Vegetable Growers News
March-April 2001, Vol. 8, No. 2
Charlie O'Dell, Extension Horticulturist, Virginia Tech
Two weeks of summer-like weather suddenly arrived in early April after a cold March. In the mountains, we went from removing winter crop covers at the end of March, right on into heavy bloom with heavy bee pollinating activity and early berry set with green fruit swelling. In the mountains, dogwood and spring flowers were about as far advanced by mid-April as in lower elevation areas east and south of us.
Then, just as suddenly as summer had appeared, with days in the high 80s and nights in the 60s, summer retreated on April 16. That night, cold NW winds roared in with night temperatures dipping into the mid-20s in our location in the mountains of SW Virginia near Blacksburg. The next 3 days and nights also were cold and very windy, days were in the high 30s to low 40s, nights continued in the mid-20s. Winds were so strong, sprinkler irrigation would have been damaging to attempt at our location.
This sudden cold event of 4 days and nights following 2 weeks of very warm, plant-tenderizing temperatures provided an excellent opportunity for testing various weights of crop covers during full bloom. Remember, weather forecasts and reported weather station temperatures are taken at 4' to 5' height above ground, but with strawberries, our reality is at ground level where temps are usually 3-4°F colder than station reports. We used crop covers of 1.25 oz/sq yd, Typar T-518 from Ken-Bar Corp.; 1.20 oz/sq yd from Atmore Industries; a heavy 2.0 oz/sq yd cover from Ken-Bar Corporation; and double covers on some plots. These were two 1.20 oz/sq yd Atmore covers, and two covers of 1.25 oz/sq yd of Typar T-518 covers applied one over the other to equal a total weight of 2.4 oz/yd (Atmore) and a total weight of 2.50 oz/yd (Ken-Bar).
When we removed the covers on Friday afternoon as the temperatures warmed, after almost 96 hours of use in very cold weather, we noted:
- Blooming plants that had been covered with the 1.20 and 1.25 oz covers had 60% dead blooms, green fruit appears to be OK.
- Plants covered with the 2.0 oz cover lost only about 5% of blooms, only those in the tops that touched the cover.
- Plants double-covered with two 1.20 oz or two 1.25 oz covers lost NO blooms to frost or freeze!
These results raise a question for cover manufacturers, researchers, extension workers and growers: were our good results with the double covers due to the extra 0.4 or 0.5 oz per square yard weight, or was it also partially due to air spaces created between the two cover layers?
Factoring in the time, labor, and cost of using the double covers, I plan to replace these lighter covers with 2 oz or even 2.5 oz single covers in the future by attrition of older covers. I'll keep cover widths to only 40 or 45 feet for easier applications. 8" cinder blocks placed every 10' around cover perimeter did a fine job of holding the covers in extremely windy weather, as did plastic mesh bags filled with large gravel.
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