It is well documented that Bartlett pears in California set fruit via vegetative parthenocarpy —that is without pollination or fertilisation.
University of California researchers described this phenomenon in 1954.
Effect of temperature at flowering on fruit set (cont from last issue)
After a five year study, they determined that in California, Bartlett pear is self-fertile, not because it pollinises itself, but because fruit growth occurs without benefit of any type of pear pollen.
They concluded that temperature during bloom was the stimulant. They found that maximum temperatures through bloom must be above 15.5C (60F), the threshold needed to stimulate parthenocarpic set.
Of course, many growers may have stories of improved set using bees during cold, rainy bloom days. This is probably because bee pollination activity will continue down to 13C (55F), which is 2.5C (5F) below the parthenocarpic set threshold.
It is easy to confuse the effects of rain versus temperature on fruit set. Rain itself is unlikely to reduce set. However, rainy periods in spring are often accompanied by cold temperatures.
Rules of parthenocarpic set
So, what are the ‘rules’ of parthenocarpic set?
Although statements vary somewhat, the consensus is that for about a 10 day period encompassing the main bloom, there must be 15.5C maximum temperatures for three days or more (72 hours).
Or more ideally, greater than 15.5C (60F) for two days (48–50 hours), or 21C (70F) for one day.
One source states that more than 150 hours of over 15.5C (60F) will set a bumper crop.
See this article in Tree Fruit Nov 2020
