“Choose Your Zone” Adventure
I communicate with growers nearly every day. Regardless of experience, there is one little detail that many of us seem to not fully understand. Zones. What the heck are zones and why do many of us identify so strongly with them?
The USDA Plant Hardiness Zone map was created as a way to help answer the age-old question, “Will this plant survive winter in my garden?” It is purely a metric to discuss the cold tolerance of plants, and nothing more. We can forget about the zone concept when discussing summer annuals as we don’t expect them to survive winter in the first place.
Zones tell us nothing about summer highs. Nothing about last or first frost dates. Nothing about precipitation, humidity, soil composition, daylength or any of the dozens of other variables that affect plant growth. They simply indicate the lowest winter temperature usually experienced in a geographic area. Plants are also categorized using these temperature ranges to make plant selection more successful in your individual growing zone.
As an example, let’s look at Zone 7. Rhode Island, North Carolina, Texas, Washington State, and Alaska all have portions that would be considered Zone 7. The frost dates, summer heat, precipitation, and daylengths vary greatly in these locations, but the one thing they have in common is that they generally drop to the defined range of 0-10F at least once in winter time. This is truly where the similarity ends.
The zone concept is very useful at pre-selecting plants that have a good chance of winter survival in your area. Cold temperatures and freezing kill plants more readily than summer conditions, so the first thing to consider in plant selection is winter survival. Of course heat and many other factors will determine a plant’s ultimate success, but we haven’t come up with a tidy system to classify summer conditions. Some have tried, but these concepts have never caught on.
Two other factors make the zone system less useful than you may have previously thought. Things are changing. Many of us are seeing warmer and warmer winters. Of course we are also seeing an increase in drought, heat waves, extreme rains, fires, etc., but let’s stay on topic. We are discussing only winter lows, and they are going up in many places. The USDA zone map is updated every 10 years, and was last updated in 2012. Things have changed dramatically in the last decade. Even in the past 7 years on our farm in Vermont we have seen a noticeable change. Negative thirty five was common in our first four winters, planting us firmly in our traditional Zone 3. But lately we’re just not getting as cold. Ten below (F) was our lowest temperature last winter, meaning Zone 6 plants would probably have survived just fine. You can’t bank on that, but these warming trends may open up new opportunities for you to try things you assumed would never live.
The other secret is that plant producers don’t always know the true hardiness of what they’re selling. It takes years of trials in dozens of locations to learn how far you can push the boundaries of each and every variety, and often enough the zone assigned to a plant is simply a good guess based on what has traditionally been grown in an area. Don’t let that plant tag stop you from giving something new a try.
Keeping your own records is invaluable. Consider keeping a journal, not only of your observed weather in your specific location, but also what plants may have survived even when they weren’t expected to do so. You may discover that you have been limiting yourself unnecessarily, and in fact you may have changed zones without even leaving home.