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Plant Hardiness Zones Explained

Posted: Saturday, November 04, 2006

by lorien1973
Online Discount Mart Home & Garden Decor

I have been gardening in some capacity all of my life. I started when I was really little, helping my mom sow the seeds and plant the flowers that we bought together at the store. I still remember wearing my little straw gardening hat and carrying my little basket to go pick the baby carrots we had grown from seed. I felt so important and so big at that moment, at the ripe old age of five. As the years passed, I believed that I knew everything I needed to know about gardening, that is until I went to pick out my own plants, without the guidance of my mom.

I scoured the greenhouse like a professional, scrutinizing every plant and radiating self-confidence throughout the process. I even ventured a few times to share my ‘gardening wisdom’ with other patrons. When I finally got down to focusing on what I had come for, I found myself dazed and confused when I read several of the plastic labels on the plants I was interested in. ‘Hardy to zone 7’ read one label. ‘Planting zones 6-9’ read another. It was like reading Greek without recognizing that it was Greek. After making such a display of my knowledge, I was embarrassed to ask an employee what it meant. I very quickly and very quietly tucked my tail between my legs and left the greenhouse, determined to unravel the mystery of this odd language. I went to the best source I knew. I went and asked my mom.

As it turned out, the concept of zones was critical to successful planting, especially when it came to perennials. I absorbed everything I could from my mom’s years of study and research, and then I did a little research of my own. Here are a few of the things I learned along the way:

What are hardiness zones? Hardiness zones (a.k.a. plant hardiness zones, planting zones) are the single most important consideration when it comes to choosing what plants to grow, because the zone numbers indicate the climate limitations for the plant. When a plant falls within the planting zone of your region, it means that the plant will survive the temperature ranges in your area.
What do the numbers mean? Each zone is given a number from 1-11. The lower the number, the colder the region that zone covers. The higher the number, the warmer the zone. Zone numbers focus on the average low temperature in an area. Zone 1, which is the coldest region, covers areas where temperatures on average drop to -50 degrees Fahrenheit (F) and below. Zone 2 has an average low temperature of -50 to -45F, and so on, until you get to zone 11, which on average never gets below 40F. This is the warmest region.
How do you determine what zone you are in? Plant hardiness zone information has been translated into a color coded map, indicating the precise location of each zone. Depending on its complexity, many of the zone maps show not only the various states but the individual counties, making it much easier to pinpoint with complete accuracy which zone you live in.
How accurate is the map information? The information used in the development of hardiness planting maps was collected over a 60 year period, taking averages of the lowest temperatures recorded throughout that time. With current changes in regional climate temperatures, the zones have been adjusted and broken down into sub-zones. These sub-zones more accurately reflect certain exceptions to the averages, like those found in the mountains of Appalachia, where temperatures can be considerably lower than surrounding areas. These sub-zones are indicated with the letters ‘a’ and ‘b’. Further sub-zones may be added in the future as regional temperatures change.
Where can I find a plant hardiness zone map? Zone Maps can be found in a wide range of places. Most plant magazines and books have maps either in the front or in the back, near the index. Also, most local farm coops can give you the specific zone information, as well as the last frost dates. Many types of hardiness planting zone maps can be found on the internet.
How do you interpret and use zone information? Remember that hardiness planting zones refer to a plant’s ability to withstand the average low temperatures of a region and survive. If a plant is labeled Zones 7-9, that means that the plant will survive the average winter temperatures for those zones. It does NOT mean that the plant cannot be planted and enjoyed in other zones. It simply indicates that it would be best to either dig the plants up and store them over the winter in a warmer location or to treat them as annuals by pulling them up in the Fall or allowing them to die in the ground.
Is there any way that a plant could still die during the winter, even if you live in the plant’s hardiness zone? There is always that chance, no matter what zone you live in. Plant survival depends on a number of variables like temperature, moisture levels, nutrient levels, and pest control. The hardiness zone maps only address the temperature lows. If the average temperature lows of a season dip below the average temperature lows of the hardiness zone maps, then plants might be in danger. To prevent unforeseen damage over the winter, it is always good practice to insulate plants with a thick layer of organic mulch. Such insulation will protect even more tender plants, offering the best opportunity for survival.

It is important to know that ‘annuals’ refer to plants that complete their lifecycles within one growing season. Annuals are not covered by the hardiness zones for this reason. Any plant that survives through any winter will not be considered an annual. If an annual is labeled ‘hardy’, this means that it is a free seeding variety, and the seeds most likely will survive through the winter.

Armed with this information, even the most inexperienced gardener can boldly step into a greenhouse and understand the plant labels they read. I learned a lot that day in the greenhouse. I learned that I didn’t know as much as I thought I did. I learned that it is okay to ask questions, even if you have a great deal of experience behind you. But most importantly, I learned that there was no better planting partner than my mom. Not only did she know a ton about plants, but she was the most fun to work with. So next time you sit down to plan out your garden, be sure to call the person who taught you to love digging in the dirt and thank her for everything she shared. Helen Stephens

Lorien1973 writes articles about home decor and garden decor for such sites as Online Discount Mart.

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