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Understanding Hardiness Zones and the difference between annuals, biennials, and perennials.

Updated: Feb 2, 2022


Knowing the difference between annuals, biennials and perennials helps you to make smarter plant purchases and gives you an idea of what you can expect from them once they’re in your yard. These terms describe the natural lifespan of a given plant. Annuals complete their entire life cycle in no more than one year regardless of what climate they’re grown in. Biennials’ life cycles take two years and perennials can live anywhere from a couple to many years. However, a key point to note is that growing an annual, biennial, or perennial where it is not hardy can significantly shorten its lifespan. That is why the plant’s cold hardiness range is usually identified in its purchase material.


Hardiness Zones

The USDA Hardiness Zone Map separates areas of the United States by the average coldest annual temperatures that happen in that area. Follow this link https://planthardiness.ars.usda.gov/ to check out the USDA Plant Hardiness Map to determine the hardiness zone for your area. This map separates the U.S. into 13 distinct zones that each cover a 10-degree low-temperature span, with Zone 1 being the coldest and Zone 13 the warmest. Using Hardiness Zone 7 as an example, the temperature will likely fall somewhere between 0 and 10 degrees Fahrenheit at some point during the year. The zone is sometimes further delineated into 5-degree increments with an “a” or a “b”. For example, Zone 7a gets down to 0 to 5 degrees while the temperature in 7b dips only to 5 to 10 degrees. When a plant is growing in a zone that gets colder than its stated cold hardiness, it will likely die once the temperature dips below its cold tolerance, making a perennial function like an annual and annual die after just a few months. If a plant needs a certain level of cool or cold temps to survive, its hardiness will be listed in a range, for example “Hardy in Zones 3-8”.



Annuals

An annual has only one year to go from seed to flower to seed, so it grows quickly and tends to put out a lot of flowers to create as much seed as possible before dying. Therefore, we commonly use annuals where we want a lot of flowers for a season. They are usually easy to grow from seed and if you allow the seeds fall to the ground, they can reseed the following year. Otherwise, they’ll need to be replanted every year regardless of your hardiness zone. Some examples of true annuals are zinnias and marigolds. Many plants that are sold as annuals are really long-blooming, tender perennials which means that although they can live longer in mild climates, they aren’t hardy in colder zones, so they’ll die like annuals when it gets cold. Some examples of tender perennials are petunias, geraniums, New Guinea impatiens, tomatoes and peppers.


Because annuals grow quickly, they tend to be sold smaller and cheaper, often in 4-packs or 6-packs. They are regularly planted in groups in gardens to create waves of color or used in containers because they produce season-long flowers. I’ll also use them as infill in perennial gardens to help provide consistent color through-out the growing season. This is especially helpful if the perennials are young, or the garden is newly planted.



Biennials

Biennials have a two-year life cycle. In the first year they will usually grow only a mound (rosette) of leaves and then in the second-year produce flowers, set seed, and then die. If you buy a true biennial in a nursery and it is ready to flower, you have bought a second-year plant. A couple of examples of biennials are Money Plant (Honesty), some hollyhocks and most foxglove and mullein. If you let your biennial go to seed at the end of the season, and the seed grows the following year, you’ll get a rosette of foliage. The year after that you should get flowers again.


If you want a biennial like foxglove to bloom in your garden every season, you can buy plants two years in a row and keep allowing them to go to seed. This will give you plants on opposite 2-year life cycles, with some first year and some blooming second-year plants every year



Perennials

The term perennial usually applies to herbaceous plants, those are plants that are not “woody”. By contrast, shrubs (although they also live many years) have woody stems. In climates where it gets cold enough to freeze, perennials will often go into a state of dormancy and die back to the ground in the fall. If a perennial is hardy where it is growing, the underground parts will remain alive, and it will regrow from the base the next spring. Some perennials are evergreen, which means they are neither “woody”, nor do they die back to the ground in the fall. They’ll keep their foliage, shape and color through-out the winter months.


If you plant perennials that are hardy in your zone, you can expect them to live for anywhere from a couple of years to many and often spread over time. Because they grow slower and live longer, they tend to be more expensive initially. If a perennial is not hardy in your zone (especially if it is sold as an “annual”), you can expect it to die once the temperature gets below what the plant can survive. While some perennials are “long-blooming” which means that they’ll bloom for a couple of months during the growing season, most will bloom anywhere from a week or two to a month or so. Deadheading can usually help extend a plant’s bloom time (I’ll cover more on that in another blog post!). However, planting a variety of perennials that bloom at different times during the season will help to give your garden a longer season of color. Another thing to note is that most perennials take a couple of years to really get rolling, often growing into their mature size and shape by year three.


Of course, there are a lot of factors that can cause a plant to die or thrive, but knowing if it is an annual, perennial, or biennial and understanding if it is hardy in your area is a good place to start towards creating a successful garden. Happy Gardening!



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