Landscape & Irrigation

January/February 2016

Landscape and Irrigation is read by decision makers throughout the landscape and irrigation markets — including contractors, landscape architects, professional grounds managers, and irrigation and water mgmt companies and reaches the entire spetrum.

Issue link:

Contents of this Issue


Page 26 of 35 Landscape and Irrigation January/February 2016 27 and can include cold, heat, elevation, drought, flooding and even wind. The lowest temperature that it can survive before freezing to death determines a plant's cold hardiness The USDA has made this fairly simple by evaluating and categorizing landscape plants into Plant Hardiness Zones. Starting at the Canadian border, Zones are assigned in 10-degree increments down to the Mexican border. Zone 1a plants need to be hardy down to negative 60 degrees Fahrenheit, while, on the other end of the spectrum, Zone 13b plants are uncomfortably chilly at 65 to 70 degrees Fahrenheit. This helps provide some guidelines on what temps a plant may be able to tolerate, but it does not mean that they can tolerate it all the time. A paper birch may be able to survive through weeks of negative 40 degrees Fahrenheit in midwinter, but would likely die in 24 hours if exposed to those temps in midsummer. Plants must first acclimate to the cold before they can withstand it. The process of getting ready for the coming winter begins with the shorter days and cooling temps of autumn. Chlorophyll production ceases, revealing the bright reds, yellows and orange pigments we all love so much. As this is going on, trees are actively moving carbohydrates into storage tissues in their trunks and roots and reallocating their moisture reserves as well. Water management is the biggest key to surviving subfreezing temperatures, as trees are comprised primarily of water. Water, of course, expands when it freezes; and this expansion will rupture the walls of plant cells causing the death of the cell, and, if widespread enough, the death of the tree. Plants all around the temperate world have evolved different approaches for surviving winter. Annuals survive winter by not surviving at all. They have developed a way to complete their lifecycle in a single season, leaving only their more storage- friendly seeds to endure the off-season. Herbaceous perennials often have specialized belowground structures that can store water and carbohydrates for the winter, and can reactivate them in the spring. This includes familiar structures like the bulbs of a tulip, the tubers of a potato, and the taproot of a carrot. Hardy woody plants, such as trees and shrubs, do not have the luxury of moving all their tissues into the soil, so they have developed a few different strategies for subzero temperature endurance, and it all starts with water management. Although trees appear solid to us, their cells are comprised primarily of water, and what they do with that water in the winter determines their survival. One mechanism involves pumping sucrose and the amino acid proline into the cell. This acts essentially like salting your sidewalks in winter by lowering the freezing point of the solution inside the cell to remain liquid. Just like salting the sidewalk, however, this is only effective for combating "warmer" freezing temps of about 20 to 32 degrees Fahrenheit. This is due partly to the effects of osmotic pressure; you could keep increasing the quantity of dissolved solutes to continually lower a cell's potential freezing point, but at a certain concentration the increased volume of "stuff " inside the cell begins to build pressure that increases its likelihood to rupture. Other plants take it one step further and undergo a process referred to as "deep supercooling." Birch trees use deep supercooling. Larix gmelinii larch TREE CARE

Articles in this issue

Links on this page

Archives of this issue

view archives of Landscape & Irrigation - January/February 2016