Gardening 101: Plant Hardiness Zones

Arum Lily

photo credit: Tom Curtis / FreeDigitalPhotos.net

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As you start getting excited about starting your garden, you’ll eventually find yourself scrolling (or thumbing) through countless pages of plant nursery catalogs, racking up quite the wish list. There are bazillions of plants out there, and they’re only a few mouse clicks away. But before you go crazy with glee, and start ordering plants all willy-nilly, be sure you understand plant “hardiness zones”. This is very important, if you have any expectation of your perennials surviving more than a single year.

What are plant “hardiness zones”?

The USDA (United States Department of Agriculture) Plant Hardiness Zone Map divides North America into 11 “zones”. Each zone represents a 10 degree Fahrenheit temperature range, and it’s the average coldest temperature range for that area.

Zone 1 is the coldest (-60 to -50 degrees Fahrenheit).
Zone 11 is the warmest (40 to 50 degrees Fahrenheit).

There are critics of the USDA Plant Hardiness Zone Map because it’s not an all-encompassing tool. There are factors it does not take into account. But it’s still a VERY useful and helpful tool.

Western states can also use the Sunset climate zones, a more robust climate tool, specifically created for the western states.

What do plant hardiness zones have to do with my flower garden?

Think of plants and plant hardiness zones as people (plants) and outdoor temperatures (plant hardiness zones). People are comfortable outdoors in a range of temperatures, but as the winter season ushers in colder, harsher temperatures, at some point, coats and gloves become a vital means of survival. Well… some plants have “invisible” apparel. Some have short sleeves; some have jackets; and some have little coats and gloves. [Don't make fun of them.] So, plants need to be in temperatures that accomodate them and their tolerance, or lack thereof, of cold temperatures.

Most plants do well in a range of zones. For instance, Astilbes can live in hardiness zones 4-8. Indiana is divided into two hardiness zones – zone 5 (northern and central Indiana) and zone 6 (southern Indiana). So, Astilbes are very happy in Indiana. Calla lilies, on the other hand, need to live in zones 9-11 (a much warmer climate), so they would not survive a zone 5 or 6 winter.

If you find a plant you really want in your garden, but its hardiness zones fall outside of your zone, there is hope. As long as the temperature in your area, at some time during the year (and for a long enough period of time), falls within the range the plant needs, you can incorporate it into your garden. You’ll just have to dig it up when the temperature reaches the limit the plant can handle. Once the temperature comes back around to what the plant needs, you can plant it again in your garden.

Example:

For instance, Calla lilies, which need a zone 9-11 (zone 9 means the temperatures never drop below 20 to 30 degrees Fahrenheit), can be planted in my Indiana yard. But once the temperature drops to 20 degrees Fahrenheit, the Calla lily bulbs have GOT to be out of the ground and in safe indoor harbor. Come spring, once temperatures rise to at least a steady 20 degrees Fahrenheit, I can place them back in the ground. However, 20 degrees is still “freezing”, so I’m not about to go out in the dead of late winter/early spring and try to crack open the ground just because the thermometer indicates, if planted now, the bulbs will survive. I think you get the picture.

In Summary

If you purchase your plants from your local nursery or garden store, most of their perennials will be perennials that are good year-round for your area. There will be exceptions though. If, however, you order plants online, knowing your hardiness zone is of utmost importance, because the nursery will be providing plants for possibly 11 zones, although I can’t say I’ve ever noticed a plant being sold for zones 1-3. (I’ve just never noticed it. That doesn’t mean you won’t find them.) It’s up to you to know which plants will survive in your garden.

And, if you happen to see a plant that lists a single zone (e.g. “zone 4″) in lieu of a range of zones (e.g. “zones 4-8″), it means the plant is good for that zone and any zone above it (i.e. zone 4 and above).

In a future post I’ll go over how you should store bulbs over the winter.

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4 comments to Gardening 101: Plant Hardiness Zones

  • Amber

    Planting, digging up, and planting again kind of sounds like you’re creating your own annual flowers out of bulbs — without the expense of buying them each year!!

  • Heather

    Thank you. I didn’t realize what Indiana’s zones were. Sometimes having things explained like this helps to answer questions I wasn’t really sure I had. I just didn’t know what I didn’t know :). I like your “don’t make fun of them” comment.

    I have considered buying bulbs that some have told me have to be dug up for the winter…but I would probably just put them in planters and move the planters into the garage or basement for the winter.

    • Holly

      LOL… I know what you mean – not realizing I didn’t know something until someone told me something I didn’t know I was suppose to know… or that there was anything to know at all on the subject. (I like your sense of humor.)

      Yes, planting bulbs in pots is a good alternative to digging them up and replanting them. I’ve wasted a lot of money on plants by not getting them dug up, or, more recently, I was finally disciplined enough to get them out of the ground, but they never saw dirt again. They’re still sitting in a box, 1-2 years later, waiting to be thrown away. :( I do best by sticking to my hardiness zones. Although, I suppose I could give it one more try by using pots, like you mentioned.

  • [...] Know your plant hardiness zone. This is especially important for flower gardening. If you’re a relatively new gardener, not understanding this concept is a sure way to get disappointed, come spring. [...]

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