“I suppose the sap freezes, and that its expansion bursts the vessels?” I said.
“Just so,” replied my uncle; “this frequently occurs, even in moderate frosts, to tender plants, especially if they are succulent. But in very severe winters even forest trees have suffered. In the great frost of 1739 and 1740, the largest branches were split from end to end, and numbers of the most hardy trees died in consequence.”
All this made me very anxious about my garden and my nice plants; I had already put stable litter on them; and I asked my uncle, if that should be frozen through, what he would recommend me to do.
He advised me to bend some long withies of sallow over them, so as to leave a small space above the surface of the litter, and over the sallows to spread either a mat or fir boughs; and he reminded me that he had explained some days ago the use of this process.
“Besides which,” said Mary, “I believe the stillness of the air under the covering helps to delay the freezing of the moisture in the ground. I recollect that the winter before last, which was very severe, Mamma had fir branches hung on the wall to cover her tender climbing plants, and long stiff straw or fern was lightly strewed round their roots, and they all lived through the winter, and looked healthy and beautiful in summer.”
My uncle told me for my satisfaction, that a long frost, if not very intense, is less injurious to tender plants, than a milder season in which soft weather and frost alternate: in open weather there is a tendency in the sap to rise; and if it is checked by succeeding cold, the sap vessels are injured, and the plant becomes sickly or decays.
“Is that,” said Frederick, “the reason why spring frosts are more hurtful than those of winter?”
“That is the principal reason; but you must also consider that the ground during the previous summer had absorbed a great quantity of heat, which helps to mitigate the winter’s cold: this has been all expended before spring, and therefore the whole force of the cold is then felt.”
Frederick said he remembered hearing Mr. Grant mention last autumn that all the potatoes had been injured by frost in Alney valley near Gloucester, while those on the side of the hill had quite escaped; and as he thought valleys must be warmer than hills, he begged of my uncle to explain the cause.
“Valleys,” he was answered, “are more sheltered from the wind; and the air in them is undoubtedly hotter in the day time than that on exposed high grounds. But in autumn, when the nights become cold, and slight frosts occur on the sides of the hills, the air that is cooled there being heavier than warm air, sinks down into the lower grounds, displaces the warm air, which rises, and accumulates in the bottom of the valley.