Yes, and no.
To begin, the zoning structure is a function of the United States Department of Agriculture (USDA). It delineates one thing and one thing only: what is on average the lowest temperature of any given region. Outside of the United States, there may be similar methods of naming, but “zones” specifically is a term used in the United States.
The Vegetable Gardener
For the vegetable gardener, knowing the geographic zone of an area serves almost no purpose. Nearly all vegetables (tomatoes, peppers, cucumbers, etc.) are annuals in most zones in the United States and will not survive killing frost or below freezing temperatures. For the warmest parts of the world that do not experience temperatures below freezing, even vegetables can be perennial if they do not succumb to disease or insect pressure.
Instead of knowing your zone, a vegetable gardener should more imporantly know their average last and first frost dates.
These dates can be obtained by googling your ZIP code followed by “last frost date” or “first frost date”. For the vegetable gardener, these dates are important because it provides a window into how many frost-free dates your area has on average. Some crops have longer maturity dates than others, so in the event you live in an area that has very limited growing time between your last and first frost, you should pick varieties that will yield a crop sooner or have a shorter maturity date. Otherwise, you risk growing a beautiful plant and never yielding any fruit.
It is important to understand that these dates are very much an average. The Old Farmer’s Almanac lists my last frost date as April 17th. I never trust this date to be true. We have had last frosts shortly after this date, but two years ago we experienced a very hard freeze almost 3 weeks after this date. The dates are important for those wanting to start seeds early. I would recommend using a date at least 1 to 2 weeks past what your last average frost date is for planning in colder climates. Then watch the 10 day forecast before planting or moving anything tender outside.
The Ornamental Gardener
For the ornamental or flower gardener, USDA Zones serve the very important purpose of helping the gardener determine whether a perennial, shrub, tree, or another plant will survive in your area throughout winter. While there are many reasons why a plant may not survive an area during winter, extreme temperatures is one of the primary reasons.
All plants are provided a zone rating in the United States and you should not make a habit of purchasing plants outside of your zone that you expect to survive in your garden from year to year. There are methods to increase a plants likelihood of survival in your zone if it is not intended to grow there. Some methods include heavily mulching the plant, providing physical cover for the plant, strategically placing the plants near brick or stone walls and away from chilling and dry winter winds.
There are likely microclimates within your own garden in areas I mentioned above that can provide enough protection for plants below your zone to survive in your garden…for a period of time.
Those plants may survive for a couple of years and then never return because they become increasingly weak over the years. Those plants may also not perform well for you if they do survive winter leading to frustration.
I garden in USDA Zone 6 in Southwest Ohio, which means the lowest temperature we should experience is -10F. That doesn’t mean we won’t get lower temperatures, but I have not experienced anything lower than that in my time living here (6 years at the time of this post). Personally (read: because I am paranoid), I try to pick plants that can survive at least in a USDA Zone 5 that I plant in the ground. I also add plants that survive only to Zone 6 in my garden, but because I want to ensure success, I prefer to choose at least a zone lower.
For anything you intend to winter over in containers, the plants should remain protected from tough winter winds and provided enough moisture. But it’s important to choose plants for containers that will survive at least two zones below your own zone. For me, that is plants that will survive in USDA Zone 4 or to -30F. Plants in containers over winter have to overcome multiple obstacles: potentially limited watering, little insulation and protection for the roots, and drying winter winds. Always choose a container that offers good insulation with thick side walls for plants you intend to overwinter outside =, place them in a location that provides some protection, and check their moisture levels throughout winter since they still rely on you to receive adequate moisture.
Changing USDA Zones
USDA Zones can change over time. Zones are estimated based on the 10 year average low temperatures in any given geographic region. Provided that the area has warmer winters for many seasons, that may lead to an increase (moving up) of the USDA Zone recommended for that area. Moving up a zone could open new worlds of possibilities for plants for your garden, but still understand that it is an average and provided a Zone has just moved up be careful of selecting plants for that zone too quickly.
