Plant Hardiness Maps

Plant  Hardiness, Mapping and Climate Change

The U. S. Department of Agriculture has issued plant hardiness maps depicting the average minimum winter temperatures across the U.S. and Canada since 1960. The maps provide guidance to gardeners in the selection of plants that will tolerate the winter conditions in their areas. The distribution of hardiness zones is based on records from the numerous weather stations in the U.S. and, to a lesser extent, Canada. The maps divide the country into 10 degree intervals  ranging from below -50o  (F.) in northern Canada to + 30 to +40 degrees in the Florida Keys. Five degree half-zones also are delineated.

The 1960 plant hardiness zone map issued by the USDA

The 1960 plant hardiness zone map issued by the USDA

The first map, issued in 1960, used temperatures for the years 1899 through 1938, except for 34 states for which data were “readjusted” based on additional data from 1931 through 1952. Presumably the readjustments improved accuracy.  Another level of data adjustment also was incorporated where qualified supplemental data were available.

 A revised  USDA map was issued in 1990 that relied on nearly double the number of weather stations than the first map. Comparison of the two maps shows that because of unusually cold winters in the 1974-86 period in Canada and the eastern states, some areas were depicted as colder than in the first map. Later, however, recognizing the warmer winters in the 1986 to 2002 period, the American Horticultural Society, undertook a revision of the hardiness zones. The resultant map, released in 2003, more resembled the 1960 map than the 1990 one, in that many spots were depicted as a half-zone warmer. Warming trends prompted the National Arbor Day Foundation to review the data and in 2006, release a map that depicted changes in zonation between the 1990 USDA map and 2006. The frequent shifts to the next highest zone (the pink areas in the map below) produced a dramatic pattern. The USDA, however, continued recommending its 1990 map, until finally issuing an updated one in 2012  based on data for the 1976-2005 period.  This map used updated data analysis techniques and should have addressed the bias in the 1990 map brought on by the aforementioned cold interval.  Indeed, the USDA states that increased sophistication of mapping methods and better data explain much of the differences in observed patterns. So, presumably, do we now (in 2013) have the most accurate representation of hardiness zones?


 Apparently not, according to Dr. Nir Krakauer, of The City College of New York’s Grove School of Engineering. He developed a new method for calculating  and mapping  the plant hardiness zones. His results recognize the warming trend embedded in the latter portion of the 1976 to 2005 data base.  Krakauer found that over a third of the U.S. has shifted by a half-zone, and a fifth of the country, a whole zone, compared to the USDA’s latest map. He also notes that winters are warming faster than is summer, a trend consistent with  global warming. Changes appear to be occurring at a rate that cannot be well captured by the USDA’s approach.

USDA's plant hardiness map issued Jan. 2012

USDA’s plant hardiness map issued Jan. 2012

Weather, is of course notoriously variable and will continue to be, perhaps even more so in a warming world.  For example, a run of unusually cold winters could occur in response to  volcanic eruptions, sun spot-cycles or shifts in the  Arctic Oscillation, to name 3 phenomena known to affect  global or regional weather. If so, the value of the current hardiness map could decrease, and it’s possible that, for example, the camellias that have survived recently in Detroit winters might be winter-stressed or even killed back. More timely maps would be the best defense against winter onslaughts and Krakauer has offered a method that would allow for annual updates. See the following for more information:  (or CUNY Newswire) andAdvances in Meteorology Vol. 2012, article ID 404876. The map depicting zone shifts is from

Note also that depending on your location, your spot may still be within the same zone even though minimum temperatures have somewhat moderated. That’s true for me. My little oasis has remained in zone 9b on all the USDA maps, 1960 to 2012, although the zone 10 boundary has crept closer. Based on my minimum yard temperatures for the period 2004 to 2011, the USDA zone 9b seems correct. The minimum temperature for the period was 26  and the number of days with below freezing temperatures ranged from 1 to 6 annually.  I still have to protect many low temperature-sensitive species in the winter but that is largely because I choose some zone 10 category species for my yard.  


