Horticulturist explains new USDA plant hardiness map

Feb 16, 2012 By Lois Stack

The recently released USDA Plant Zone Hardiness Map, updating the previous 26-year-old zone map, reflects rising mean temperatures throughout the country, including Maine, where gardeners must carefully consider possible temperature dips as they anticipate spring planting.

University of Maine Cooperative Extension professor and ornamental horticulture specialist Lois Berg Stack is available to explain what the new map might mean for Maine gardeners, in addition to suggestions about preparations for early planting.

The hardiness zone map has always been a suggestion for plant selection, not a guarantee, says Stack. Lots of other factors – length of growing season, accumulated heat during growing season, water availability, nutrient availability, soil type, general plant maintenance, etc. – are equally important.

“I usually tell people that the hardiness zone rating is a starting point,” she says. “And, although we do have climate change, and the new reflects some ‘warming’ in some parts of Maine, it is based on one thing: the single coldest each winter, averaged over several years. have to survive each day, one by one. Although our winters may be warmer on average, we still have cold temperatures every winter.”

Stack also says that February is the start of the gardening season – the time to start onion seeds for next winter’s storage, time to take cuttings of geraniums that have overwintered on a windowsill, time to visit a local garden center to for inspiration, or the time to take a gardening course or read a gardening book.

February also is the perfect time to cut tree and shrub branches to bring indoors to force their flowers into color, she adds.

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