“To overcome this dormancy, plants require a period of exposure to low temperatures. The process is called vernalization and results in the formation of flower heads. Wheat head size, or the number of spikelets per head, is determined when the vernalization requirements are met,” she says.

 In the Southeastern United States, most winter wheat varieties require three to seven weeks of vernalization, according to Ortiz. To vernalize, early maturing varieties require less time (less exposure to cold temperatures) than do later-maturing varieties.

Both warm and cold weather conditions can affect wheat growth development, she says. Prolonged warm winter weather can cause wheat to lose some accumulated vernalization units. In such weather, varieties with longer vernalization requirements (approximately six weeks) will not fully vernalize.

“A wheat variety receiving only a partial amount of cold weather required for vernalization either will produce few heads — between 1 and 10 percent — or it will produce full heads, but mature when the weather is very hot, resulting in poor yield and test weight,” she says.

In the Southeast, maximum vernalization occurs around 40 degrees F. An ideal vernalization day would have a nighttime low of around 38 degrees F. However, vernalization occurs even if maximum temperatures on a cold day are above 35 degrees F. and nighttime low temperatures are below 50 degrees F.

The required length of low-temperature exposure decreases with colder temperatures and advanced plant development.

Day length — the number of hours between sunrise and sunset — also can affect the length of time required to reach flowering and heading stages, says Ortiz. Variety selection should closely tie to a day length’s anticipated effect on heading time.

There are some other special issues to consider with day length, she says.

“For example, day length interacts with vernalization and heat units, which makes it difficult to predict its effect on wheat development, especially during mild winters. Also, day length–sensitive wheat varieties are long-day plants, which means that flowering is induced by longer days. The longer the days, the fewer heat units are required to initiate flowering.”

When a long-day wheat variety is exposed to above-average temperatures, early flowering and a short grain-filling period may occur, she cautions. This might result in low yield and reduced grain quality.

Late planting may cause yield losses, especially on medium and late-maturing varieties, says Ortiz. Moreover, late-planted wheat has less time to tiller during the fall.

“Most tiller development occurs in the spring with late planting of late-maturing varieties, though spring tillers contribute less to yield potential than fall tillers. Late planting also may shorten the grain-filling period and delay it until the weather is warmer.”

By contrast, early planting causes excessive tillering during fall and spring, which increases the risk for spring freeze injury, she says. “Excessive growth also may cause wheat to grow taller in the spring, which promotes lodging. The impact of planting date on final yield varies by location in Alabama.”

Data from an Alabama field study conducted between 2010 and 2012 demonstrated the negative effects of delayed planting on wheat yield.

Three wheat varieties, AGS 2060 (early maturity), AGS 2035 (medium maturity), and Baldwin (late maturity) were planted at different planting dates, each one 15 days apart, in north, central and south Alabama.