Diabase Bedrock: The Foundation for Unique Native Wildflowers at Penny’s Bend

By Wendy Diaz EMGV

This article was first published in The Gardeners Buzz, a newsletter of the Extension Master Gardeners of Durham County in June, 2022.

On April 2, 2022 my husband and I joined the ‘Spring Botanizing at Penny’s Bend‘ tour organized by Duke Gardens Education Program for an easy hike at Penny’s Bend Nature Preserve[1]. Duke University, Department of Biology professors Paul Manos and Alec Motten guided us and answered our questions. The botany at Penny’s Bend is unique because of the geology of the site. The restoration of the rare Piedmont Savanna at Penny’s Bend is ongoing by the North Carolina Botanical Garden at the Preserve[2].

Water flows downhill, along a path of least resistance, and that is one reason rivers meander. Penny’s Bend is a local example of a pronounced sharp curve in the course of the Eno River. The reason for this abrupt change and subsequent re-correction of the river course is that it passes around the hard, erosion-resistant igneous rock called diabase. The easterly flowing Eno River encountered the diabase promontory exposed at the surface and abruptly changed course – flowing south and then for a short distance, north – along a diabase sill outcrop forming a pinched horseshoe shape. 

From left to right: Photo of trail head map at the entrance Kiosk at Penny’s Bend Nature Preserve, Durham North Carolina. Looking north from the top of diabase sill outcrop along Eno River. All Photos by Wendy Diaz, April 2022

A sill is the term for a geologic structure forming a sheet-like igneous body that is parallel to the bedding planes and solidified between the sedimentary rock layers of the host rock; whereas, a dyke is a sheet of igneous rock that cuts across bedding planes of the host rock. There isn’t a lot of exposed bedrock in Durham County but there are a few outcrops of the diabase rock, such as along the Eno River at Penny’s Bend and the old rock quarry walls of the bear enclosure at the Museum of Life and Science. I also noticed there are some transported diabase boulders (not outcrop) along the side of Brigg’s Avenue Community Garden.

Diabase bedrock wall near the bear enclosure (former quarry) at the Museum of Life and Science, Durham, North Carolina.

While the diabase rock may not be unusual, geologically speaking, the soil derived from it did stimulate the growth of a group of unique plants that flower in the spring and summer at Penny’s Bend. Dutchman’s Breeches (Dicentra cucullaria)[3], the endangered Smooth purple coneflower (Echniacea laevigata), Prairie blue wild indigo (Baptisia aberrans) and Hoary puccoon (Lithospermum canescens) are some of the unique plants that require neutral to alkaline soil conditions. While it was too early in the season to see the blooms of most of these unique wildflowers, we did see large patches of Dutchman’s Breeches along the north side of the rock face and the east bank of the Eno River.

Photo of Dutchman’s Breeches (Dicentra cucullaria).

Some spring ephemeral flowers that we saw on our walk were Spring Beauty (Claytonia virginica), which have pink stripes on the petals to guide pollinators to the nectar, Rue Anemone (Anemonella thalictroides) and Sessile Bellwort (Uvularia sessilifolia). A large colony of Painted Buckeye (Aesculus sylvatica) shrubs was also in bloom near the diabase cliff along the north end of the bend. There was also many Trout lily (Erythronium americanum ) seed pods and leaves along the hiking path. 

Yellow blossoms of the Painted Buckeye (Aesculus sylvatica).

A Brief Geologic History of North Carolina

In Durham County, we live among the rolling hills of one of three physiographic regions, known as the central Piedmont, which is located between the Blue Ridge Mountains to the west and the relatively flat Coastal Plain to the east. Each region can be divided into geologic belts or ‘terranes’ that have similar rock types and geologic histories[4]. Furthest west, the oldest Blue Ridge Belt is a mountainous region with rocks dating all the way back to 1.5 billion to 1 billion years. These rounded mountains were thought to have peaks as high as 20,000 feet before millions of years of erosion took their toll. The youngest belt is the Coastal Plain-a wedge of marine sedimentary rocks which thicken closer to the coast and formed from sediments eroded from the mountains to the west between 65 million to just a few million years ago[5]. The bedrock which underlies the Piedmont consists of several different rock types formed over several hundred million years of geological history. Piedmont bedrock age ranges from the metamorphic Inner Piedmont Belt of 500 to 750 million years old to the sedimentary rocks of the Late Triassic Basins formed between about 235 million and 200 million years old. Most of the southern half of Durham County is underlain by one of three Triassic Basins, named the Deep River Basin which extends from Granville to Union County. Younger still, are the diabase dykes and sills intruded into the Late Triassic sedimentary rocks during the Early Jurassic Age[6] around 200 million and 175 million years ago when the supercontinent Pangaea was breaking apart forming what would eventually become the Atlantic Ocean. (This is illustrated in the YouTube video by California Academy of the Sciences[7]). Yes, the Jurassic Period, known as the peak age of the dinosaurs. When the continental plates diverged, the earth’s crust thinned when it was stretched, then the crust over the mantle became less dense and the reduced weight promoted the uppermost mantle to partially melt producing basaltic magma which intruded into the Triassic sediments.

About 220 million years ago when the Deep River Basin (rift valley) began to form, the continental tectonic plates pulled apart, forming cracks in the earth’s crust, roughly perpendicular to the direction of the moving continents. This basin was bordered by faults. Over millions of years it gradually filled up with eroded sediments from the much higher Blue Ridge mountains and the Piedmont Plateau to the west of the basin. These sediments solidified and formed mudstones, siltstones and shale. Other cracks continued to widen until they filled with ocean water. The pulled apart edge of early North Carolina (minus the coastal plain which was deposited after the pulling apart of the continents that formed the supercontinent of Pangea) roughly fits with the coastline of Africa from Mauritania and Morocco. At this point in geologic history the North American continent was closer to the equator.