Lawns- a few numbers to ponder

Lawns  |  a few numbers to ponder:

From the Environment Protection Agency  (, we learn that an American family of four  uses 400 gallons of water per day; about 30% of that is used outdoors. Over half  of the outdoor  water is used for irrigating lawns and gardens. The total water use nationally for this purpose is more than 7 billion gallons daily!

Today 30 to 40 million acres are given over to lawns in America, 20 million acres of  which are residential.  Think of what is required to maintain a so-called healthy lawn-gasoline to run the mowers, fertilizer and supplemental water to keep the grass dense and actively growing , pesticides to control plant and animal competitors (designated as pests), and time and effort to keep on top of lawn management. Looking further, fossil fuels are, of course, burned to generate the electric power needed to make the equipment  and to manufacture fertilizer and pesticides (for a lawn that’s the envy of the neighborhood, you’ll need to buy insecticides, herbicides and maybe fungicides, rodenticides and nematicides!). And, of course, anytime fossil fuels are combusted, residual chemicals are emitted into the atmosphere. One estimate is that 5% of the nation’s air pollution is attributable to lawn maintenance. Operating a typical gasoline-powered mower for one hour emits the same quantity of smog-forming hydrocarbons as driving a typical car almost 200 miles.  And, of course, these emissions add to the greenhouse gas loadings that affect our climate.

In citing these figures, we grant  that lawns provide benefits- exercise,  picnics, barreling around on a tractor mower,  jobs, and for some, a reason to be outdoors.  On the other hand there are even more concerns: For example,  wildlife  poisonings.  Over 200 pesticides  are approved by EPA for lawn care although  most applications are restricted to abut 35 of these.  Homeowners use up to 10 times more chemical pesticides per acre than farmers do on crops, according to the U.S. Fish & Wildlife Service. 

Recent Canadian studies found that pesticides applied on farmland may explain three to 14 bird deaths per acre.  In the State of New York, testing of dead birds provided for West Nile Virus assessment  in 2001 found that, of the more than 80,000 birds tested,  the leading cause of avian deaths was pesticide poisoning not the virus (  Other concerns include insect pollinators (especially bees and  butterflies) essential for many crops such as squash and tomatoes, which are declining across much of the country.  One of chief causes appears to be pesticides. Fish and amphibians also are apparently declining and pesticide applications are partially responsible (

Much more could be said about confirmed and suspected impacts of lawn-applied chemicals upon aquatic and terrestrial  environments.  There is considerable controversy over the efficacy of treatment approaches recommended to home owners  and over  the effects upon non-target species and ecosystems.  In fact, it is a vast topic, which does not include  human health effects and well water contamination which are  real concerns but  beyond the purview of this site. I mention only that at least the state of Connecticut has recognized the need to prevent pesticide applications (with exceptions for mosquitoes and biting or stinging insects) in and around day care centers and schools ( And  lastly, what  about all that noise! Couldn’t  less intrusive mowers, blowers, pruners and power saws be offered in the market (electric mowers are quieter, of course, but are limited to small lawns on flat terrain and still fossil fuel-based)?

Here are a few simple approaches to advancing your yard to a point where it can begin to be thought of as habitat for plant and animal wildlife.  The first photograph shows a patch of a native wild flower that can succeed in lawns without becoming a “problem”. It is growing  in a portion of a lawn that has not been subsidized via pesticides, fertilizer, or water for at least seven years.  Yes, the photo is taken in the late summer of central Florida when rain is plentiful. But the patch has also gone through the hot dry season each year  when water is limiting. And by late April, water stress effects will be evident.  But with the rainy season, growth recovers nicely.  The site was mowed once every several weeks roughly from May through October.  Six  plant species are visible here in a patch of 500 cm2 competing with the grass but not overwhelming it and  forming a much more diverse and to my eyes, more pleasant view.


The photograph centers on the rosettes of Erigeron quercifolius, a native biennial  that produces pleasing aster-like flowering heads and succeeds in the coastal plain of Georgia and Florida where lawns are not mowed too closely. In the spring-summer flowering period, the plant bolts to about 20 cm high. Because it tends to occur in groups mowing can avoid them at this period. The plant produces seeds which can be collected or left to disperse, and then dies, limiting any invasive tendencies. Its first year is spent as a basal rosette shorter than most mowing heights. Comparable species are likely to be available in other states.

