HOW LATE CAN I PLANT WINTER WHEAT?
Using weather data to predict vernalization.
Ernie Marx, Russ Karow Oregon State University
What is vernalization?
Some plants will not flower until after they have been exposed to cold temperatures. This exposure to cold temperatures, resulting in physiological changes in the plant, is called vernalization. Not all plants have a vernalization requirement, and the degree of vernalization required can vary within a species.
What are "vernalization days"?
Vernalization days (VD) are a measure of cold temperature, similar to growing degree days (GDD) as a measure of warm temperatures. Contrary to popular belief, the best vernalization temperatures are in the 40 F - 50 F range, not at colder temperatures. Vernalization is a biological process and plants have to be biologically active - not frozen - for it to occur.
Does wheat have a vernalization requirement?
Winter wheat does have a vernalization requirement. Most spring wheats do not, or have very low vernalization requirements.
Do all winter wheats have the same vernalization requirement?
No. The table below groups varieties with low, intermediate, and high vernalization requirements. Cultivars with a higher vernalization requirement need more exposure to cold temperatures. Most of the current varieties commonly grown in Oregon have intermediate vernalization requirements.
Low vernalization requirement:
Whitman triticale, Hoff, Hyak, Oveson,
Intermediate vernalization requirement:
Gene, MacVicar, Madsen, Malcolm, Rod, Stephens, W301
High vernalization requirement:
Eltan, Hill 81, Kmor, Rohde, Yamhill
How late can I plant a winter wheat?
Assuming satisfactory field conditions, you can safely plant any winter wheat before January 1 in all regions of Oregon with minimal risk. After January 1, the answer depends on where you are, how much yield you are willing to lose, and how much risk you are comfortable with.
To have a 50-50 chance of reaching 70% of your normal yield, you need at least 70 VD, regardless of whether you are growing a variety with a high or low vernalization requirement. There is a difference, however, in the risk associated with high or low vernalization cultivars. Suppose, due to warmer than average weather, you fall short of the required 70VD. Yield reductions for the high vernalization varieties can be extreme. It is very possible that much of the crop will not head, and the crop could be a total loss. With a low vernalization variety, however, chances are good you will still get 40-50% of your normal yield. Varieties such as Stephens, Madsen, and Rod are between these extremes, meaning you can expect large yield reductions but not a total crop failure if warm weather keeps the crop below 70 VD.
Figure 1 is a map of Oregon with the latest planting dates for which 70 VD can be expected. These dates are based on 30 year averages (1961-1990). These are the latest dates winter wheat should be planted. The later you plant, the greater the risk.
Figure 1. Latest planting dates for which 70 vernalization days can be expected for location in Oregon. Also shown are spring wheat yields as a percentage of winter wheat yields. Yield data is from three years (1995-1997) of the Oregon Statewide Variety Testing Program.
When should I plant a spring variety instead of a winter variety? In general, you should probably consider switching to a spring variety for any planting after January 1. The risk is lower and yields will usually be as good or better than a late-planted winter variety.
Spring wheat as a percent of winter wheat yields are shown on Figure 1. Notice that spring yields are at or above 70 percent of winter yields for most locations. Data is from four years of the Oregon Statewide Variety Testing program.
The risk associated with planting a spring crop is usually lower than for a late-planted winter crop. The primary risk factor for spring wheat is low rainfall. Earlier planting dates are less risky. So if field conditions allow you to plant after January 1, consider an early planting of a spring variety instead of a late planting of a winter variety.
Last updated Dec. 21, 1998.
Crop & Soil Science WWW, Oregon State University
(http://www.css.orst.edu/cereals/)
