United States Hardiness Zones
Based on the average annual minimum temperature for a given location, the USDA map provides an easy guideline for categorizing locations suitable for winter survival of a rated plant in an “average” winter. Since temperatures in the non-coastal-adjacent areas of the continent rarely present a consistent experience from year to year, and occasionally present a major—and often agriculturally devastating—deviation from the average minimum, the map has limitations for much of the country as a basis for using with long-term reliability, at least in areas close to the margin of a plant’s rated hardiness-zone.
The USDA first issued its standardized hardiness zone map in 1960, and revised it in 1965. A new map was issued in 1990, based on U.S. and Canadian data from 1974 through 1986 (and 1971–1984 for Mexico). The 1990-issue map was based on nearly double the number of stations, and it divided the temperature zones into five-degree a/b zones for greater accuracy. This revised map identified many areas as colder than did the 1960 map, due chiefly to a number of severely colder winters in the central and eastern U.S. in the 1974–1986 data-gathering period, as opposed to the mid-20th century data-sampling period used in the 1960 map; for instance, many locations in the southeastern U.S. set all-time record lows during January 1985.
The 1990 map shows 10 different zones, each of which represents an area of winter hardiness for the plants of agriculture and the natural landscape. The USDA introduced zone 11, representing areas that have average annual minimum temperatures at or above 4.4 °C (40 °F) and that are therefore essentially frost-free.
In 2003, the American Horticultural Society (AHS) produced a draft revised map, using temperature data collected from July 1986 to March 2002. This was a period of warmer winters than the 1974–1986 period, especially in the central and eastern U.S. The 2003 map placed many areas approximately a half-zone higher (warmer) than the 1990 map had. Reviewers noted the map zones appeared to be closer to the original 1960 map in its overall zone delineations. The 2003 AHS draft map purported to show finer detail, for example, reflecting urban heat islands by showing the downtown areas of several cities (e.g., Baltimore, Maryland; Washington, D.C. and Atlantic City, New Jersey) as a full zone warmer than outlying areas. The map excluded the detailed a/b half-zones introduced in the 1990 map, an omission widely criticized by horticulturists and gardeners due to the coarseness of the resulting map. The USDA rejected the AHS 2003 draft map; the agency stated it would create its own map in an interactive computer format.
As of August 2010 the AHS and the National Arboretum websites still presented the 1990 map as current.
In 2006, the US National Arbor Day Foundation completed an extensive update of U.S. hardiness zones. It used essentially the same data as the AHS. Once the Foundation analyzed the new data, it revised hardiness zones, reflecting the generally warmer recent temperatures in many parts of the country. The Foundation’s 2006 map appears to validate the data used in the AHS 2003 draft. The Foundation also did away with the more detailed a/b half-zone delineations.
In 2012 the USDA updated their plant hardiness map to reflect the warmer observed temperatures in the past thirty years.
Protecting your Cold Hardy Palm from damage or death – bottom line – protect the bud.
Palms grow from the inside out. The new growth will show up as a long spine, then split into multiple leaves. At Suwannee River palms we protect our P. sylvestris, European Fan, Windmill and Pindo palms by spraying copper fungicide prior to expected cold weather. This application adds a level of protecting to the bud from disease. We can’t cover and heat our inventory; this is our best protection.
When an extreme cold event is experienced, we witness some leaf browning. This is the palms normal reaction; the browning is a result of leaf “Desiccation”.
The plant losses water faster than the roots can uptake. Root absorption is reduced or prevented when the soil is cold or frozen.
Antidesiccant mixtures are available at your local garden centers. However, research has shown that these compounds degrade rapidly and are of little value to the palm.
To prevent damage, avoid late summer or early fall fertilization while plants are still actively growing. Avoid late summer pruning which stimulates new, tender growth and reduces the supply of nutrients available to the plant through the winter.
At Suwannee River Palms our Cold Hardy inventory consist of four Cold Hardy Palm varieties with different levels of cold tolerance. Our Pindo’s, Windmills and European Fan Palms may exhibit little to no leaf browning while the P. sylvestris will have some browning at the leaf edges or the feathers of the leaves.
A simple quick pruning will satisfy most visual damage to the leaves. This should be accomplished after the first flush of new growth shows in the spring.
The advantage Suwannee River Palms has over the mid to southern Florida growers, we have a winter and our Cold Hardy Palms are Truly Cold Hardy. We record minimum and maximum temperatures during the winter months – December through February. We have recorded as low as 9 degrees F. Our palms are grown and managed in this environment.
When you plant your new Cold Hardy Palm, give thoughts to where and what is around your landscape addition. If possible plant in a sunny protected area. Even in an area where other trees / shrubs are located. This environment will provide additional warmth, thus providing additional cold protection.
For additional information check out Betrock’s Cold Hardy Palms Hardcover – September 1, 2005 by Alan W. Meerow