Thomas G. Chastain

One question that is on the minds of seed producers is how much has the cold weather set crops back?  Over the 123 years of weather records at Corvallis, there has been only 15 times that the month of March has been colder than we experienced in March 2012.  Cold spring weather has been a phenomenon that’s been observed here in the Willamette Valley over the past few years (Fig.1). This cold weather was accompanied by near record wet conditions (Fig. 2).

Figure 1. Monthly average temperature at Corvallis (click to enlarge)

Leaf development and the passage of time are related in crops – the phyllochron is the interval between the appearance of successive leaves on a tiller.  From our research we’ve learned that perennial ryegrass tillers will produce a new leaf every 118 growing degree days (GDD) at a base temperature of 32˚F (Fig. 3).  With normal March weather, the valley averages 454 GDD meaning that perennial ryegrass tillers will produce 3.8 new leaves in the month of March.  But in March 2012, only 356 GDD were recorded and perennial ryegrass crops produced only 3.0 new leaves.  Development of perennial ryegrass seed crops was retarded by about 20% by the cold weather.

Figure 2. Monthly average precipitation at Corvallis (click to enlarge)

In tall fescue, the number of GDD to produce a leaf is 135 (Fig. 4).  An average March would have permitted the tall fescue tillers to grow by 3.4 leaves, but in March 2012, the number of leaves produced was 2.6.  Tall fescue crops were set back by 25% by the cold this past March.

Figure 3. Relationship of leaf development and growing degree days in perennial ryegrass (click to enlarge)

Looking ahead to April growth, perennial ryegrass seed crops will grow by 4.6 leaves and tall fescue by 4.0 leaves.  With cooler and wetter than normal weather persisting into mid-month, a repeat of the conditions experienced in the past few years is possible.

Figure 4. Relationship of leaf development and growing degree days in tall fescue (click to enlarge)

Crop management operations that are dependent on the developmental stage of the crop might be pushed to later in the spring than growers are accustomed to by the cold weather.

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