Fall, winter, and spring bring the danger of frosts and freezes to Texas gardens, orchards, and landscapes. Although sporadic and unpredictable, these cold spells have left their mark on horticulture in Texas by wiping out peach crops, freezing pecans in their shucks, forcing the replanting of spring vegetables, killing valuable landscape plants, and necessitating the replacement of beloved avocado, citrus, and fig trees. Home gardeners and commercial growers can minimize these losses by understanding how cold affects plants and implementing diverse strategies to protect them. When the water inside plant cells freezes, ice crystals form that can pierce and damage the cell walls, killing the cells. As temperatures rise, fluids leak out of those cells and they begin to decay. In the coming weeks, I will be addressing weather related issues including cold spells as our winter season unfolds.
Freeze damage first appears as dark, water-soaked tissues that later turn brown or black and dry up many ornamental and edible plants have mechanisms to resist freeze damage. Trees and woody plants that go dormant, such as pecan and peach, can tolerate very low winter temperatures. However, they can be injured if they are too slow to stop growth in the fall or begin growing too quickly in the spring. Many species of woody evergreens, such as hollies, can tolerate tremendous cold. Subtropical plants such as citrus and palms have variable levels of cold hardiness; some can withstand mild to moderate subfreezing temperatures. Their survival depends on their age, condition, size, and genetic mechanisms for acclimation (gain in the ability to withstand freezing) and the depth and duration of the cold.
Some species of herbaceous (non-woody) plants are cold tolerant, enduring all but the most severe cold in Texas. Examples are perennials like lilies and irises and annuals like violas and sweet alyssum. However, many spring- and summer-growing annuals and perennials may be damaged at, near, or even above freezing (32°F). Likewise, most fruit and vegetable structures have little resistance to freezing temperatures, prompting a quick harvest when the forecast calls for frost. Because Texas weather is often erratic, these guides do not always predict plant performance in freezes exactly. They also cannot account for a particular farm or landscape being colder or warmer than its surroundings because of its topography, urban microclimate, nearness to bodies of water, etc. Plants can generally acclimate to freezing weather when they are exposed to consistent, gradually colder weather. Intermittent warm periods, not uncommon in Texas, can cause the plants to deacclimate, leaving them more vulnerable to damage in a frost or freeze. To help plants withstand frosts and freezes, do not fertilize with nitrogen or harshly prune them in late summer, which will stimulate growth and make them less winter hardy.