This mild winter and spring, along with welcome rainfall, is helping to push the drought and heat of 2011 out of the minds of gardeners. While we should remain cautious regarding the potential return of drought conditions, it is hard to resist the urge to plant the garden, or to replant and replace drought-damaged plants.
In case you have not seen or heard, the United States Department of Agriculture (USDA) has just published an updated Plant Hardiness Zone Map (PHZM). This is a map which graphically shows the average annual minimum winter temperatures, divided into 10-degree F zones. This map has been in the works for several years, and reflects a lot of input not only from a multitude of weather stations, but also from folks in the horticulture industry. The last time the USDA PHZM map was updated was in 1990, and prior to that 1960, which was the first time the USDA produced a hardiness zone map. Arnold Arboretum has also produced cold hardiness zone maps in past years. The 1990 USDA map has been criticized because it only used a data set of 13 years of observations, while this new map uses a 30 year data set, with many more reporting stations, making it a more accurate and representative map than the 1990 version.
As you might expect, the cold hardiness zones have shifted warmer from the 1990 map. For the Tyler area, we moved from being on the border of zone 7b/8a (approximately 10 degrees F average minimum temperature) to the border of zone 8a/8b (approximately 15 degrees F average minimum temperature) – a 5 degree shift. Since I have lived in Tyler, I would say that 15 degrees is a lot closer to reality than 10 degrees for our average lows.
The new online map on the USDA web site has several new features, including being interactive, whereby you can zoom in to street level to see the climate zone, and is also searchable by zip code.
The PHZM web site is: http://planthardiness.ars.usda.gov/PHZMWeb/
Keep in mind that average minimum cold temperature tells only one part of the story regarding plant hardiness. For example, a plant may be rated cold hardy to zone 8a. But, if we had a very mild fall, with little cold temperatures to encourage plants to go dormant, followed by a sudden and severe outbreak of subfreezing cold, even hardy plants could be severely damaged.
Cold hardiness can also vary based on the duration plants are exposed to certain temperatures. A brief dip for an hour or two to 10 degrees followed by a warming trend might not harm a plant, while an extended exposure, or repeated exposures, to the same temperature might cause severe damage or even kill the same plant.
Some plants can handle cold temperatures if the soil is not wet, while others may rot when exposed to cold and wet soils.
You also have microclimates around your house, where cold air may collect in low lying areas, or pockets of warmer air occur near structures. Each of these can influence a plant’s hardiness in a specific location.
Still, knowing a plant’s hardiness in a typical winter is an important first step in selecting plants for the backbone of your landscape. It is fun to “stretch the zone” – growing plants that might be marginally hardy for our area – but such plants should be used with caution and not counted on for the permanent landscape display.
The average first freeze for our area is mid-March, still a week away, and this mild weather has everything coming out of dormancy. Winter may not be completely over yet. Remember it snowing in April in recent years? Hopefully, though, we have seen the last of freezing temperatures until next winter.