hardiness zones established in the USA, Zones 1 and 11 represent the extremes. The bulk of the United States, though, fits into Zones 5, 6, 7, and 8. Originally, the zones were conceived to be 10°F apart.
As you study the map or chart, say you find that you live in USDA Zone 7. So you determine that this means
You should be able to grow any tree, shrub, or perennial labeled “hardy to USDA Zone 7.”
You probably can’t grow plants that are less cold-hardy, such as Zone 8 or 9 ones — your colder winters may harm or kill them.
You can grow plants labeled for farther north, even more cold-tolerant ones said to be “hardy to Zone 6 or 5.”
However, every rule has an exception. Most gardeners can stray one, maybe two USDA zones from their own when making plant choices and the gamble will pay off. See “Zoning Out: Breaking the Zone and Growing Season Rules,” later in the chapter.
You often see a plant’s projected USDA Hardiness Zones expressed as a range. For instance, most clematis hybrids are said to be “hardy in Zones 3 to 8.” This statement means anyone gardening in Zones 3, 4, 5, 6, 7, or 8 ought to be able to grow one; the plant should survive your winters.
Basic zones are based on the average annual minimum temperature — in other words, as cold as winter gets. Thus, in the USDA Plant Hardiness Zone Map, Zone 6’s lowest winter temperature (on average) is –10°F (–12°C).
Why base zone maps on cold temperatures? After all, other things doom plants, such as high heat, lack of water, too much water, too much or too little sun, and the wrong soil. But over the centuries of gardening, people found that cold is a better predictor of what will survive than any of those other (admittedly important) factors.
A HOT ISSUE: CLIMATE CHANGE AND ZONE CHANGES
The USDA revised its map in 2012. If climate change continues to accelerate as many people fear and as more and more scientists believe, the averages will rise. But for now, expect the numbers of the zones to remain the same, even if the details of each zone changes.
The most important revisions made to the existing zone maps so far have been to list finer details. For example, in the USDA map, the more central zones have been split into halves, so you can also see Zone 5a and Zone 5b and so on, with the a being somewhat colder and the b being somewhat warmer. Canada’s map includes even more split zones.
Sometimes gardeners blame heat or cold for the death of their plants, but other reasons may be the causes, such as following:
Poor drainage: Poor soil drainage is big killer of plants especially during a wet winter.
Dry conditions: A very dry winter can also cause a lot of damage and even plant death for evergreen plants that continue to use and lose water during the winter. These plants should be watered when the ground isn’t frozen, if needed. Sometimes it’s winter drought, rather than winter cold, that kills a garden plant.
Winds: Drying winds can be very damaging. Some gardeners provide windbreaks to relieve this problem.
Varmints: Don’t forget varmints, especially voles and mice that can do invisible underground root, stem, and bulb damage that occurs most in the winter when less of their natural food is around.
Climatological data comes from the horticulture industry, university researchers, botanic gardens, and so forth. Sometimes when a new plant enters commerce, the zone rating is conservative and after a while — with more people growing the plant in different areas and with more research — they find the plant to be more or less cold-tolerant than they originally thought.
Fathoming Frost Zones and Growing Seasons
Whether a plant can survive the winter isn’t your only concern. You know annuals are going to live for only one season, but you also want to know how long that season will be. After all, you may not be pleased if your dahlias die before flowering or your tomato plants freeze before producing much fruit. Unfortunately, hardiness zones don’t tell you much about the length of the growing season. Enter the frost zone map.
Zones are determined not only by temperatures but also by the climate, which combines temperature readings, rainfall, humidity, wind, air pressure, and other factors. Climates in frost zone maps are generally determined by growing season, the time during which — hold onto your hats — plants add new growth. The last spring frost and the first fall frost bookend the growing season, marking a nice period of frost-free days. Basically, this time period is your window of opportunity to plant and nurture and enjoy your home landscape, whether you’re growing flowers or edibles. Make the most of it!
In really mild areas, such as parts of Southern California, Florida, and the Gulf Coast, the entire year is likely to be frost-free and wide open. The rest of us may feel we’re not as lucky (but just remember that those folks, and their plants, have to contend with intense summer heat and humidity, not to mention bugs that are never killed by frost). Look at it this way: If you have a colder winter, you get a break, a chance to stop and relax — and a chance regroup and plan for an even better garden next year, after the last frost is past.
The length of a growing season varies somewhat from year to year but is generally about the same. You probably already have a sense of your growing season, but if you really need to know, finding out is fairly easy. Call your nearest Cooperative Extension Service office, ask a knowledgeable gardener or garden-center staffer, or watch your local newspaper for the frost dates (which can vary from one year to the next). Many gardeners also use frost zone maps, like the ones here:
Bonnie’s Plants (https://bonnieplants.com/gardening/first-and-last-frost-dates/)
National Gardening Association (https://garden.org/apps/frost-dates/)
Here’s how to calculate your growing season: Suppose you live in Denver, Colorado; your last frost is May 3, and your first fall freeze is October 8. That gives you 157 days in which to garden. Or suppose you live in Las Vegas, Nevada; your last frost is March 7, and your first fall one is November 21. You get 259 days of growing.
Zoning Out: Breaking the Zone and Growing Season Rules
Despite all the zone maps and all the research, hardiness remains an inexact science. Although some plants turn out to be surprisingly tough, others succumb unexpectedly. The easiest thing you can do is to set your sights on plants said to be appropriate for your area. Here are some tips on deciding