The second photo shows the tiny Dichondra carolinensis, a stoloniferous species of the morning-glory family that gets along well with lawn grasses where there’s some protection from summer sun and and , of course, from  herbicides. Throughout the southeastern coastal plain this species will thrive where soils are wetter and shadier than optimum for grasses. 


Where trees are present, possibilities multiply.  Beneath hardwood trees, lawns generally fail even if sparse grass plants hang on. Fortunately they can be replaced by ferns which are perennial, shade-tolerant, need no fertilizer and are naturally pest-resistant. Below see  two ferns, a Nephrolepis and a Thelypterus established at the base of a live oak in a central Florida yard. This  small patch of semi-natural habitat has been observed to give shelter and,or food to anoles, toads, Black Racers, Florida Brown Snakes, gray squirrels, birds, and invertebrates. 

Each of the photographs has illustrated three simple ways a lawn can be given greater interest and diversity while at least decreasing if not eliminating some of the fossil-fuel derived lawn maintenance products conventional lawns depend on. 


Some backyard fungi


Mycorrhizal fungi are ancient plant mutualists, i.e.  they are engaged in a mutually beneficial physiological relationship with plants.  The mycelia (the thread-like filaments that make up the vegetative, below-ground phase of mycorrhizal fungi) grow in close contact with plant roots, often fully enveloping them.  The mycelia absorb minerals like phosphorous and nitrate from the surrounding soil which become available to the “host” plant ( usually a tree or shrub)and in exchange obtain  organic carbon which the fungi then metabolizes for energy.  The carbon can be traced back to the host’s foliage which has converted atmospheric carbon dioxide to organic carbon compounds by means of photosynthesis.  Myc are widespread and form associations with most terrestrial plants including crops. One estimate is that perhaps 80% of all vascular plant species are mutually involved with mycorrhizal fungi!  The mycorrhizae enable plants to colonize nutrient-poor soils that would otherwise be less favorable for plant growth.  Most of our crops also benefit from mycorrhizal fungi thus incorporating  land areas into cultivation that would be otherwise inadequate for human food production. Interestingly mycorrhizal fungi generally do not occur independently of host plants and in fact, some are found only in association with a single tree species.

Mycorrhizal fungi produce basidiomata (“mushrooms”), a cap (pileus), or fruiting bodies) like some other kinds of fungus. These can be found in walks through the woods or even in yards and fields. Some species are edible while  others are poisonous to humans.


This group of fungi inconspicuously carries out the vital function of  recycling of nutrients through the decay of  wood and other plant tissues. Here is a photograph of a colony of  Armillaria tabescens.  a gilled mushroom  that sprang from an expansive growth of mycelia that sprang from underground roots of a red maple in our back yard.  Unfortunately for the maple, the mushroom is an aggressive invader of living and dead trees, and can be an important agent in the decline and mortality of forest trees. In cases where the fungus attacks living trees, they would be designated as parasitic rather than saprophytic.  The host tree (not visible) is in decline, with dead main branches, a well-thinned canopy and  sloughed-off bark at base.  Downy and Red-bellied Woodpeckers are frequent visitors and the former species successively (and exhaustively) excavated a nest-hole in a secondary (dead) branch. The tree has hosted multiple colonies of  Antennaria tabescens for at least 5 years, but likely was under siege via root-rot, for longer.

Other fungi such as the oyster mushroom  have  advertised their presence through their mushrooms but only the Armillaria   ‘root-rot’  has appeared predictably every year.  The Armillaria colonies occur mostly in autumn,  quickly passing through the above-ground phase in a  week or so. They are first seen in a small  “button” phase, although undoubtedly closer inspection (hands and knees) would have revealed even smaller phases.  Several days later the  button stage has expanded into  fully opened nearly flat caps with exposed gills lining the underside of the cap.  Along the gill surfaces are lined countless small, microscopic  cells, each bearing  four spores capable of  developing in to the next generation of fungus.

A new colony of Armillaria that emerged at the base of a dying red maple, 2012.


Mature Armillaria colony. Note whitish bloom on some caps from released spores.