Nick Christians
October 31, 2014
I often get questions at this time of year about creeping
bentgrass turning a purple color. The
symptom of phosphorus deficiency is initially a reddish color, followed by a
purple discoloration. Figure 1 shows a
study at Michigan State University a few years ago. It shows true P deficiency. Notice how the problem disappears when small
amounts of P are added to the area.
Figure 1.
In the fall, however, grasses can take on s similar purple
appearance in response to cool temperatures.
What you are seeing is a plant pigment called anthocyanin. This is quite common in creeping bentgrass,
particularly some of the old, stolonized varieties. Figures 2, 3 and 4 were taken on a golf course
in Des Moines, Ia. last week. This course was
established more than 60 years ago to a stolonized variety called “Washington”. Washington bentgrass was known for turning
purple every fall when temperatures dropped.
The purple patches are Washington bent and the surrounding greener bentgrass
is Penncross and a few other newer varieties. This shows how persistent some of these old varieties are and how long they can be found in greens.
So, how can you tell the difference between true P
deficiency and grass turning purple in the fall? Soil tests can help. I generally do not see P deficiency symptoms
on bentgrass unless P levels fall below 10 lb/acre (5 ppm). The greens on the course in Des Moines have
levels of 250 to 300 lb/acre. The other
way is to simply apply P to the area in a test strip.
Usually 0.5 to 1 lb P/1000 sq. ft. will be sufficient. If the grass turns green again, it was P
deficiency. If it stays purple, it was a
response to cold temperatures.
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