Authors: Mary Knapp, Romulo Lollato

There has been a considerable decrease in vegetative activity in southwest Kansas compared to last week. Figure 1 below is a comparison of NDVI (Normalized Difference Vegetation Index) readings between the week of March 15-21 and March 8-14. NDVI is an index of plant “greenness” or photosynthetic activity, and is one of the most commonly used vegetation indices. The reduced level of NDVI readings in southwest Kansas from the week of March 8-14 to the week of March 15-21 is striking, and illustrates the significant effect of the hard freeze on wheat, alfalfa, and other vegetation actively growing at the time of the freeze.

This area of reduced NDVI activity corresponds remarkably well to the area where some of the coldest temperature persisted for the longest time (Figure 2). In southeast Kansas, where temperatures remained milder, there was actually an increase in photosynthetic activity from the week of March 8-14 to the week of March 15-21.

Because NDVI is directly related to leaf greenness, the decrease in NDVI readings as compared to last week is a direct consequence of leaf burn, leaf senescence, and overall decrease in crop vigor experienced by wheat, alfalfa, and other crops as result of the extremely low temperatures. A general leaf burn and decreased crop vigor as result of below-freezing temperatures was expected for most of the state, and should not necessarily result in decreased grain yields, provided the growing point was not affected. The decrease in NDVI readings as compared to last week cannot be correlated with death of the growing point at this time. Producers should be able to assess the actual effects of the freeze to the growing point over the next few days.

Reports from agronomists and producers from across the state correlate well with Fig. 1. Thus far, leaf damage has been reported throughout the state but very few growing points have been affected. Damage to the growing point has generally been restricted to extremely early tillers in few fields, such as that shown in Figure 3 by Doug Shoup, K-State Southeast Area Agronomist. The affected growing point in Figure 3 was from an isolated area of more advanced plants within a field in Butler County, in south central Kansas, where the flag leaf was emerging out of the whorl. The unaffected growing point in Figure 3 was sampled from the same field, but from a plant that was still in the second node stage of development at time of the freeze.

For more information on symptoms of freeze damage in wheat, please refer to eUpdate issue 555 of March 21st, 2016, “Diagnosing late winter/early spring freeze injury on wheat”. For more information on symptoms of freeze injury in alfalfa, please see accompanying article in the current eUpdate issue “Managing freeze-damaged alfalfa.”

Figure 1. Vegetative activity from the week of March 15-21 compared to the previous week. Source: K-State Precision Agriculture Laboratory.
Figure 2. Coldest minimum temperatures from March 15-21, 2016. Source: K-State Weather Data Library.
Figure 3. Wheat growing point damaged by freeze from an area of the field in an advanced stage of development (Feekes 8, flag leaf emergence, upper plant) versus an undamaged growing point from a plant in a different area of the same field. The lower plant in this photo was still at Feekes 7 (second node) at time of freeze event and had no apparent damage at the time of the photo. Photo by Doug Shoup, K-State Research and Extension.

Mary Knapp, Weather Data Library

Romulo Lollato, Extension Wheat and Forages Specialist