On a warm Saturday morning in April, the Wimmer family was in the field just north of Alexandria, planting a taste of summer.

You may have seen their bins of sweet corn around town in early- to mid-July, extremely early for locally grown sweet corn in this part of Minnesota. Often, sweet corn isn't sold at farmers markets until August.

The Wimmers can plant early thanks to an old Ford cultivator that Bernice Wimmer's brother modified into a device to easily cover seed rows with clear plastic, creating a greenhouse effect that gives the corn seed a head start.

"It gives us about a two week jump," said her husband, Joe Wimmer. "Everybody's hungry for corn then."

First they plant seed in furrows separated by a ridge of dirt. Then a tractor pulls the modified cultivator over the seed beds. Wheels press the plastic into the dirt, while a smaller wheel tucks the edge in and blades scoop dirt along the sides to keep the plastic from flying off in the wind. Family members shovel dirt over the ends of the sheets, while John, 19, rides on the machinery.

Creating the ridge of dirt between rows creates a parabolic curve similar to an airplane wing, so that the wind slides over the plastic instead of shredding it. It was one of the many things they figured out as they went along.

The temperature beneath the plastic is about 20 degrees warmer than the air temperature. They leave it on until the plants get about 12-18 inches high. Then it's a group effort to remove them. By that time, the soil is so wet, Bernice said, that family members get covered with mud.

April is not the earliest they have planted sweet corn. They have planted in March before, but wished they hadn't because they had to remove the plastic and then got another spell of cold weather.

They also have to make sure they don't leave the plastic on too late, or the plants will cook.

One year John, then 3, came home from visiting the corn and said, "It's great mom, but some of it is sleeping."

Laughing ruefully, Bernice recalled, "It was sleeping — permanently."

The Wimmers began growing sweet corn in the early 2000s as a way to keep their four kids busy in the summer.

"No way was I going to let them watch TV all summer," said Bernice, who grew up growing sweet corn in southern Minnesota along with her 17 siblings.

Their kids are now ages 19 to 30, and they have been able to use their sweet corn money to pay as much as 40% of their college tuition, she said.

The earliest they've sold sweet corn was July 6, she said. They provide corn for local events, which may not even be held this year because of COVID-19, as well as the fairgrounds and outside several local businesses. They also grow asparagus.

The Wimmers don't farm full-time. Bernice works at the Alexandria Senior Center, although she is now laid off since it is closed, and Joe manages several Pro-Ag locations. The brother who modified the cultivator, Larry Ravenhorst, is an agricultural engineer who has also developed machinery and tools to help Haitians harvest breadfruit, she said.