What is going on with these warm 60-degree days in January and February? Is it spring already? Many daffodils are in full bloom on south-facing slopes in Chattanooga. Maple trees have big red buds. The grass never did get brown this winter. Unusual temperatures like this cause us to investigate what is normal.
Hardy plants
Plant scientists and climate experts put their knowledge together from temperature data collected from 1976 to 2005. They made a map of 13 hardiness zones for perennial plants of the United States. Each zone is the average of lowest winter temperatures for a given location for this time period. The map is based on the average annual minimum winter temperature, divided into 10-degree Fahrenheit zones.
Zone 1 would be places such as Alaska, while Zone 13 would be tropical, such as Hawaii. The greater Chattanooga area is in Zone 7, which has an average annual minimum temperature of between zero and 10 F. We can subdivide into Zone 7a (higher elevations around the city) and slightly warmer Zone 7b (the lower Tennessee Valley areas). Most garden centers will have charts or plant tags stating which perennials are hardy for each zone.
Plants are survivors
Plants are really smart. They have been surviving for millions of years, so they know what they are doing. They constantly check the external environment to determine when to start flowering. Several factors including light and temperature are known to affect the timing of flowering. Plants can adjust to all of these factors.
The unusually mild winter here in Chattanooga seemed to confuse many plants and shrubs into thinking it was spring. However, this was just our human opinion or point of view. Plants are actually well adapted to survive extreme or unusual conditions. When to flower is an important decision in the life cycle of a plant. They need to get it right most of the time to survive.
Some plants use day length as a signal that they should start making flowers and produce fruit. They can distinguish between 16 hours of light and eight hours of light. This process places the likely odds that fruit or seed will be produced in the best weather. Day length gradually increases in the spring.
The alarm clock for flowering is set differently for all plants. This is a survival technique for plants. It also helps provide food for us over a broad range of conditions. Some plants also use a process by which they are able to sense that they've been exposed to cold for a very long period of time. They're able to sense that essentially winter has passed. Thus, there may be more than one timekeeper in the control of flowering.
Trees and shrubs that bloom early will probably not put out as many blooms as normal. These trees are putting out blooms that were formed last summer. A very cold spell will affect both the bloom and the early leaves. The trees will generally have sufficient stored energy so that the spring damage will be virtually unnoticeable by summer. Some trees can be damaged by a sudden freeze after a warming trend. This occurred here in 2007. Many landscape trees were damaged. Many forest trees died but, overall, nature recovered.
The last word
Maybe there are better ways to predict nature. According to German folklore, if a groundhog sees his shadow on Groundhog Day, there will be six more weeks of winter. If not, an early spring is expected. Pennsylvania's Punxsutawney Phil saw his shadow this year. However, our own Chattanooga Chuck didn't see his shadow. So we get springlike weather for the remainder of the winter months, according to the legend. Maybe we simply need to listen more to our most knowledgeable plant and animal friends as we travel on our spaceship Earth.
To see USDA Plant Hardiness Zone Map for 2012, go to http://planthardiness.ars.usda.gov.
Contact Tom Stebbins at tstebbins@utk.edu or 423-855-6113.