Tidewater Gardening - March 2012

Whacky Winter Weather


K. Marc Teffeau

Daffodils blooming in February? Red maple buds swelling and turning red? What’s happening here? Well, we had an unusually warm January, and now into the second week of February the colder temperatures of winter have finally seemed to settle in. But, as you know, we can get slammed in February with very cold temperatures and snows into March. Are the warm temperatures the result of global climate change? Sunspots? El Nino? Who really knows?
The important impact of the warm January is that some of our woody plants did break bud and come somewhat out of cold dormancy. Hopefully the flower buds on the fruit trees did not break enough to experience damage, especially if we experience bitter cold in the next couple of weeks.
I noticed that in some protected areas of the landscape forsythia bloomed and flowering cherry and plums opened up and showed color. The good news is that the January warm spell is not expected to negatively impact the cherry trees around the Tidal Basin in Washington, D.C.
It is interesting that during this warm spell the USDA Agricultural Research Service released the long-awaited revision of the Plant Hardiness Zone Map (PHZM) on January 25th. This release provides an update of a very useful tool for gardeners and researchers for the first time since 1990. The map has greater accuracy and detail than the 1990 map and is a collaboration of between the Agricultural Research Service and the Oregon State University’s (OSU) PRISM Climate Group.
For the first time, the new map offers a Geographic Information System (GIS)-based interactive format and is specifically designed to be Internet-friendly. The map website also incorporates a “find your zone by zip code” function.
Static images of national, regional and state maps also have been included to ensure the map is readily accessible to those who lack broadband Internet access. A ccording to Catherine Woteki, USDA Undersecretary for Research, Education and Economics, the map is the most sophisticated Plant Hardiness Zone Map yet for the United States. The increases in accuracy and detail will be extremely useful for gardeners and researchers.
A little explanation is in order to help understand the map. Plant hardiness zone designations represent the average annual extreme minimum temperatures at a given location during a particular time period. They do not reflect the coldest it has ever been or ever will be at a specific location, but simply the average lowest winter temperature for the location over a specified time. Low temperature during the winter is a crucial factor in the survival of plants in specific locations.
The new version of the map includes 13 zones, with the addition for the first time of zones 12 (50-60 degrees Fahrenheit) and 13 (60-70 degrees Fahrenheit). Each zone is a 10-degree Fahrenheit band, further divided into 5-degree Fahrenheit zones “A” and “B.”
To help develop the new map, USDA and OSU requested that horticultural and climatic experts review the zones in their geographic area, and trial versions of the new map were revised based on their expert input.
Compared to the 1990 version, zone boundaries in this edition of the map have shifted in many areas. The new map is generally one 5-degree Fahrenheit half-zone warmer than the previous map throughout much of the United States. This “warming” is not the result of climate change but is mostly a result of using temperature data from a longer and more recent time period; the new map uses data measured at weather stations during the 30-year period 1976-2005. In contrast, the 1990 map was based on temperature data from only a 13-year period of 1974-1986.
However, some of the changes in the zones are a result of new, more sophisticated methods for mapping zones between weather stations. These include algorithms that considered, for the first time, such factors as changes in elevation, nearness to large bodies of water, and position on the terrain, such as valley bottoms and ridge tops.
Also, the new map used temperature data from many more stations than did the 1990 map. These advances greatly improved the accuracy and detail of the map, especially in mountainous regions of the western United States. In some cases, they resulted in changes to cooler, rather than warmer, zones.
As a member of the USDA technical review group for the map, I can vouch for the very sophisticated way it was developed and its accuracy. A couple of years ago the National Arbor Day Foundation put out their own “revised” plant hardiness map for their promotion and marketing efforts. When I called and talked to the person in charge of their map development he would not answer my questions as to how the map was developed or what statistical methods or weather data sets were used. The new USDA map has been peer reviewed and verified for its accuracy.
Now, I know that there are always “outliers.” I have already seen some comments posted in the nursery trade press saying that the map was not accurate for “their location.” We have to remember that there are always unique locations and microclimates that vary from the general map.
For example, there is a location near Salisbury that is equally distant between the Chesapeake Bay and the ocean which is historically a few degrees colder in the winter than the surround area. My county of Caroline is divided between zone 7a and 7b. Where I live near Bethlehem is zone 7a, while you go five miles north to Harmony and it is in zone 7b. In fact there was a question as to the accuracy of the map for the Eastern Shore. The design team for the map verified with local sources that the zones indicated were correct for the area. If you look at the map of the Delmarva Peninsula you can see how the Bay and the Atlantic Ocean influence the temperatures.
While about 80 million American gardeners, as well as those who grow and breed plants, are the largest users of the USDA Plant Hardiness Zone Map, many others need this hardiness zone information. For example, the USDA Risk Management Agency uses the USDA plant hardiness zone designations to set some crop insurance standards. Scientists use the plant hardiness zones as a data layer in many research models such as modeling the spread of exotic weeds and insects. Many nursery crop producers reproduce the map in their sales catalogs for their plant material and we also see it on seed packages, and in gardening magazines and other publications
In the past, a poster-sized version of this map was available for purchase from the government. With the new Web-based format, anyone may download the map free of charge from the Internet onto their personal computer and print copies of the map as needed. To access the map go to http://planthardiness.ars.usda.gov/PHZMWeb/Default.aspx and put in your zip code to get your specific zone. You can download the zone map of your area and also zoom in and out of the map locations.
Assuming the March temperatures will be normal this spring, now is the time to start working the vegetable garden. Garden peas, radishes, onions, spinach, turnip greens and collards will all grow well in a cool soil, which means they can be planted towards the end of March. Other cool-season crops include broccoli, cauliflower, kale, and cabbage. Potatoes and salad vegetables, such as lettuce and carrots, round out the cool-season planting. You can seed spinach later on in the month as it does well in cool weather.
March and early April are the best times to transplant all bare-root plants including fruit trees. It is important that the roots become well established before their buds break into active growth. In order to develop and grow properly, leaves and young developing stems require a constant supply of water and nutrients. These needs can only be met by transplanting the plants early, before growing conditions become favorable for new leaves to appear.
Although you may not realize it, roots of most woody trees and shrubs begin to grow when the soil temperature reaches 40ºF. This is also an excellent time to plant balled and burlaped and container-grown plants into the home landscape. This will give them time to become established before the hot weather appears.
Happy Gardening!