Some of the many environmental benefits provided by perennials are:
Perennials also use as much as 50% less water than annuals‚ and sequester carbon into the soil for decades instead of days or weeks‚ by developing roots that reach six feet or more underground․ They also provide pollinators with habitat over the course of the entire year‚ which annuals‚ because they die back every year‚ do not․ If you garden in one of the USDA zones 6a through 7b in Tennessee‚ replacing some of your annual beds with perennial plantings is one of the best things you can do to improve your slice of the environment․
That's the short answer․ But the full story of what's going on underground is more complicated and more important than most gardeners realize․
Carbon Storage Happens Underground‚ Not in the Leaves
While most people think of a canopy as the main carbon storage feature of a plant‚ perennial garden plants are also effective at sequestering carbon‚ as they build up more root mass year after year‚ while annual plants develop a shallow root system and then die․ Many species‚ including purple coneflower and native asters‚ continue to develop their root systems for three to five years or more until full maturity․
As the roots grow‚ they roil carbon compounds in the soil through a process called root exudation to feed soil microbes‚ which in turn stabilize the carbon into slower-turnover forms․ After five years‚ a perennially planted bed can store meaningfully more carbon than the same bed re-planted each spring with petunias․ This is one reason that many prairie restoration projects in the Midwest and Southeast use deep-rooted natives rather than turf grass: dramatically better soil carbon banking․
Soil structure improves with each growing season․
In Tennessee‚ clay soil is hard-packed‚ especially in Middle Tennessee․ It is aerated by perennial roots‚ which create holes for water and air to penetrate the soil․ This is better than allowing water to move across the surface or run off during the heavy spring rains․
As perennial plants die back in autumn‚ their decomposing leaves add organic material to the soil and build soil structure without the need for bagged soil amendments․ Those gardeners who have battled compacted‚ poorly draining clay soil for years have often been surprised at how much the soil has improved in the 2 or 3 growing seasons after converting a garden bed to daylilies‚ black-eyed Susans or ornamental grasses․ You're not just putting in flowers․ You're slowly rebuilding the soil profile․
Pollinator and wildlife habitat requires reliable rather than prolific flower production․
A big difference between my kind of flower garden and general horticultural advice is that pollinators need flowers in bloom for all of the growing season‚ and places to overwinter․ Annuals are in bloom for several months of the year‚ then die․ Planted carefully‚ a perennial bed could have flowering plants providing nectar resources from March to October of any given year․
In Tennessee‚ that might look like: - Early spring‚ creeping phlox and columbine - Late spring to summer: coneflower‚ black-eyed Susan‚ and bee balm - Late summer through fall: asters‚ goldenrod
Native bees will often spend the winter inside hollow stems or leaf litter in the crowns of perennial clumps․ This is why the best reason to leave some dead perennial stalks standing all winter (rather than cutting everything down before a November snow) is that a lot of well-meaning garden cleanup destroys overwintering habitat right when it will be needed․
Water Conservation Is About Root Depth‚ Not Just Species Choice
The reason for this is almost entirely root structure․ The roots of annual bedding plants are fibrous and near the surface․ These roots dry out quickly and require daily watering in a Tennessee July․ Established perennials have roots that dip below the top few inches of topsoil to moisture levels that annual root systems cannot reach․
It doesn't happen overnight․ Newly planted perennials still require watering during their first full season after planting while the roots are still establishing themselves․ By the second year‚ most perennials that are well-suited to our climate will need watering only during extended periods of drought․ Coneflowers‚ yarrow‚ and Russian sage will even endure several weeks without rain during an average Tennessee summer․
Natural Pest Management‚ with a Corrective
There are some perennials that repel pest insects‚ at least in the sense of making it more difficult for them to consume the vegetation․ For example‚ lavender‚ catmint‚ and marigold (technically a tender perennial in warmer zones‚ but usually grown as one) are all aromatic plants that can be less appetizing to some insects․ Even more importantly‚ most flowering perennials attract predatory and parasitic insects such as ladybugs‚ lacewings‚ and parasitic wasps that feed on aphids and caterpillars‚ controlling pests‚ without the use of chemicals․
The net effect is to produce a garden that requires fewer sprays and also doesn't harm the pollinator and helpful insects you are trying to attract․
Perennials for Planting in Tennessee Gardens
For the gardener in our area‚ however‚ a few plants prove more reliable with these environmental conditions․
Purple Coneflower (Echinacea) - This plant is very drought-tolerant once established, and blooms most of the summer․ Seed heads will provide food for goldfinches through the fall months․
Black-eyed Susan (Rudbeckia): Tough‚ adaptable to clay soil‚ and one of the most reliable pollinator magnets you can plant in zones 6-7․
Once established‚ daylilies are nearly indestructible‚ and their roots help bind earth and control erosion in hillside plantings․
Native Ferns - Great for shady‚ moist places․ These plants often stabilize soils․ After a good first year of establishment‚ they require little or no additional moisture․
Ornamental Grasses - With deep‚ fibrous root systems‚ they are among the best plants for erosion control on slopes or streambanks․
TN Nursery sells all of these as plants‚ not seeds․ The roots on the plant are well established, and the perennials can establish and create a functional soil and water ecology in one growing season‚ as opposed to perennial plants that take two or three growing seasons to do so․
The Bottom Line
Beyond their low maintenance demands‚ perennials improve the garden ecosystem every year they live in the same spot‚ by sequestering carbon‚ rebuilding and improving soil structure‚ holding moisture in the soil‚ and supporting insect life that is at the base of the food web․ The case for perennials becomes stronger for each year that a perennial flower bed lives in the same place‚ so there's no reason to wait for another perfect perennial planting season․
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