Understanding Hardiness Zones
USDA Plant Hardiness Zones are one of the first and most important things you will need to understand if you expect to have a landscape that is full of healthy, thriving plants. Plant hardiness zones are an essential guide to knowing which trees, shrubs, and perennial plants will survive in what areas. As you can imagine, plants that may thrive in Wyoming may really struggle to grow in Virginia, and even though Washington and Montana are nearly neighboring states, they both have wildly different plant hardiness zones. Each state even has multiple hardiness zones within themselves, and knowing which plant hardiness zone you are in will determine which plants will grow well in your location. So, if you are planning to add some new plants to your landscaping and want to make sure that you choose plants that will grow well in your particular area, we are here to help you with the planning process and make sure that the plants you have selected are suited to your USDA zone.
What Are Hardiness Zones Anyways?
Plant hardiness zones are geographical areas designated to contain a range of climates relevant to the specific growth and survival needs of different plants. The United States Department of Agriculture (USDA) developed a map that splits the United States into thirteen different zones based on the average annual minimum temperature of each area. The USDA map even goes so far as to break these zones up into sub-zones (ex. Zone 2a, Zone 2b, etc.) so that gardeners and landscapers across the country can be sure to only plant what will survive in their area. Each zone has a 10 degree difference, with the average coldest temperatures for USDA Hardiness Zone 7 ranging from 0-10 degrees Fahrenheit, Zone 8 ranging from 10-20 ˚F, Zone 9 ranging from 20-30 ˚F, and so on. The hardiness zone of any area is affected greatly by elevation. For example, the hardiness zone number for Denver is usually around 5b-6a, whereas the plant hardiness zone just an hour away near Keystone where the elevation rises significantly drops to zone 4. This change in elevation changes the growing environment significantly, making it impossible for some plants to survive the cold winter temperatures. Perennial plants grown outside of their USDA hardiness zone may survive during the warmer seasons of the year, but they will behave as other annuals do, dying off as soon as fall and winter temperatures drop below what they are able to tolerate.
Do Hardiness Zones Really Matter?
You might be asking, does it really matter if I am in Hardiness Zone 6, but I want to buy a plant that is specific to Hardiness Zone 7? Many people may be tempted to buy a plant outside of their zone because they like the way it looks or the colors it displays, and then get frustrated and disheartened when the plant does not survive or when it loses all of the beautiful features that drew you to the plant in the first place. So, the answer to this question is yes, it does matter!
All plants present a lovely display unique to their variety, and often this display, whether it is vibrant color, beautiful flowers, deep green foliage, etc., is dependent on the climate and the growing environment which the plant is in. The 10 degree difference in zones, though it may seem insignificant, is easily enough to affect the way your plant grows, and cold temperatures can easily cause irreparable damage. If the plant does not get the right amount of sun and warmth, it will lose its color. Or if a plant gets too cold, it may not flower. That is why the Hardiness Zones were developed– to help people know which plants will grow best in which climates. When it comes to subzones, most plants are usually able to adapt. Some plants may not even be labeled so specifically that subzone is mentioned. So, if you know you are in zone 9a and find a plant that only says it is suited to zone 9, the chances that your plant will survive are very high. Even if the plant in question is labeled as suited to zone 9b, most plants will likely be fine once it has become adapted to the area. Overall, though, it is always best to not push the limits of any plant when it comes to planting it somewhere outside of the area it is best suited for, as climatic conditions may not be suitable for the growing needs of the trees, shrubs, or perennials you have in mind.
So, while buying a plant that is marked for Zone 8 when you live in Zone 5 (or any zone that is warmer than the one you are in) may not result in the immediate death of your plant, it definitely will affect the health of the plant, and therefore the plant will likely lack many of the qualities that made it so beautiful and intriguing. You can, however, plant something that is hardy to zone 3 in zone 5. This is because a zone 3 plant will be able to withstand temperatures much colder than zone 5 will ever experience, so the plant will be fine. So, you can almost always plant something from a colder zone in a warmer zone, but you cannot plant something from a warmer zone in a colder zone, as a warm-weather plant will not be able to withstand the extreme temperatures of a hardiness zone that is colder than the one they are rated for.
How To Read a Plant Hardiness Zone Map in the USA
Reading a USDA hardiness zone map is really very simple. If you do a Google search, the first result will likely be a map of the designated hardiness zones with a list of USDA zones on one side, each with individual colors that are correlated to each USDA zone. As can be seen in the zone map below, the map of the United States is colored based on the average annual minimum temperature of locations across the country.
Whether you are in Florida, Oregon, Oklahoma, Texas, Hawaii, Alaska, or any other state, simply find your place on the map and the color of that region will tell you the annual average extreme minimum temperature range for that planting zone. There are also many interactive maps that allow you to enter your zip code to be told the exact USDA plant hardiness zone for your location. Here in Fort Collins, Colorado, we are in Zone 5b, with extreme temperatures reaching as low as -10 to -15 degrees Fahrenheit, on average.
Our Advice
So, our advice to you, if you want to have a thriving and beautiful landscape, would be to stick to the plants that are within your plant hardiness zone. Using a USDA hardiness zone map as a general guide when choosing plants for your gardens and landscaping is the first step you should take to ensure that the varieties of plants you choose will tolerate your growing environment. Not all garden centers and plant nurseries may have plants marked specific to the sub-zones, however if you live in Zone 5b, purchasing a plant that is hardy to zone 5 will be just fine. And if you are simply dying to have banana trees and crotons incorporated into your outdoor landscape, consider keeping these lovely plants as indoor houseplants. You can even place them outside on your patio during the warmer months, given your sun/shade is suitable to the requirements of the plant.
Questions?
If you have any concerns about specific plants or questions about what will grow in your area, please reach out to us through our website! We are here to help with all your questions and are happy to provide you with the important information you need to determine which plants to grow in our region. Also, here at Bath Garden Center in Fort Collins, we only sell what will grow and thrive in the area! So, if you are purchasing plants for your yard and you live nearby, swing by for a visit. You won’t have to worry about whether your plant will survive, since all our plants are hand-picked and specific to our climate here in Northern Colorado. We look forward to seeing you and helping you pick out the perfect plants for your yard!