This was a classic radiational freeze. Calm, clear night, with lows in the mid-30s. Some find it strange that you can have frost when the air temp is above 32 degrees. It happens because some objects (e.g. grass) lose heat to the surrounding atmosphere through radiation. Not the Chernobyl kind. The kind that you feel when you’re near a fire and the glowing coals emit their heat. Only it’s far more subtle than that, thankfully. The result is that the grass actually becomes cooler than the atmosphere, and dew freezes to form ice crystals on the grass. That’s frost. If you notice, under trees, shrubs or just about any object, frost often doesn’t occur on nights like these. That’s because the heat radiated by the grass is reflected back down, keeping things just a few degrees warmer.
The cold grass often refrigerates a thin layer of air at the surface (which tends to pool in low spots) and so you get what’s called an inversion layer. Air is colder at the surface, and gets a bit warmer as you rise. That’s why, as you see in this photo, you can have frosty grass, but everything more than a few inches off the ground is unfrozen. This photo was taken two weeks ago, and those marigolds are still bloomin’ their heads off. If it’s a windy night with a cold front that’s bringing in sub-freezing air, the atmosphere is mixed thoroughly and everything freezes equally. That’s a very different thing—and usually does a better job killing off the whole garden.
To be sure, some radiational freezes are cold enough to take out the garden. But I don’t mind a light frost like this, especially when it’s followed by a long, lovely Indian Summer, with plants ablaze with the reds, yellows and russets that make autumn landscapes so gorgeous.
Sometimes I’m disappointed by the marketing claims: that first-ever “blue-flowering” variety may not be all that blue, that new perennial may not turn out to be as hardy as they say, or that plant with those gorgeous blossoms may be a really shy bloomer.
But then the plants come along that live up to their hype. One I’m pretty psyched about right now is Lavender Falls wisteria (Wisteria floribunda ‘Betty Tam’).
What’s so cool about it? The supplier, Greenleaf Nursery, told me it’s a reblooming variety. And it actually is — I took this photo last week.
I’ve had Lavender Falls for two years now in my Zone 5a garden, so it seems to have passed the hardiness test (though it did die back nearly to the ground last winter). It hasn’t bloomed in spring for me, but I have enjoyed the lovely flowers in July, August, and September. I’m hoping for a batch yet this October.
The bad news is that it’s an Asian wisteria, so it may show invasive tendencies in some areas (especially the Southeast and Mid-Atlantic). That means I’ll keep an eye out on it, and I may need to rip it out if it looks like it may become a problem in Iowa (happily, I’ve not yet seen it produce a single seed).
When I purchased my 1920 Arts & Crafts bungalow in a turn-of-the-century neighborhood in Central Iowa, I also inherited stewardship of a handful of the most majestic of forest trees. What I did not anticipate was the total inundation of my property with their hard-shelled progeny. We’re talking acorns here, and bushels of them. My yard and garden is currently being showered with a bumper crop of White and Bur Oak acorns. Not only do these seeds smart when they bean you on the head, they also leave behind their calling cards as annoying little dings on cars that dare park beneath their laden branches.
According to the Morton Arboretum in Lisle, Illinois, oaks trees are dropping an exceptionally large number of acorns this year. “This is one of the heaviest years I’ve seen for acorns, not just in the Chicago region, but in other states as well,” says Kris Bechtell, Vice President of Collections and facilities.
A year of such heavy production, called a mast year, is a natural, cyclical process that apparently helps to guarantee a species’ survival, and does not harm the trees. Each fall, tree roots store energy that’s used the following spring to expand buds and create twigs and leaves. A mast year does not leave oaks with a large energy deficit of otherwise harm the trees, according to Dr. Gary Watson, Senior Scientist and Head of Research.
Why a mast year? Experts theorize that if oaks produced the same number of acorns every year, predators would eventually become so numerous and consume so many acorns that there would not be enough to both feed the predators and grow a new generation of seedlings. During mast years, predators are over-saturated with acorns, providing enough acorns for the predators as well as for the future of the species.