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Why microclimate cup runneth over


Beautiful gardens by William Johnson

IT WAS Friday, January 6. Nighttime temperatures dropped to 27°F for a few hours. On that night, I set out a single 16oz Solo cup that I had filled to the brim with tap water. To no surprise, the water in the plastic cup had frozen by the morning.
Nighttime temperatures for the following night were forecast to drop below freezing again so I had time to construct a better-designed experiment.
I spared no expense conducting this one – I used four 16oz Solo cups and added to my water bill filling all four cups!
The first cup was placed out in an open area on the patio. The second was a few inches from a brick wall on the patio. The third cup was placed under the canopy of a 15-foot-tall loquat tree growing near a wooden fence. The fourth was under the dense canopy of a blue plumbago shrub that was still flowering.
While I have no delusions about submitting the findings of my rather rudimentary experiment to a revered scientific journal for publication, the findings nevertheless can provide some insights to home gardeners on what happens when a cold snap rapidly arrives after winter’s weather conditions have been fairly warm and balmy.
So what were my findings on Sunday morning? The water in the cup placed in an open area of the patio was frozen to a depth of nearly three quarters of an inch by the morning. The water in the cup near the brick wall had only formed a paper-thin layer of ice on the surface. The ice was easily broken with a gentle push of my finger.
None of the water in the cup under the evergreen loquat tree was frozen. The same held true for the water in the cup under the shrub canopy.
What are the implications of this study? There can be subtle microclimates in a given area. Microclimates are the little weather variations in neighboring areas – from one side of a hill to another, from one street to the next and even within different parts of the same yard.
The presence or absence of shade, wind exposure, a water garden – or the Gulf Of Mexico – and even a nearby brick wall or the canopy of a tree or shrub can produce a microclimate. South-facing brick walls warm up earlier, reach higher temperatures and have greater variations in temperature than north-facing brick walls because of their greater exposure to the sun’s rays.
Last week, I described my agony with moving my cold-sensitive plants indoors before the arrival of the year’s first cold snap. The first to be moved inside were my plumeria plants as they are very sensitive to the cold. Of course, my Aggie Maroon plumeria was the first to be moved, followed by other varieties.
By the time I had moved the fourth plant, I was debating the merits of moving yet another large shrub. The fifth and remaining plumeria was almost six feet tall, so I decided to leave it outside against a nearby brick wall. Remarkably, it sustained only minor damage to two growing tips.
The occasional cold snap makes many gardeners scramble to protect their cold-sensitive plants. Tropical and subtropical plants can be used effectively in the landscape but they must be protected or replaced when necessary. The best idea is to plant a good balance of tropical and winter-hardy plants so that your landscape is not totally devastated in the event of extremely cold weather.

Frost flowers make an ice show

THE COLD SNAP in early January provided an additional and quite rare spectacle known as frost flowers. They are also known as ice castles, ice blossoms or ice ribbons and, until then, I had only read about them.
Frost flowers aren’t real flowers at all. They are natural ice formations that are so named because they often form into exquisite patterns that curl into “petals” that resemble flowers.
They are formed when thin layers of ice are extruded from long-stemmed plants under certain cool conditions and some people describe them as looking like spun glass or cotton candy.
Their formation is dependent on freezing weather occurring when the ground is not already frozen. When that happens, plants can still draw water from their roots up into their stems, where it quickly freezes because of the cold air around the stem.
Because the outer wall, or epidermis, of plant stems is thin, ice crystals that form inside the stem will then push their way out through its walls.
Depending upon the structure of the plant’s stem, the ice crystals might form as thin strands like a ribbon or as thin curling sheets. When these ribbons and sheets join together, they can create a shape like a flower petal.
Although frost flowers are stunningly beautiful, they’re also incredibly delicate. As the sun rose that first frosty morning and the temperature started to rise, my frost flowers melted away and vanished in an instant —but not before I could grab my camera and take some photos.
William Johnson is a horticulturist with the Galveston County office of Texas A&M AgriLife Extension Service. Visit his website at

This month’s cold snap produced a quite rare spectacle known as frost flowers, a natural ice formation that can take the form of petals on a plant stem. William Johnson

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