Sweet corn is a popular, warm-weather crop that rewards the gardener with a taste of summer. Cultivating it successfully in Wisconsin requires careful attention to timing and technique. The state’s short growing season and unpredictable spring weather demand a localized approach for optimal results. This guide focuses on the best time to sow seeds and the methods that ensure a plentiful harvest.
Determining the Earliest Planting Window
The calendar date offers only a rough suggestion for when to begin planting sweet corn in Wisconsin. Gardeners must first consider the region’s average last spring frost date, which typically ranges from mid-May in the south to early June in the northern areas. Sweet corn is a tender crop, and its seedlings are highly susceptible to damage or death from frost, establishing this as the hard limit for early sowing.
Soil temperature is a more precise indicator than the calendar. Sweet corn seeds require consistently warm soil to germinate successfully and avoid rotting. While germination can occur at a minimum of 50°F, the ideal temperature for robust emergence is 60°F or higher, especially for supersweet (sh2) varieties. Planting below 50°F can cause imbibitional chilling, which damages the seed during initial water uptake and results in poor stand establishment.
To encourage warmer soil temperatures earlier, gardeners can employ simple techniques. Covering the planting area with black plastic sheeting for several weeks helps absorb solar radiation and significantly warms the soil. Regularly checking the soil temperature with a thermometer, usually in the early morning at a depth of one to two inches, provides the confirmation needed to proceed. Waiting for this optimal soil warmth minimizes the risk of poor germination.
Planting Techniques and Depth
Once the soil temperature is adequate, sweet corn seeds should be planted using specific guidelines. The standard method is direct sowing into the garden bed at a depth of approximately one to one-and-a-half inches. Planting too shallowly can cause the seed to dry out, while planting too deeply, particularly in cooler soil, can delay or prevent emergence.
Proper spacing and arrangement are crucial because corn is wind-pollinated, relying on pollen blowing from the tassels to the silks. Planting in long, single rows is highly discouraged, as much of the pollen may blow away, leading to ears with missing kernels. Instead, sweet corn should be sown in a “block” formation, consisting of several short rows, ideally four or more rows wide.
Within these blocks, seeds should be spaced about 8 to 12 inches apart in the row, with rows separated by 24 to 30 inches. This dense arrangement maximizes the chance of successful pollination. While transplants can be used to gain a slight head start, sweet corn does not tolerate root disturbance well, so starting seeds in biodegradable pots or large cells is recommended.
Extending the Harvest Through Succession Planting
To avoid having a large crop mature all at once, gardeners can implement succession planting to enjoy fresh sweet corn over a longer period. Succession planting involves scheduling multiple, smaller plantings throughout the season. In Wisconsin, this typically means sowing a new batch of seeds every 10 to 14 days following the initial planting.
This process can continue until early July, allowing enough time for the final planting to reach maturity before the first fall frost. An alternative strategy is to plant varieties with different Days to Maturity (DTM) simultaneously. For example, planting an early-maturing variety (around 65 days), a mid-season variety (around 75 days), and a late-season variety (around 85 days) on the same day will naturally stagger the harvest window.
When using this mixed-variety approach, it is important to check if the varieties are of the same genetic type, such as standard sugary or supersweet. Planting different genetic types that tassel at the same time can cause cross-pollination, which may reduce the desired sweetness and flavor of the more advanced varieties. Strategic variety selection and staggered planting dates ensure a continuous supply of sweet corn from mid-summer until the end of the growing season.