Sunday, February 24, 2008
The Central Basin
The hills of Warner Park as seen from Cheekwood mansion
Right in the center of Tennessee is the Nashville Basin/Central Basin. The Central Basin is an elliptical shaped depression that is approximately 120 miles long and 50 to 60 miles wide. Davidson County, where I garden, is in the Central Basin. It is a land of gently rolling hills with the taller hills of Radnor Lake to the south and the Warner Parks to the west.
The Central South lies within a humid mesothermal climate region. Which means we have four distinct seasons with fairly mild winters. That doesn't mean we don't have some cold winter days, but before long the temperature returns to the pleasant high 30s or 40s. Very tolerable until we have an unpredictable cold spell right in the middle of some pleasant spring weather...zapping the early bloom. Our summers are hot, humid and lately, very dry.
We get most of our rainfall during the spring and winter months. Even when we get enough rainfall, my shallow, clay soil and sloping yard have a significant impact on how much water actually gets to the plants. Another factor I have noticed is the effect the intensity of the rainfall has on a garden. A gentle rain often cannot penetrate the tree canopy to get to woodland gardens ....and the very hard rains hit the clay soil and runs off into the streets and drainage ditches before it has a chance to be absorbed. This is especially true when the soil is very dry; conditions similar to last summer.
Extreme weather is probably the most important factor in the Central South: dogwood winter, blackberry winter and cotton britches winter are names given to unexpected cold weather that nips spring bloom in the bud. But last year's drought with weeks of extremely hot and dry weather was even more damaging to our gardens than the late frost. It seemed that some neighborhoods were hit harder than others. My neighbors lost many magnolias, sugar maples, shagbark hickories and pine trees. I lost several dogwood trees and a native holly all too far from the water spigot to water regularly.
Underneath our clay soil is a substrate of limestone that is 500 million years old. Now that is a cool factoid! Well, I could describe how the shallow sea that once covered a large part of North America receded and plant succession created our beautiful vegetation, but that could bore you and you didn't stop by to read a geological report. Would you accept my word that this limestone bedrock is cool and an extremely important factor in the development of soil and plant communities? Here is an example of how important it continues to be: Last fall Mike Berkley (Growild) and I were talking about how hard the drought hit my neighborhood. He said that the fissures and cracks in the limestone bedrock in the West Nashville area allowed the water to be carried away.
Our limestone bedrock is on display all the time: you drive by it every time you travel on any of our interstates...blasted open for all to see. Isn't it beautiful?
The central south was once a magnificent forest with pockets of Cedar Glades. But city growth and sprawl have replaced the forests and destroyed many cedar glades. For the most part Nashville is considered to be drier and support oak (Quercus) and Hickory (Carya) forests. The Warner Parks are good examples of the hilly forested land. My yard has dozens of oaks, hickories and other native trees. We joke that we have plant succession going on in our front and back yards because the leaf litter continues to alter the soil composition and every bird dropped seed wants to grow and turn this yard completely into a deciduous forest. Since I let the leaf litter remain in place my soil composition is in the process of changing from clay to a more humus rich clay....who knows what will grow there in a few years. I just wish that honeysuckle and privet weren't so successful.
Each yard has its own unique plant community. Whether your yard is sloped, flat or has been altered by construction; whether the soil is neutral, acidic, shallow or deep; whether water and sunshine can reach your plants are the components that add up to create your very own unique plant community.
I decided to make peace with my shallow, dry, clay soil and plant natives that will thrive here. The big canopy trees, oaks (quercus) hickories (Carya) hackberries (celtis), and others were here and there were some terrific understory trees including red buds (cersis) dogwood (cornus) and rusty blackhaw (viburnum rufidulum). There was no shrub or herbacious layer so I added shrubs like Grow Low sumac (Rhus aromatica), St. John-wort (Hypericum frondosum), Oak Leaf Hydrangea (Hydrangea quercifolia) and wild hydrangea (Hydrangea arborescens) and grasses/perennials/wildflowers including aquilegia canadensis, heucheras and dodecatheon meadia (shooting star). I wanted the yard to have a woodland look and feel that I could see and enjoy from my front porch. This woodland is a Central Basin woodland...with dryer, heavier and more neutral soils.
Anyone who has tried to grow azaleas, even native azaleas, knows we have heavy, nearly neutral limestone soils. Soil that can dry concrete hard or be so wet it is a slick, sticky glob that drowns/smothers a plant. The soil can be shallow or fairly deep. Mine is shallow with a few exposed rocks and when you dig you will unearth perfect rocks for walls. Once you remove the rock you have a nice deep hole. This is a good time to amend the soil with nice fluffy composted soil; unless you are planting Tennessee Coneflower (Echinacea tennesseensis) and other plants that have learned to adapt to harsher conditions and don't thrive when pampered. Then plain, not too fertile garden soil with sharper drainage works just fine.
Considering our nearly neutral soil, don't expect to find acid loving wildflowers here. But do expect to find some really lovely natives. Shooting Star (Dodecatheon meadia), Wild Columbine (Aquilegia canadensis), Rose Verbena (Verbena candensis), wood asters, Trout Lily (Erythronium albidum), any many, many more grow very happily in our soil.
When I moved to this property 20+ years ago, I found patches of False Garlic (Nothoscordum bivalve) Blue Eyed Grass (Sisyrynchium augustifolium),Downy Wood Mint (Blephilia ciliata) and Western Daisy (Astranthium intergrifolium). Also Mayappple (Podopyllum peltatum) was romping in the woods behind our house and Hairy Beard-Tongue (Penstemon hirsutus) was thriving in a soggy, shady spot (it didn't mind having its roots wet all winter or dry all summer). Finding those plants was a delight and a gift.
It's true, I have often wished for deeper, richer, less rocky land. Especially when I start a new bed and hit that darn rock or turn over that gooey clay! But as my neighbor said to me when I complained to her that a new neighbor was "actually cutting down a red cedar so he could grow grass, when everyone knows grass is nearly impossible to grow in our neighborhood and why was he cutting down that beautiful tree with all is winter color... snarl, snarl". She said, calmly and wisely "Isn't it amazing that others are getting rid of the very thing we want?"
Did I say I had made peace with my garden, it's a process.
**Disclaimer and credit: I credit Thomas Hemmerly's excellent books for educating me about gardening with wildflowers. Any and all factoid mistakes I have made are mine.