A bummer summer for gardeners could come to an abrupt end Wednesday night, with the Twin Cities and much of southern and central Minnesota under a freeze warning.

That's a freeze, not a frost, as in limp black basil and cold-scarred tomatoes that rot instead of ripen. And it could happen with more than a week of summer left on the calendar.

"Oh, that's a bummer," said Mary Meyer, University of Minnesota professor and extension horticulturist. "People who are first-time vegetable gardeners, they need a couple of breaks. This has been a challenging year."

If it does freeze, it would happen more than three weeks earlier than the median freeze date of Oct. 7 at Minneapolis-St. Paul International Airport and more than two weeks ahead of Chaska's Oct. 2.

Wednesday night will have perfect conditions for a freeze that could extend from Alexandria and St. Cloud to Red Wing, Albert Lea, Mankato, Redwood Falls, Morris and Mora, National Weather Service forecaster Tony Zaleski said. The Twin Cities area is included in that zone.

A ridge of high pressure with dry conditions and very light winds and no cloud cover "create a perfect frozen storm," Zaleski said. Though Minneapolis and St. Paul may dip only to about 36 degrees, he said, the suburbs around them "will be close to freezing and the outlying suburbs will be freezing." The freeze warning runs from midnight Wednesday until 8 a.m. Thursday.

Meyer noted that the early cold coincides with a waning full moon, which she said can cast enough heat to clear the sky of clouds.

"This and the springtime are really the only times a full moon can give us a hard time," she said.

Gardeners who want to save warm-weather crops should choose the plants that have fruit that will soon ripen, Meyer said. It's probably best to harvest delicate basil, but tomatoes, peppers and eggplants can be protected with a sheet or fleece blanket.

Cold-weather plants such as lettuce, bok choy and Swiss chard will be fine, and vegetables like Brussels sprouts actually like a touch of the cold.

Coleus, begonias, impatiens and other tender annual plants should be covered with sheets. Plastic does not make a good shield for plants because it doesn't breathe, can collect condensation and can blow off, Meyer said.

Zaleski said temperatures should moderate after Wednesday night. That's just fine with Meyer.

"This is a little early," she said. "I would like to see [gardening] going through at least October 1."

Mary Jane Smetanka • 612-673-7380