Sweet corn is an annual crop cultivated for its high sugar content, which provides a distinct sweetness compared to field corn. The timing of harvest is crucial because sweet corn must be picked at a specific, immature stage before its sugars rapidly convert to starch. Depending on the variety and environmental conditions, sweet corn typically reaches maturity within 60 to 100 days from planting.
Sweet Corn Varieties and Expected Maturity Times
The genetic makeup of the sweet corn variety planted is the primary factor determining the expected maturity timeline. Plant breeders categorize varieties based on their specific sugar genes, which dictate the sweetness level and the duration of the harvest window. These timelines are estimates provided on seed packets, assuming ideal growing conditions throughout the season.
The Standard Sugary (su) varieties, the oldest type, generally mature within 60 to 85 days. Since they convert sugar to starch quickly after harvest, they are best consumed within 24 to 48 hours of being picked. Sugar Enhanced (se) varieties offer a mid-range maturity, typically requiring 75 to 90 days. They contain more sugar and maintain their sweetness longer than su types, providing a slightly extended harvest period.
Supersweet (sh2) varieties, characterized by shrunken kernels and high sugar content, often mature in a range similar to se types, from 66 to 85 days. The sh2 gene requires warmer soil temperatures for successful germination, often necessitating a later planting date. These varieties are highly valued because their sugar converts to starch at a significantly slower rate, allowing for several days of storage without major quality loss.
How Temperature and Environment Influence the Timeline
The calendar days listed on a seed packet are a guide, but development is highly dependent on accumulated heat. Plant scientists use Growing Degree Days (GDDs) or heat units to accurately track the plant’s progression, as growth is driven by temperature. GDDs are calculated daily based on air temperature, using a base of 50°F and a cap of 86°F, since growth stops below 50°F and slows significantly above 86°F.
Initial growth is especially sensitive to soil temperature. While some varieties germinate just above 50°F, the supersweet (sh2) types prefer a minimum of 65°F for reliable emergence; planting too early can cause poor stands. Once established, the cumulative ambient air temperature dictates the speed of development. A summer with sustained, moderate heat brings corn to harvest on schedule, while a cool summer can easily add 10 to 20 days to the expected maturity date.
Water availability, particularly during the reproductive stages, also heavily influences the final timeline and kernel quality. During the silking and kernel fill stages, adequate moisture is required for successful pollination and to ensure the kernels plump up properly. Drought or severe moisture stress during this period can stall development, reduce kernel size, and ultimately impact the final harvest date and quality of the ear.
Recognizing the Signs of Ripe Sweet Corn
Regardless of calendar days or accumulated heat units, the final confirmation of readiness must come from physical indicators on the ear itself. The first sign is the browning and drying of the silks emerging from the tip of the ear. Green or wet silks indicate that the kernels are still developing and have not completed pollination.
The ear should feel firm and full when gently squeezed through the husk, with a rounded, blunt tip indicating the kernels have filled out to the end of the cob. The most reliable method to confirm readiness is the “milk stage” test, performed on a single ear. This involves gently peeling back a small section of the husk and puncturing a kernel with a fingernail.
If the liquid that squirts out is thin and clear, the corn is still underripe and needs a few more days to accumulate sugar. The corn is at its peak of sweetness and tenderness if the liquid is milky and translucent. If the liquid is thick, starchy, or pasty, the ear is past its prime, meaning the sugars have begun to convert into starch. This peak harvest window is short, often lasting only a few days, and typically occurs 18 to 24 days after the silks first appear.