Gardeners have always been interested in the weather, and with the advent of major media coverage of climate change, weather is an almost daily concern for people everywhere. Weather is especially critical to those of us who grow plants because the weather affects not only the health and growth rates of our crops but also determines to a large extent which plants we can grow well in the first place.
When you’re choosing plants and designing gardens, keep in mind that there are climatic variations within a geographic region and even within each garden. Your garden’s immediate climate may be different from that of your region overall. Factors such as altitude, wind exposure, proximity to bodies of water, terrain, and shade can cause variations in growing conditions by as much as two hardiness zones in either direction. It is also important to realize that your area’s climate can change over time.
Cold Hardiness And Heat Tolerance
Hardiness is the ability of a plant to survive in a given climate. In the strictest sense, this includes not only a plant’s capacity to survive through winter, but also its tolerance of all the climatic conditions characteristic of the area in which it grows. Still, most gardeners refer to a hardy plant as one capable of withstanding cold and to a tender plant as one that’s susceptible to low temperatures and frost.
In order to help growers determine which plants are best for their regions, in 2006 the National Arbor Day Foundation, using data from 5,000 National Climatic Data Center stations, released an updated version of the 1990 USDA Plant Hardiness Zone Map. The USDA is also at work on an update to the 1990 map; it was due to be released in 2010, but you know how that goes. Hardiness zone maps divide the United States into 11 climatic zones, based on the average annual minimum temperature for each zone. Zone 1 is the coldest, most northerly region (in Alaska), and Zone 11 is the warmest, most southerly (in Hawaii). If you live somewhere in Zone 6 and a plant is described as “suitable for Zones 5–9” or “hardy to Zone 4” you can expect the plant to do well in your area. If you live in Zone 3, on the other hand, you should select a more cold-tolerant plant. You can find out which zone you live in by referring to the map.
In addition, recognizing that hot weather can also limit plant growth, the American Horticultural Society (AHS) has released its AHS Plant Heat-Zone Map, based on 12 years of climatic data ending in 1995. On this map, the United States is divided into 12 zones based on the number of days the region experiences temperatures above 86°F (“heat days”). Zone 1 experiences less than one heat day; Zone 12, over 210 heat days. Use this map the same way you would the Hardiness Zone Map.
Many food and ornamental plants are native to warm climates and can’t withstand freezing temperatures. Others go dormant for winter. Thus, the primary growing season for most North American gardeners is between the last frost in spring and the first killing frost of fall.
Air temperature is only one of the factors that determine whether or not plants will be damaged by a frost. Sometimes when the temperature dips a little below freezing, the air is sufficiently moist for water vapor to condense (in the form of ice crystals) on the ground and on plants. When water condenses, it gives off heat and warms the air around plants, protecting them from extensive damage. On clear, windless, star-filled nights when the forecast calls for near-or below-freezing temperatures, it’s wise to protect plants. Heat is lost rapidly under these conditions, and frost damage often occurs. When temperatures fall more than a few degrees below freezing, frost damage to growing leaves and shoots is likely no matter how humid conditions are.
Frost damages plants when the water in the plant’s cells freezes and ruptures the cell walls. Different plants and parts of plants have different freezing points. Plants that are native to Northern regions have many ways of protecting themselves from the cold. Many perennials die down each fall. The roots buried in the insulating soil remain alive to sprout again the next season. Some plants such as kale have cold-tolerant leaves that will survive unharmed under a blanket of snow, but not when exposed to drying winds. Deciduous shrubs and trees drop their leaves each fall and form leaf and flower buds that stay tightly wrapped in many layers and go dormant until spring comes again. Or, like cold-hardy evergreens, they may have a natural antifreeze in their sap that helps prevent them from cold injury.
Using A Rain Gauge
Natural rainfall is an important factor in which crops you can grow and how to take care of those you do grow. Keep track of the rainfall in your garden with a rain gauge. You can buy a gauge, or simply use an empty tin can and a ruler. An inch of rain in the can equals an inch of rain on the garden.
Frost heaving of soil can also cause problems for gardeners. Soil moves as it freezes, thaws, and refreezes. This action can push newly planted perennials, shrubs, or other plants that don’t have established root systems out of the soil. Mulch heavily around these plants after (not before) the soil freezes to prevent thawing during sudden warm spells in winter or early spring.
Our ancestors understood the role weather played in growing food. They saw that nature gave ample warning of approaching rain, storms, and frost. The sky is filled with weather indicators, especially cloud formations. For example, “When ye see a cloud rise out of the west, straightway cometh the rain” (Luke 12:54) refers to the fact that weather fronts usually move from west to east.
“Rainbow at night, shepherd’s delight. Rainbow in morning, shepherd’s warning” refers to the same phenomenon. A rainbow seen in the evening to the east is caused by the setting sun shining from the west, indicating fair weather in that direction. A morning rainbow, caused by the rising sun from the east, indicates rain to the west, heading your way.
“If the sun goes pale to bed, ’twill rain tomorrow, it is said” is another saying that involves cloud patterns. High cirrus clouds in the west give the setting sun a veiled look. When appearing as bands or mares’ tails, they signal an approaching storm.
Finally, who among us will argue with “Clear moon, frost soon”? Cloud cover acts like a blanket over the earth, keeping temperatures from dipping as low as they would on a clear night.
The skies are not the only aspect of nature filled with weather signs. Animal and plant behavior also indicates changes. Here belong all the sayings about the thickness and color of an animal’s coat, the bark on a tree, or the skin of a vegetable, such as “When the corn wears a heavy coat, so must you.” A related saying is “The darker the color of a caterpillar in fall, the harder the winter.”
Certain animals and plants do respond in a consistent way to a change from a high-to a low-pressure system, which often brings rain. This is why a saying such as “When the sheep collect and huddle, tomorrow will become a puddle” is reliable. “The higher the geese, the fairer the weather,” a saying that applies to all migratory birds, also refers to this phenomenon.
Many plants are sensitive to drops in temperature and to high humidity. “When the wild azalea shuts its doors, that’s when winter temperature roars” refers to the fact that azaleas and rhododendrons draw their leaves in when the temperature drops.
Cold weather lore often merges weather phenomena with common sense. Snow, for example, is known as “the poor man’s fertilizer,” which may be because it acts as mulch, protecting plants and keeping nutrients in the soil that rain would otherwise wash away. Frost is “God’s plough” because it breaks up the ground and kills pests.
Going beyond folk wisdom is phenology, the study of the timing of biological events and their relationships to climate and to one another. Such events include bird migration, animal hibernation, and emergence of insects, and the germination and flowering of plants.
Phenologists have found that many plants and insects within the same region or climate pass through the stages of their development in a consistent, unified sequence. The budding of a given plant, for example, may correlate with the hatching of a particular pest insect. Variations in weather from one year to the next affect the timing of such events, but the order in which they occur tends to remain the same.
As a result, it’s possible to foretell when conditions are right for a crop to germinate or an insect to appear by learning to read the various growth stages of indicator plants. You may want to experiment with phenology in your own yard. For example, you could plant a variety of perennials as indicators that will provide a steady succession of blooms throughout a season. Then observe and record the indicators’ growth phases, along with weather data, the appearance of insects or diseases in your garden, and the progress of food crops.
Sooner or later, patterns will emerge. You may notice that daffodils always begin to bloom when the soil becomes warm enough to sow peas, or that Mexican bean beetle larvae appear at about the same time foxgloves open. You can then use that information in subsequent years to help you decide when to plant peas or when to start hand picking beetle larvae. Your observations may also help you discern if and how the local climate in your area is changing over time.