I thought I would share some of my observations from this spring and see if I could stir up some discussions.
I will start with my observations. Today it is 90+, yet less than 2 weeks ago I had 3 mornings with ice. These late frosts were right when pollination of some apple trees was taking place. They were a very strange, localized frost. I would go out of the house when I first woke up and check for frost. There was none. The thermometer read 38. Then I would shower and get ready for work. When I came back out the thermometer still read 38. But, there was ice on the car glass and on the grass. This happened within 20-25 minutes, just before or at sunrise. Ok, the thermometer was wrong. I went and bought 2 more thermometers. Now I have 2 digital and 2 analog thermometers. The next morning, I get up and look at all 4 thermometers. Lowest reading was 37 and highest was 39. No frost or ice to be found. After my shower and heading off to work, my car window is iced over. I go and check the thermometers. All about the same, one analog might have been at 36. These were the gorgeous days when I would come home from work and watch the mason bees doing their work. They were pollinating and filling holes like crazy. I walked by my trees and could see them working there. Ok so this happened a third time too. Has anyone seen this type of frost activity? I am not describing a 'light' frost. These were hard enough that the windshield wipers needed fluid to clear the ice off. They just didn't push the ice off. So, now on to the real meat of the discussion I hope.
I have been walking around looking at my trees. The pear trees were already setting fruit before this and I have ok pollination and fruit set on them, both low to ground and higher up. The apple trees were the ones being pollinated. I can see that I have ok fruit set happening on the trees above 8-10 feet, but I haven't found 1 apple set below that height. Could it be that these 'flash' frosts only occurred low to the ground and as such the lower blossoms were frozen? I was planning on pruning my trees down in size to make them more manageable, but now I am wondering. I think I have determined my average last frost date is May 31 (according to the local airport near my house). I live in a prairie type situation and so I think we could have our own micro climate. I plan to look at the other fruit trees close by later and see if their fruit set is higher up also. Here is what I hope to stir up some discussion with. Since heat rises, could the height of the tree 'help' a tree set fruit through these late frosts? Or is what I am observing just an fluke? Next year I plan on having a weather station hooked up to my old laptop to record any of these 'quick dip' freezes.
I don't know that what you're calling late frosts are really that - your thermometers are almost certainly correct; what you are seeing is actually caused by evaporative cooling. When you get a heavy dew and the temperature is cooler, the dew evaporating back into the air cools the dew off; sometimes enough to form ice - but the air temp itself is usually above freezing! The ancient Egyptians actually made ice that way for their royalty, despite the air temperature never actually being below freezing there:) And it happens more on exposed highly temperature conductive surfaces such as glass than on tree wood; it probably wasn't forming any frost on your trees at all, just on your windshield:) And if you start observing the conditions it happens in, you'll probably find that it's mostly on clear nights when a dew can really form; but also goes back into the air in the morning.
gkowen,
There might be something to what lotus is saying and maybe it is only effecting the glass and not the trees. To play it safe try keeping the ground bare and moist because on 30ish nights following the good days like your talking about the ground will return the heat back in the form of radiation;
[url:2jx47ws3]http://www.ipm.msu.edu/cat08fruit/f03-18-08.htm#6[/url:2jx47ws3]
At least under these circumstances of trapping sun energy the lower branches should actually fare better.
gkowen,
I live in Rochester as well and looked back into my records. My last frost was April 24, although there was definately some mornings in May that were PDC (pretty darn cold). I cannot give you any fruit pollination info from my place. We just moved here and all my trees were planted this year.
Neighbor does have a really good set on his dwarf apples and asian pears. The trees are very dwarf - 100% harvestable from the ground by a man about 5'8". I think he did cover them a couple times this year.
I am on the west side of 5, north of 12, between Sargent and Joslyn. My "orchard" is small and contains 7 trees (plums, peaches, cherries, apricot, and fig). I have two newly grafted very dwarf apples in pots. Plus, I have grapes, kiwi, raspberries, blue berries, and strawberries surounding my veggie garden. Again, very small scale as I am on just less than an acre.
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