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Plant Zone Maps 101
When planting annuals, perennials, grasses, shrubs, and trees in the ground, they need to be able to survive the winter in your area. To understand the plant's hardiness, double-check the plant zone information for the plant and match it with your location. To find this information out, check your local plant zone map.
USDA hardiness plant zone maps were introduced in the mid-1900s by the US Department of Agriculture for North Americana and Mexico. The lowest annual minimum temperature groupings separate the zones into 10° increments. For all the die-hard fans of the minutia, additions were added in the '90s to break down into 5° increments. The additional zones note them as 'a' and 'b.' For instance, the 5a plant zone will be slightly cooler than the plant zone 5b.
We have thirteen USDA plant zones covering the entire US, including Hawaii, Alaska, and Puerto Rico. Zone 1 starts in Alaska, registering bitter cold winters at -60°F to Zone 13, with tropical temperatures not lower than 60°F in some areas of Hawaii. Therefore, the lower the plant zone number a plant is associated with, the colder temperatures a plant can tolerate and still live and flourish.
An Example of a Plant Zone Range
If you're planting in the northern states of Montana, North Dakota, or Minnesota, the planting zone is 3a, which means winter temperatures can go as low as -40° to -35°F! Brrrr! A plant you choose must be able to withstand this type of cold and survive outside. Any plant information card that comes with your plant will show the hardiness zone range for the best-growing results.
For instance, a perennial Primrose (Primula) likes partial sun and grows in plant zones 3-8. This range means it can tolerate the cold temperatures in Montana and North Dakota as well as thrive in the lesser cold temperatures of 10°-15°F in Zone 8a as far south as Alabama.
What Plant Zone Do I Live In?
Once you know your plant zone (look it up on this map by your zip code), you can determine plant choices that are right for your area. Depending on your plant zone, you may choose plants that are not in your hardiness zone and treat them as annuals or indoor plants.
For instance, you might enjoy some petunias in your container pots this summer, and they grow in zone 10-11, but you're in a zone 7a. Consider them an annual in your area because they are not hardy in the winter months. No worries, though! You can still enjoy it throughout the spring and summer season, but realize it will eventually die from the first fall frost.
Container Planting and Plant Zones
If you're putting your plants in containers throughout the year, you always want to make sure the shrub, perennial, or evergreen is two zones colder tolerant than your current growing zone. For instance, if you're going to plant a shrub that grows in plant zones 6-9 and you're living in the plant zone 6, you'll choose a plant that can tolerate cold temperatures of zone 4 (-25° to -30°F). The root system won't be protected with as much insulation in the container as it is in the ground, so it serves as a buffer to help your plants survive.
Microclimates within Plant Zones
Depending upon where you live and the contributing factors, you may be working with microclimates around your garden. Microclimates are spots with special conditions that may contribute to a different planting zone pocket. An example may be planting near a body of water. The humidity level is always higher around water, which raises the temperatures. What might get frostbitten a mile away in the same plant zone, may not suffer the same outcome near the water because of the slight change in temperature due to humidity. This area would be considered a microclimate. Additionally, south-facing walls next to a house exterior are warmer than beyond in the yard. Areas near concrete or asphalt will also radiate higher temperatures and affect plant hardiness ranges.
Determine First and Last Frosts
When planting annuals and vegetables, consider the last frost date in early spring before planting outside in containers or the garden beds. Many get eager to plant as soon as the snow has melted. Warm days beg us to get out and dig in the soil. Checking the last frost dates will help you determine the best time to sink those plant babies in the ground to enjoy all spring and summer. Don't be tempted to plant early as you never know what Mother Nature will bring. Many times, gardeners have been taken off guard by a late frost, and it's damaged or killed their newly planted flowers.
When choosing the best plant for your location, perennial flowers, shrubs, and trees grow best within their appropriate USDA plant zones. Winter damage may occur if you plant out of this range. Avoid marginally hardy plants, as they too may suffer weak growth and winter damage. Use these perimeters in planning your landscape and enjoy your garden!