Soil temperatures in Oklahoma can be hot when planting in late August to early September for forage-only or dual-purpose wheat (Figure 1). Seed that was planted into soils with temperatures above 85° F may result in delayed germination or prevent wheat seedling emergence. In addition to the soil moisture status since planting, listed below are two factors that may cause poor early stand establishment when wheat is sown into hot soils.
Figure 1. Maximum soil temperature at a 4 inch depth under bare soil over the past three weeks near Altus. We can assume that the maximum soil temperature at shallower depths was likely higher. Data is from Oklahoma Mesonet.
High temperature germination sensitivity: This is a more elaborate way of saying that some wheat varieties do not germinate well in hot soil conditions. This is not to say that the seed will not germinate at all, but it may not germinate until the soil temperature has lowered. Keep in mind too that this sensitivity can vary from year to year. For example, a sensitive variety like Ruby Lee may germinate fine in 90° F soils one year and only produce a 10% stand in the same soil conditions the next. When sowing early, it is best to plant varieties first that do not have high germination sensitivity (e.g., Duster, Gallagher). Soil temperatures typically begin to cool by about September 20 due to lower air temperatures and/or rainfall events. Waiting until at least mid September to plant sensitive varieties can help reduce the risk of this issue. A rating of high temperature germination sensitivity for wheat varieties can be found in the OSU Fact Sheet PSS-2256 Factors Affecting Wheat Germination and Stand Establishment in Hot Soils.
Coleoptile length: The coleoptile is the rigid, sheath-like structure which protects the first true leaf and aids it in navigating and reaching the soil surface. Once the coleoptile breaks the soil surface, it will stop growing, and the first true leaf will emerge. If the coleoptile fails to reach the soil surface, the first true leaf will emerge below ground and usually takes on an accordion-like appearance (Figure 2A-B). If this happens, the plant will die.
Figure 2A and 2B. Example of two different wheat seedlings in which the coleoptile failed to break the soil surface. The first true leaf emerged below the soil surface and resulted in this accordion-like appearance.
The coleoptile length for most wheat varieties today can allow for the seed to be safely planted up to 1.5 inches deep. Under hot soil conditions though, the coleoptile length tends to be decreased. Therefore, “dusting in” early-sown wheat at ¾ to 1 inch depth and waiting on a rain event may result in more uniform emergence than trying to plant into soil moisture at a deeper depth if soil moisture is not available in the top 1 to 1.5 inches of the soil profile. A rating for coleoptile length for wheat varieties can be found in the OSU Fact Sheet PSS-2142 Wheat Variety Comparison (P.S. we are working on updating this).