Here are two stalks of asparagus growing just a foot apart. Both are of the same thickness and height. After an early morning temperature of 28 degrees F, one stalk is frozen and one is not. I have seen this happen many times. Anyone know why?
This spring, when temperatures went from ridiculously high levels much of winter and early spring, and then plunged on some nights in late March and early April as much as fifty degrees in 24 hours into the twenties, fruit farmers are at their wits’ end. Early warm weather usually means lots of killing frost later on. One market gardener told me a month ago while we were both enjoying our lovely blossoming trees that I could kiss my peaches goodbye for this year. I thought so too, but strangely, against all odds including at least seven nights of below freezing weather since then, I still have peaches. Trying to figure out why provides a whole bunch of observations about frost, none of them suggesting anything conclusive except that official weather reports of the highs and lows for the day don’t always apply equally to every local area within the reported area.
So many factors influence frost killing that I doubt actual official temperatures are very meaningful. Sometimes the slightest little change in one farm’s micro-climate compared to another can make a huge difference. I don’t know for sure why my peaches survived. First of all they got through winterkill, the usual destroyer of peaches here, when a couple of warm days in January or February can cause buds to swell. Then really cold weather kills them. Having survived that, most of all because it never got really cold, these peach trees came into full bloom in early March, a month ahead of schedule. The warm spell lasted six days, enough so that the flower petals began to fall, and the tiny little peaches form. These little peaches, however tender, can withstand a degree or two more of below freezing weather than open blossoms. (Professional peach growers say, even in bloom stage, a peach tree can handle 4 hours of 27 degree F weather but no lower or longer.) Not all the peach blooms survived. Some died and fell off, but the trees were overloaded with blossoms anyway, so that was good.
My trees are surrounded by large forest trees and grow next to the barn buildings. The trees can keep the temperature a degree or two warmer in their immediate surroundings, so experts tell me. Also barn buildings give off a bit of heat during the night that they absorbed from a preceding sunny day. That’s why some gardeners espalier peach trees to the sides of their houses or against rock walls. Another frost protector is wind. Wind motion can literally blow frost away which is why in eastern Pennsylvania you find commercial peach orchards on steep mountainsides where one would think frost would be inescapable. At night, cold air sinks pulling warmer air higher up the mountain slope down through the orchards to fend off frost. In my situation, a northerly wind at night can sometimes keep frost away even at below freezing temperatures. Another frost defense is a large body of water nearby. Fruit orchards flourish along Lake Erie because the lake effect can keep the frost away that causes mischief farther south. I wonder if fruit trees avoid frosts if grown on the banks of farm ponds.
Frost effect can be exacerbated by mulch. In my earlier, innocent days I over-mulched part of my garden too early in the year—in May. Sure enough we had a late frost and it killed the squash plants that were heavily mulched but not anything where I had not yet mulched. The mulch had prevented the warmth of the soil from rising up and fending off the frost.
All well and good, but none of that explains one frozen spear of asparagus just a foot away from an unfrozen spear. I like the mystery of it. If we knew the answer to everything, life wouldn’t be fun anymore.