers need such high soil temperatures, most Minnesotans start their own seeds indoors or buy plugs (small plants) rather than sow these seeds directly into the ground.
Of course, the roots of plugs are sensitive to low temperatures, too. But Tom Michaels, a horticulture professor at the University of Minnesota, estimates that you can plant most plugs when soil temperatures are 10 to 15 degrees lower than if you were planting the same plant from seed.
To test soil temperature, pick up a soil thermometer at a garden center for about $10.
Calendars, moons, odds
If you're at all cautious, planting warm-weather veggies on Memorial Day is a pretty safe bet.
Soil temperatures should be fine for almost all plugs, as well as most seeds. And the risk of us getting frost after this day (May 31 this year) is low. But you don't have to plant by the calendar alone.
My friend Meleah Maynard, a master gardener and local garden writer, has been studying lunar planting. According to this practice, plants that produce their edible parts above ground should be planted while the moon is waxing (or getting bigger). Plants with edible parts below ground should be planted while the moon is waning. Nothing should be planted on the first day of the new moon and flowers should be planted during the moon's first quarter. This year, if you planted broccoli by the moon cycles, you would have planted on May 12 -- and missed the frost we got on May 8. If you want to use lunar planting, you need to use some common sense, as well. Planting anything outside during a waning moon in February isn't going to work.
If you prefer to plant by percentages, the National Environmental Satellite, Data and Information service has a handy chart (available at www.ncdc.noaa.gov/oa/climate/freezefrost/frostfreemaps.html) that lets you know the date at which there is a 10 percent chance of temperatures dropping below 28 degrees, the temperature at which vegetables sustain damage. It differs across Minnesota, but here in the Twin Cities that date is between May 1 and May 15. So if you waited until May 15 to plant, there is a less than 10 percent chance that a frost will kill your plants.
Planting is always a bit of a gamble, whether you go by the calendar, look to the skies or play the percentages. But with homemade salsa on the line, you win big when you get it right.
Jeff Gillman is an associate professor of horticulture at the University of Minnesota. He's also the author of three books, "How Trees Die," "The Truth About Garden Remedies" and "The Truth About Organic Gardening" (Timber Press, $12.95).