It’s easy for gardeners to become lulled into feeling like they can grow anything, especially if they’re cultivating a vegetable garden. Part of the reason many of us start vegetable gardens in the first place is to grow the things we usually buy at the grocery store so we can save money and enjoy tastier produce. While we may be proud of our green thumbs, there are still some plants that give us trouble. Either they don’t grow as well, don’t grow at all, don’t produce as much as we had hoped, or go to seed too quickly. More often than not, this happens because the veggies you're trying to grow are outside of their zone.
What is a Hardiness Zone?
The hardiness zone map was developed by the United States Department of Agriculture to help farmers and gardeners cultivate the plants that grow best in their areas. The map categorizes areas of the U.S. into twelve zones based on their minimum temperatures, or their average annual extreme lows. In zone 10, for example, the minimum temperature is -1.1 degrees Celsius, or 30 degrees Fahrenheit. These numbers come into play when purchasing seeds or starters for vegetables, shrubs, and trees. Often, a plant's hardiness zone will be printed on seed packets alongside the rest of its planting information. You may notice tags that read “hardy to zone 7” or “Zone: 7-11.” This just means that that particular plant will survive in colder climates, but it’s unlikely to do well in sub-zero temperatures.
Of course, the USDA’s hardiness zone map isn’t perfect. In fact, it's come under fire for not taking other factors into account, such as the summer heat, plants' minimum daylight requirements, elevation, rainfall, and humidity. While the USDA may also publish a list of indicator plants to complement its map, it doesn’t tell the whole story in a lot of regions, especially in the western part of the country. Nonetheless, the map is still universally used.
Sunset developed a series of climate zone maps for the western part of the United States that takes into account each area's average summer temperatures, elevation, latitude, wind patterns, growing season length, and more. As a result, even more zones were created to correspond to the microclimates within individual regions.
Why Are Zones Important?
If you’ve ever wondered why the lettuce in your garden bolts by May or why your hot pepper plants never produce peppers, your zone may be to blame. Gardeners that live in places with hot and toasty summers may have trouble growing lettuce that doesn’t bolt in the late spring unless they use a variety that's been specifically bred for that climate. On the other hand, anyone that lives somewhere with a temperate summer will have more trouble with warm-season crops, including peppers, tomatoes, and some melons. Additionally, northern latitudes tend to have shorter growing seasons than their southern counterparts, making them better environments for plant varieties that also have short growing seasons.
Sometimes, plants will grow outside of their ideal hardiness zones, but they may not be as tall or produce as much as they normally would. For example, an avocado tree may grow in zone 7, but it will require extra care, such as being grown in a pot instead of in the ground or being brought in during the colder winter months. Unless the variety was bred to withstand colder temperatures and the unfavorable weather that winter brings, the tree will not grow as tall or produce as much fruit as it would if it were planted in a zone where avocados thrive.
How to Grow Outside of Your Zone
What if you have your heart set on growing plants that aren't known to thrive in your zone? Well, you may still be able to grow them if you can recreate their optimal environment. If your zone doesn’t have hot summers, for instance, you can still grow warm-season veggies, fruit vines, and trees if you place them in a greenhouse or, if you have the space, a hoop house. Since trees and shrubs tend to take up more space as they grow, plant them in containers to ensure that they won't outgrow the greenhouse. Greenhouses and cold frames also extend the growing season for those who live in northern climates by providing shelter from the cold and wind and cover from snow and precipitation.
Growing great lettuce, beets, and other cool-season crops in the heat of the summer is a matter of reducing the temperature around them by providing them with some shade. Nestling cool-season crops in between warm-season varieties like tomatoes, corn, and squash will keep them from suffering in the heat of the mid-day sun. Similarly, dark green mesh row covers can help filter out light and keep crops cooler than they would be if they were directly exposed to the sun. Additionally, keeping plants well-watered will prevent the soil around them from drying out and increase their chances of thriving.