Sorghum is a versatile grain and forage crop, recognized for its tolerance to heat and drought, making it a valuable choice in warm climates. Planting timing is the single most important factor for a successful harvest. For optimal growth and yield, sorghum requires specific environmental conditions, especially a warm soil environment, to begin its life cycle. This timing must be managed to ensure the plant matures before the first frost.
The Critical Role of Soil Temperature
The decision to plant sorghum is primarily governed by soil temperature, not the calendar date. Agronomists recommend waiting until the minimum daily soil temperature reaches at least 60 degrees Fahrenheit (15.5 degrees Celsius) at the planting depth. The ideal temperature for rapid emergence is closer to 68 to 70 degrees Fahrenheit (20 to 21 degrees Celsius) and rising. Planting below this minimum threshold will significantly slow down germination.
When soil temperatures are too cool, emergence can be delayed by two to three weeks, exposing vulnerable seedlings to soil-borne pathogens and pests. This delay often results in poor stand establishment, uneven emergence, and a weakened population susceptible to disease. To accurately gauge conditions, the soil temperature should be measured at the intended planting depth (typically 1 to 2 inches) at the same time each morning. This morning measurement provides the lowest daily temperature the seed will experience, offering the safest planting benchmark.
Timing Based on Sorghum Type and Intended Use
While all sorghum types share the same minimum soil temperature requirement, the ideal window is influenced by the product’s final use. Grain sorghum, harvested for its seed, requires a long enough season to achieve full maturity before the autumn frost. Selecting a hybrid with appropriate maturity is necessary to ensure the grain reaches the black layer stage, indicating physiological maturity, in time.
Forage sorghum is often planted later than grain types, sometimes used for double-cropping after a winter or spring crop. Its purpose is to produce high volumes of biomass for silage or grazing, allowing for a more flexible planting date, even into July in some areas, provided sufficient heat units remain for substantial growth. Sweet sorghum, cultivated for its high sugar content in the stalks, needs the longest consistent warm season to develop maximum sugar before harvest. This type benefits from planting earlier in the optimal window to maximize its vegetative growth period.
Adjusting Planting Windows for Climate and Region
The optimal calendar window for planting sorghum varies dramatically across geographical locations. In the Southern United States, planting often begins as early as mid-April, as soil temperatures warm up sooner, allowing for a long growing season. Moving north into the Upper Midwest, the safe planting window is compressed, typically shifting to late May or even mid-June.
Planting should occur well after the region’s last expected frost date, as even a light frost can damage young, emerging seedlings. The concept of Growing Degree Days (GDD) plays a role in regional planning, as it is a measure of accumulated heat the plant needs to develop. Growers use GDD totals to estimate when a specific hybrid will reach maturity in their climate.
The goal is to time planting so that the critical flowering stage occurs before the hottest, driest part of the summer in areas prone to high heat. This strategy minimizes the risk of heat stress during pollination, which can severely reduce the final grain yield. Planting dates must be a compromise between getting the crop in early enough to mature and avoiding cold stress on the seedlings.
Consequences of Planting Too Early or Too Late
Planting sorghum too early, before the soil temperature has adequately warmed, carries several significant risks. Slow emergence in cold soil increases the vulnerability of the seed and seedling to soil-borne diseases and fungal infections. Cold stress also stunts early growth, leaving seedlings exposed to increased insect damage, such as from chinch bugs, which thrive on stressed plants.
Conversely, planting too late can drastically reduce the crop’s yield potential. A late start shortens the growing season, which may prevent the plant from reaching full maturity before the first fall frost. If frost occurs when the grain is still in the soft dough stage, the yield loss can be severe, resulting in shriveled kernels and low test weight. Late-planted sorghum is also more susceptible to drought stress during the critical flowering period, often exposed to the hottest and driest conditions of late summer.