If you’ve been a gardener even for a short time, planted seeds out of a seed packet, or read a plant tag on shrub, you’ve either seen a planting zone map or seen a reference to one. Because we see it so much, the USDA plant zone map can become so commonplace that it’s easy to overlook its significance, or to know what it’s designed to tell us.
The official name of the map published by the USDA is the “Plant Hardiness Zone Map”, or “PHZM”. The name gives a strong clue to the primary purpose of the map — providing guidance on what types of trees, perennials and biennials will survive over winter in an area — hardiness. The PHZM provides a standard for seed companies, nursery stock producers and retailers to indicate what plants are likely to endure in a zone. The zones, which are numbered, are separated by 10-degree (Fahrenheit) differences in average night time winter lows. When you see a zone further notated with an “A” or “B” suffix, that zone is being further subdivided from its neighbor by another five degrees (F).
The map was first published in 1960, and updated in 1990 and again in 2012. The latest version is web-interactive. The Hardiness Zones themselves are based on the average annual minimum temperature during a 30-year measurement period in the past. Because the map is using averages, a zone will not reflect the lowest temperature that has ever occurred in the past in that zone, nor could it forecast the coldest temperature that might happen in the future. So, if you are planting varieties that are on the fringe of a zone, you may do fine for many years, but lose that variety if an extreme low occurs long enough to kill the plant.
Depending on what version of the map you reference, you will find that Tooele lists as “Zone 7B”. That was a surprise to me, as I’ve been using Zone 6 as a basis for my planting decisions. When I looked more closely, I discovered that the zip code for Tooele is generating the reference. If you look at a detailed zone map, along with the “A” and “B” coding, you will find that Tooele and the surrounding areas can have quite a bit of variance, showing up as 6a, 6b, 7a, and 7b.
Lesson learned? The maps are guides, not absolutes. I was at my friend Bruce’s house last week pruning fruit trees with him and taking a look at the beautiful grounds he and his wife have created. The conversation turned to planting times in the spring and how late you can grow things into the fall. I learned that he has an interesting microclimate because his land sits a bit lower than the sites of some other gardening friends. He also has a river of colder air that crosses his land coming out of the field and hillside just to his east. This means that he has to start a bit later (a couple of weeks) in the spring, and end up earlier in the fall. As far as the hardiness map goes, the prudent approach is to take its basic information and localize it just for you.
Other factors can create microclimates as well, including soil moisture, amount of snow on the ground, wind exposure, humidity, soil type and sun exposure. Add structures such as fences, sheds, shade or reflected light by houses, and you have further modification to the zone map. A diary of your gardening efforts, including varieties, locations, successes and challenges, will go a long way it getting your garden set up yearly for strong success.
Another “game changer” is rapid temperature changes. In late fall, plants become more hardy as the days grow shorter and average temps drop. If a really cold spike happens, you can get some winter kill on the tree or shrub because it hasn’t properly prepared for winter. This happened a few years back when we had a 30-degree downward swing near Thanksgiving, essentially going from a warm fall to a harsh winter inside a week. There were a lot of boxwood hedges lost that year! Damage of this type tends to be a lot less in the wild temp swings of spring as many plants “wait and see” if it’s really going to stay warm. I wish I could teach that trick to my apricot trees, though. You who have apricots know what I mean.
Simply put, USDA map focuses on cold hardiness more than it does with how the plant is cultivated during the summer growing season. As gardeners we know that our success is affected by lots of things, including humidity, rainfall, cloud cover, amount of frost-free days, daily highs and nightly lows (you really see the effects of this last one in tomato production), wind, and orientation of your crops to the sun. None of these are addressed by the PHZM.
As for the maps you typically see on the back of seed packets, they are related to the USDA map, but they are not one in the same. The packet map is further generalized and averaged and is designed to give guidance regarding when it’s advisable to plant a specific variety based on last typical days of frost, plus a safety factor thrown in for good measure. That’s where the “plant two weeks after all danger of frost” language comes in. Remember, these are general observations and need to be fine-tuned over time.
While the USDA map usually gets the most attention, there are other plant zone maps in common use. Some of these come from the USDA themselves and can be quite detailed. Visit planthardiness.ars.usda.gov to view the U.S. hardiness map as well as maps for specific states. There are also maps based on heat index (the inverse of the PHZM), as well as a broader spectrum of area characteristics that directly affect how well a plant, tree or shrub will flourish in a given area.
To easily access these maps, visit the American Horticultural Society’s (AHS) website at ahs.org/gardening-resources/gardening-maps. Besides the USDA PHZM, you’ll be able to view the AHS Heat Zone map. This map focuses on temperature highs, which are also a limiting factor of what can be grown in differing areas. The map assumes that adequate water is supplied to the root zone of the plants at all times, as even a single episode of extreme dryness can lead to the quick demise of the plant.
While extreme cold can exceed a plant’s hardiness and kill it in short order, heat extremes can exceed the plant’s tolerance and usually lead to a prolonged death. Grisly, but true. The plant will adapt to its poor situation by curling leaves (to preserve moisture), dropping blossoms, even halting chlorophyll production or dropping leaves, all to “buy time”. More and more plants are being indexed to the Heat Zone map, allowing the gardener to choose varieties that will do well in a temperature range between a set of hot and cold average parameters.
In addition, the very popular and highly-referenced Sunset garden books use their own system, called the Sunset climate zone maps. You can access these maps through the AHS web address listed just above, or you can visit www.sunset.com/garden/climate-zones/climate-zones-intro-us-map and get a good overview of the factors being considered as well as looking up your particular zone map. Great stuff!
As you may recall from other articles, I’m a strong believer that you can’t know it all when it comes to even a small category of horticulture. So, the next best thing is knowing where you can readily put your hands on vetted information. Having ready access to the USDA PHZM, AHS Heat Zone Map, and Sunset’s climate zone maps can make me look pretty smart. Those that know me best would say that’s quite an accomplishment.
Jay Cooper can be contacted at email@example.com, or you can visit his website at dirtfarmerjay.com for videos and articles on gardening, shop skills, culinary arts and landscaping.