Understanding Plant Hardiness Zones

The USDA's list of plant hardiness zones categorizes geographical locations across the U.S. based on their estimated lowest winter temperatures.  The lower the number on the list, the colder the zone will be.

For example:  Zone-10 has an expected low temperature range of between 30 and 40 degrees F. (pretty temperate).  Zone-9, which is one number lower, has an expected low temperature range of between 20 and 30 degrees F.

The numbers change in 10 degree increments all the way down to zone-1 with a range of -60 to -50 degrees F (burrrr).  

Zones are further broken into 5 degree designations.  Zone-9a has a more precisely defined low temperature range of between 20 to 25 degrees F, while zone-9b has a range of between 25 to 30 degrees F.

All of this is based on data compiled by the USDA (United States Department of Agriculture) between 1976 and 2005. It doesn't take into account things like high temperatures, rainfall, humidity or soil conditions.

When you look at plant catalogs or seed packets, you often see recommended zone ranges for particular plants.  This is excellent general information that will help you decide, at a glance, whether or not a plant is well suited to the growing conditions in your garden.

I say general idea, because some areas across the U.S. are made up of microclimates, small pockets where the environmental conditions are a little warmer or colder than the norms on the hardiness zone chart.  Some areas may be protected from the elements by hills, making them warmer in winter, or they may be downwind of lakes or rivers that could make them a few degrees colder than they would be otherwise.

If you live where there's an atypical microclimate, the folks at your local nursery (or the local gardening club) probably know it.  You can also consult the USDA's Cooperative Extension Office in your area to obtain more information. 

Just because a plant will grow in your hardiness zone doesn't necessarily mean it's a good match, or that it won't require some TLC.  Remember, things like humidity and the expected high temperatures for a region have a big impact on how well certain plants will fare.  It's true that with the right care you can probably grow almost any plant, either outdoors, in a greenhouse or indoors.  When you're choosing plants, though, making sure you have plenty of easy-care varieties can help make your gardening efforts more fun than work -- and more successful, too. 

Here's another thing to remember:  Plant hardiness is a determiner of winter conditions.  It's typically a rating reserved for trees, shrubs and perennial plants (the kind that come back year after year).  If you're growing annuals (plants that flower, set seed and die in a single season), zone ratings aren't a big consideration unless you typically experience a very early frost in your area.

Next time, I'll post a list of perennial herbs and their hardiness zones.  It'll help you plan your herb patch this year.

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