Before acquiring a plant, gardeners usually want to know if it will survive winter. The USDA Plant Hardiness Zone Map (PHZM) is one of our most valuable garden resources because it is a fast way to get an idea about a plant’s winter hardiness.
For most of us, understanding what winter hardiness zone we are in and then referring to information provided by resources such as this website or plant labels immediately gives us an idea of whether a plant has a good chance of surviving winter in our area.
The USDA Plant Hardiness Zone Map is a great place to start.
USDA winter hardiness zone maps change over time.
The USDA plant hardiness zone map represents an average of the lowest temperatures recorded over a thirty year period.
In Athens, Georgia for example, we were zone 7b for many years but are now considered to be in zone 8a. Winter hardiness zone maps are not a catalog of lowest temperatures ever recorded. They are designed to give us a good idea of what the potential lowest temperature is for our region in any given year by averaging a lot of years together.
If you choose plants for your zone and a record-breaking low occurs, you may still lose them. For example, New Orleans, which is in zone 9b, has a 30 year average of 25 to 30 degrees. In January 2018 the temperature plunged as low as 15°-20°F in the region. Plants were lost. This is the risk gardeners and farmers take, and I don’t make this statement flippantly.
Sometimes we lose fruit or flowers but not the plant. On a regular basis, we hear in the news that farmers in Florida are concerned about their orange crops. Rarely are the orange trees themselves in danger, but a late damaging frost can damage the crop. Indica azaleas in zone 7b can survive winter just fine but every once in a while this azalea loses the flowers for the season due to low temperatures.
I would not recommend being ‘extra safe’ by choosing plants based on a zone colder than where I live. If you choose your plants according to your zone, the chances of losing the plant to cold are minimal. The vast majority of plants die in winter as a result of root rot from poorly drained soil during their dormant period.
As the USDA correctly indicates, the plant hardiness zone map is just a starting place.
When it is time to make a decision about whether to buy a plant, microclimates in your garden should be considered. For example, walls may shelter a plant from winter winds as well as provide a reflective surface that provides more radiant heat from the sun. Conversely low, shady areas of the garden susceptible to wind may be a few degrees colder than other areas that are more protected.
The most valuable tool at your disposal is your own experience in your garden over time. Every gardener loses plants, whether it be to winter cold, drought, or our own mistakes. As you garden in a place you will learn what works through trial and error. This is fine.
Reputable nurseries often provide information about what zone a plant is in. Some nurseries may not include hardiness zones on their labels. Good nurseries should include genus and species as well as common name.
A quick search online will quickly reveal the hardiness zones for the plant you are considering. For example, searching for ‘joe pye weed hardiness zone’ on Google quickly reveals zones 4-9. If you are in these zones, joe pye weed should survive winter in your area. In this example, I used the common name for my search, but you are always safer searching by genus and species.
What zone am I in? How to use a plant hardiness zone map
- Find out what plant hardiness zone you are in. You can cross-reference your location by viewing a picture of the USDA Plant Hardiness Zone Map here. If you wish to view a more detailed map, the USDA has provided you the ability to view state maps, regional maps or to search by your zip code.
- Commit your zone to memory. This may seem nonsensical but it is important. We cannot expect nursery employees to follow us around describing the winter hardiness of each plant. What we can expect however is that plants will be labeled for their winter hardiness zones or genus/species. By giving the plant label a quick look, one can learn very quickly whether it will survive winter where we garden.
- Consider the specific location you might place a plant. Consider how narrow or wide the winter hardiness range is. What microclimates exist in your garden?
A short example of using plant hardiness zone maps
The photograph of golden columbine above is pretty fetching, just the kind of photograph that catalogs use to convince us to buy. The photograph does not tell us where golden columbine is native, or if it will survive winter in our own garden.
If I were considering golden columbine for a Georgia garden, a plant label or this website might inform me it is hardy zones 3-9. This range covers winter average lows from well below zero to barely freezing. This is a wide swath of North America and I would move on to other criteria to decide whether to purchase golden columbine.
The nice thing about plants that have a large geographic distribution is this may indicate they are highly adaptable. It could be a warning sign a plant might be invasive as well, of course. Check with online resources, local gardeners, or nurseries before planting anything you are not familiar with.