On Hardiness Zones

tomato seedlings, circa 2010

If you want to drive me absolutely guano loco tell me you’re planning a vegetable garden and then ask me what type of broccoli or tomatoes or green beans or corn or peppers or any other common annual vegetable you should plant for your zone.

Go ‘head. I’ll wait.

And then I’ll take you firmly by the shoulders, look you deep in the eye and tell you, “For the love of all that is good and right in the world, your zone has nothing to do with it!”

Why not? Your USDA hardiness zone is based on one thing and one thing only: the average lowest annual temperature in your area; the very coldest of the coldest temperatures your locale reaches in the heart of winter. And since your tomatoes (or corn or beans or peppers or broccoli or…) will not be grown over the winter your zone is of absolutely no consequence to your summer vegetable garden plans.

Don’t believe me? Consider this:

Juneau, Alaska…


…and Tulsa, Oklahoma…


… are both in USDA Hardiness Zone 7a. This, despite the fact that Juneau’s record highest temperature ever (90F) is lower than the average temperature in Tulsa for more than 16% of every single year (93F). This, despite the fact that Juneau averages more than twenty inches more precipitation and twenty fewer frost-free days per year than Tulsa. This despite the fact that Tulsa is often almost as warm in the winter as Juneau is in the summer.

This, in a nutshell, despite a whole host of drastic differences between the two locations that actually are relevant to your summer veggie garden; humidity, length of the growing season, day length, and summer high temperatures, for instance.

So, to what kind of growing is hardiness zone relevant? In a word: perennial. Fruit trees, rhubarb, asparagus, artichokes, strawberries, fruit bushes and brambles, some herbs, and many ornamentals. Anything that must be overwintered, that is kept alive during the winter either in a dormant or active state, to grow and produce again in the following year.

And what should you consider when choosing varieties for your summer vegetable garden? First, find your frost dates — both last in the spring and first in the fall. These dates will tell you how many days your area experiences frost free, which will help you roughly determine how many days there are in your growing season. Different types of veggies have different season length requirements. From radishes that only take a few weeks to produce to eggplants and winter squashes that can take well over one hundred, the amount of time it takes to grow something to its ripe state is one of the most important considerations you can make.

Another is your average summer temperatures. Juneau, for example, may have just shy of two-hundred days frost free, plenty to grow eggplant, but the city’s high temps are only occasionally over even seventy degrees, presenting a potential problem for the heat-loving nightshade.

Still others include your area’s average summer humidity and precipitation levels; the fungal, bacterial, and insect pests that are most commonly a problem for gardeners nearby; and your region’s day length and relative sun strength. Some varieties may be more or less resistant to excessively wet or dry conditions and certain pests. And some crops are dependent on plenty of sunshine and day length; onions, for instance, begin bulbing only when the length of daylight reaches a certain point dependent on the type of onion.

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