Pushing planting zones

Zones in the 2012 USDA Plant Hardiness Zone Map (PHZM) are based on 1976–2005 weather data. (A trial check did not find that the addition of more recent years of data made a significant difference in the definition of these zones.) Each zone represents the mean extreme minimum temperature for an area, calculated from the lowest daily minimum temperature recorded for each of the years 1976–2005. The previous edition of the USDA PHZM, revised and published in 1990, was drawn from weather data for 1974–1986. The longer period (30 years) of data was selected for the review of the latest revision as the best balance between smoothing out the fluctuations of year-to-year weather variation and the concept that during their lifetimes, perennial plants mostly experience what is termed "weather" rather than "climate."
Compared with the 1990 version, zone boundaries in this edition of the map have shifted in many areas. The new PHZM is generally one half-zone warmer than the previous PHZM throughout much of the United States, as a result of a more recent averaging period (1974–1986 vs. 1976–2005). However, some of the changes in the zones are the results of the new, more sophisticated mapping methods and greater numbers of station observations used in this map, which has greatly improved accuracy, especially in mountainous regions. These changes are sometimes to a cooler, rather than warmer zone.

Recently updated USDA Pant Hardiness Zone Maps might suggest that a gardener  could now consider plants that wouldn't have fallen within their previous zone.  Here, near Appleton, Wisconsin we are considered to be clearly in hardiness zone 5a, while we were close to the border with 4b on the older map.  When we lived north-east of here in the Green Bay area close to over 15 years ago, we were selecting plants that were hardy to at least zone 4b, while the area is now considered to lie in zone 5a as well, opening up the possibility of trying plants considered to be a bit less hardy in cold climates.

“Based on the new zone map, gardeners may be tempted to grow marginally cold hardy plants advertised in mail-order catalogs or at ‘big box’ stores,”  said Laura Jull, University of Wisconsin-Madison Associate Professor/UW-Extension Specialist for Woody Ornamentals. “However, hardiness zones are based on an average minimum temperature not the coldest it has ever been or will be in the future.”       

But is hard for most of us as enthusiastic gardeners to resist the temptation to experiment with growing some of the less winter-hardy plants featured in the catalogs appearing in our milboxes as wintr drags on.  So recommendations to follow when attempting to successfully "push planting zones" include:

• Alkalinity and water-retaining clay are not optimal conditions for pushing your zone. Add sulphur twice a year along with a "tremendous amount of light, friable material such as shredded bark, pine straw, sand or peat moss. 
• Better drainage improves any plant's chances of surviving a spell of adverse weather.
• While one building can block a steady wind, several can create a wind tunnel.
• Winds from the Northwest are the most damaging.   It's better to use evergreens as a windbreak rather a wall or fence to facilitate better allows air circulation. 
• A south-facing slope has a longer growing season, but you should also consider in its greater exposure. Frost flows downhill and may nestle in dips that otherwise seem protected.
• Shrubs don't reflect heat, which can bring plants out of the ground too early. Tender new growth is more vulnerable to a late cold snap, which can easily kill a plant that otherwise would have survived the winter. Consistent cold and an insulating snow cover provide optimal conditions for survival of marginal plants.
• Observe areas where plants survive a light frost, where reflected light brings a late boost to a crop or where some bulbs sprout earlier than others for indicators of unique micoclimate conditions.
• Try new breeds. Many of the new plant introductions expand the regions and growing conditions of some old favorites.

And it's not just the heat and cold that limit our growing potential. Those gardening in acidic (low-pH) soils may tire of liming, while those alkaline (high-pH) areas long for blue hydrangeas, blueberries and red maples. The bigleaf hydrangea's flowers are pink in our alkaline soil, but adding alumnium sulfate helps turn the flowers blue like the ones grown in acidic soil where aluminum is more readily available. I'm planning to move our remaining bludeberry plants tocontainers, where it should be easier to prepare a more acidic soil for planting.


2012 presented gardeners in Wisconsin with some callenging conditions.. With a premature spring arriving in March, followed by a blustery, wintery April, plants that had budded, leafed out or flowered were devastated by freezing temperatures. Tender vegetative growth and flowers were killed on many plants and apple and cherry yields were reduced drastically. May, and the traditionally wettest month, June, were excessively dry. Trees planted in 2011 and 2012 struggled to get established if no additional watering was applied. Exceptionally high temperatures during the summer baked plants.  Dwarf conifers, in particular, were scorched on top even though their rootballs were moist. Some established trees required extra watering.
The drought of 2012 was an excellent opportunity to test species for heat and drought tolerance. Some plants that did not do well included the Japanese Tree Lilac, hydrangeas, roses and many exotics.Plants that did well included established Junipers and Oaks, the purple-leaved Eastern Ninebark, Black Chokeberries, Sumac, Diervilla, the native St. John’s wort; Russian Sage and many  ornamental grasses. The drought- successful plants should be considered for more garden plantings, especially in
water-restricted areas, and for commercial landscape plantings.  No species should be considered
drought tolerant until they have established their root systems;  that may take 2-3 years for shrubs and 3-5 years for trees.
An extremely wet and unusually cold spring statewide followed in 2013 with the fourth wettest April  and the twelfth wettest May since record keeping began in 1895. Snow and standing water and multiple  freezes damaged plants.  A number of plants in our garden succumbed to the difficult conditions.

We lost quite a few of our shrub roses, particularly those exposed to the harsh winter winds off the lake.

By summer of 2013, we realized we had lost
 all 3 of these Sea Foam shrub roses 

Groupings of red and pink knockouts and red double knockouts
edging the landscaping beds in the front yard.
The roses in the foreground already show somewhat stunted growth
relative to other groups that are more protected from cold winter winds.

I moved some of the remaining shrub roses to fill in some gaps.  I'm still contemplating giving Sea Foam roses another shot.  But possibly I can find something with appealing white flowers that hardier and more likely to tolerate the persistent cold winds off the lake.

We also lost quite a few of the butterfly bush that were blooming profusely in 2012.

Butterfly bush

But butterfly bush has proven to be invasive on both U.S. coasts. Birds spread the seed to the point that it's popping up in unmanaged fields and roadside areas. A native of Asia, it has no checks and balances in the U.S.  butterfly bush has no evolutionary history with native insects.  There are other plants more beneficial to native butterflies and insects that go a lot further toward attracting and maintaining insect populations.
Suggested replacements include pasque flower, arrowwood viburnum, pale purple coneflow, purple prairie clover, tall blazing star, and aromatic aster, planted to provide a succession of purple blooms  When one key pollinator plant stops blooming, another turns on to take its place to mimic the long bloom time of butterfly bush,
Joe pye weed is an alternative for a tall display of pink billowy flowers, hosting over three dozen species of butterflies.

"One of the worst mistakes you can make as a gardener is to think you're in charge."

-Janet Gillespie