Good Conditions This Week for Strawberry Pollination and Fruit Set – Get Row Covers OFF (9:30am, 3/31/15)
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I have not had any frost reports this morning. At 7 a.m., Michael Beal, near Siler City (Ramseur, NC), sent this text message:
“Yep, last night no frost and 33-34 F at 6 a.m.”
Fig. 1. Cronos network at 6:59 a.m., 3/31/15. Some areas in central piedmont dropped to 32 F air temp before sunrise, including Siler City (according to CRONOS system), but a grower in this vicinity had no frost this morning.
My early morning impression is that if Siler City did not see frost this AM, it is doubtful that anyone else did. I know at my home in N. Raleigh, we had only dew this morning on the lawn. There may have been some frosty spots in the Raleigh-Durham area this morning, but not my place. Here is the AWIS Hourly Forecast for Raleigh-Durham (7:18 a.m. EST).
Table 1. This AWIS forecast shows that RDU area might have had conditions favoring frost this morning? Winds were 0 mph since midnight, and very high dew points (31 F). But, I am not aware of any Frost occurring in Raleigh-Durham area. And, looking ahead, I don’t see that much reason to worry about frost the rest of this week! On Thursday morning the minimum will be 40 F in Raleigh (vs. 37 this morning). So, I am not worried. And, all the more reason for growers to get those row covers off NOW. Row covers quickly become YOUR ENEMY DURING BLOOM IF THEY REMAIN ON ON GREAT DAYS LIKE TODAY FOR POLLINATION.
Great weather on tap today and much of the week!
Fig. 6. In Raleigh area, a nearly perfect week of weather for strawberry bloom stage (ABC forecast last night). A lot of great stuff starts happening when we get up into low 70s with sunshine, and I am hoping you see some honeybee activity in your crop today with these warmer conditions. We recommend 2 active hives/acre during bloom.
Warmer temperatures and light conditions favor pollination and fruit set: It is a lesser known fact that some varieties of strawberry will not release pollen at temperatures below 57 F (14 C), so that’s another good thing about these warmer temperatures this week (Guttridge, C.G. Anther failure main cause of poor fruit set in Redgauntlet, Grower, March 15, 1979, p. 38-39).
Fig. 7. There has been research showing that pollen is not released with temperatures below 57 (14 C). When the pollen is released from the stamens, it is viable for 2-3 days. And, once the pollen is released, it is available for transfer to the pistil (female flower part). The pollen can be moved by wind, gravity and insects to the stigma surfaces at the tips of the pistils.
Though honeybees are not essential to the pollination and fruit set process, wind is critical. And, when you leave row covers ON on ideal days like today, you run the risk of producing bumpy, malformed berries in another 25-30 days.
I will never forget an early May visit that I had in Orange County, NC, many years ago to a piedmont farm where they left the row covers on for a solid week during bloom (not intentionally), and this grower had some of the worst looking fruit I saw that season due to pollination problems caused by the row covers staying on the crop at peak bloom.
How much honeybees can help with the pollination process is subject to question, but in the textbook, Handbook of Fruit Set and Development, Shaul E. Monselise, CRC Press, 1986, it states on page 424:
“Self-pollination (by gravity) and air currents often are NOT sufficient and insects improve pollination, especially in cultivars with stamens shorter than the receptacle (e.g. Chandler).” This book also points out that the stigmas (at the tips of pistils) are receptive BEFORE the anthers release the pollen, THUS, cross-pollination by insects is favored.
They share some data form a trial where mean fruit weight was reduced by nearly 20% without bees, and the amount of malformed fruit (of the entire crop) was 21% with and 49% without bees in this particular field experiment (J.B. Free, The pollination of strawberries by Honey-Bees, J. Hortic. Sci., 43, 107-111, 1968).
Get row covers OFF. It does not appear that they will be needed for frost on Thursday morning in most areas, but you can check to see if AWIS has a frost forecast “up” for your location in NC below. When it comes to the pollination process, the worst thing you can do is leave a row cover ON during bloom – the covers interfere with wind movement.
Early season Chandler blooms are generally not the best in structure, and stamens (male part) are frequently recessed, or even missing on some flowers. Further, the stigmas (tips of the female flower part) are receptive before the anthers on the same flower, thus there can be real benefit from cross pollination by honey-bees.
Two active honey-bee hives per acre can really help with fruit shape and berry size.
Supplemental weather information (check your city/town for FROST risk over next 10 days, and take appropriate measures to frost protect, if needed):
Raleigh, NC 27695-7609
Sidney J. Harris, In: Reclaiming a Lost Heritage – Land-Grant & Other Higher Education Initiatives for the Twenty-first Century