Rock Mineralogy

Diabase is a dark grey to black, medium-grained, mafic igneous rock composed of Iron and Magnesium-rich minerals such as pyroxene. The exposed rock of the diabase sill at Penny’s Bend weathered and formed a soil markedly different in chemistry and properties to the surrounding acidic soil which was derived from the Triassic sediments. 

Close up of unweathered medium-textured diabase rock. Rust-colored surface of weathered diabase rock in the background.

When the minerals of the diabase rock breakdown they form clay minerals that form ‘sweet’ or basic soil with high pH and richer in iron and magnesium that what many other native plants can tolerate2. The soils are also rich in calcium from the plagioclase feldspar mineral[8]. Medium-grain size in igneous rock indicates the basaltic magma intruded and cooled underground near the earth’s surface (microscopic grains would indicate the molten rock cooled quickly on the earth’s surface). The diabase rock is more resistant to erosion because it is igneous rock that solidified from molten rock and not from cemented together grains of the surrounding less-resistant sedimentary rock. 

Fractured and weathered boulders from the Diabase sill outcrop along the east bank of the south flowing part of the Eno River.

The old diabase bedrock beneath Durham formed millions of years ago, and is now the foundation of our current natural world and determines the native plants we enjoy today on our spring and summer walks through the Penny’s Bend Nature Preserve. If you have not yet visited this site, I recommend a short walk along the Eno River, especially in the spring time. You will be rewarded when you see the unusual flora and fauna – and don’t forget to take a closer look at the bedrock that is exposed along the river bank which makes this area so special.

Black snake resting on dead tree stump along the Pyne trail at Penny’s Bend Nature Preserve. The new snake skin scales reminds me of the texture of unweathered surface of diabase igneous rock

DRIVING DIRECTIONS to Penny’s Bend from Durham (3710 Snow Hill Road, Durham, NC 27712)

Head north on Roxboro Street.

About 1.5 miles north of I-85, turn right onto Old Oxford Road.

Stay straight for 3.2 miles until you cross the Eno River.

Turn left onto Snow Hill Road and on the left is a gravel parking area on the left side of the road.


[1] https://www.enoriver.org/what-we-protect/parks/pennys-bend/

[2] A River Runs Around It: Restoring the Rare Flora of Penny’s Bend by Emily Oglesby, Conservation Gardener Magazine, Spring/Summer 2022, North Carolina Botanical Garden, The University of North Carolina.

[3] https://www.wildflower.org/plants/result.php?id_plant=DICU

[4]Generalized Geologic Map of North Carolina, N.C. Department of Environmental Quality https://deq.nc.gov/about/divisions/energy-mineral-land-resources/north-carolina-geological-survey/ncgs-publications

[5] Crossroads of the Natural World. Exploring North Carolina with Tom Earnhardt. University of North Carolina Press: Chapel Hill, 2013, 314 pages.

[6] Geologic Map of the Southwest Durham 7.5 Minute Quadrangle, Durham and Orange Counties, North Carolina by Charles W. Hoffman and Patricia E. Gallagher, North Carolina Geological Survey Open File Report 2001-XX

[7] https://www.youtube.com/watch?v=ADsjdu27WaM California Academy of the Sciences

[8] Exploring the Geology of the Carolinas, Kevin G. Stewart and Mary-Russell Roberson. The University of North Carolina Press, Chapel Hill, 2007, 298 pages.

If I Knew Then What I Know Now

by Andrea Laine, EMGV

In a short time I will relocate to a place with entirely different land features and growing conditions than I have enjoyed in Durham County. Of all the places I have lived (three states and six dwellings) my current home is where I have had the biggest amount of land on which to garden and ample time each week to spend gardening. It is also where I learned a lot more about gardening: as a volunteer at Sarah P. Duke Gardens, as an Extension master gardener, and through trial and error in my own yard.

Reflecting on my gardening experiences has brought forth a list of recommendations I would like to share. Each recommendation is followed by the reason it made a difference to me and a tip about implementation. Of note, my garden is primarily ornamental and includes two natural areas, the property (1.74 acres) is fenced (so, deer-free but I contend with my share of rabbits and voles), and I have no outdoor pets.

If I knew then what I know now, I would:

Plant on top of the soil.  Digging through clay and rock is not fun for anyone and, often enough, not even successful, resulting in improper planting. When I first heard this tip, I dismissed it as cheating. Years later I gave in and tried it, and I haven’t looked back. Yes, I still attempt to dig a proper hole first. But if it proves too difficult, I dig what I can and make up the difference with commercially bagged garden soil or compost piled on top of the hole and mixed with the native soil.

Add a dose of compost every spring. As with planting on top of soil, before laying down compost rough up the soil surface a few inches deep. It will encourage the existing soil and the compost to mingle and improve the soil more effectively. Great gardens begin with great soils (and soil tests)!

Mulch every other year. Did you know that you are supposed to rake off old mulch before applying new mulch? I have too much garden for that chore! Yet not doing it while mulching every year (as I did for a while) does no good; layer upon layer of undisturbed mulch becomes compacted. Compaction causes a barrier where water runs off and air pockets beneath the soil line are compressed. Lately I’ve compromised by giving the mulch an extra year to break down. I poke and turn it with a pitch fork the days before new mulch is applied. This option is easier on my wallet, too.  

Weeding grass out of flower beds is no fun!

Lawns … a) Seed fescue grass every other year (alternating with mulch years) unless it really needs it. b) If ornamental beds haven’t been mulched in a while, don’t seed the lawn (see photo). c) Skip fescue entirely and plant zoysia or another warm season grass. It’s too hot here for fescue to thrive, especially without a lot of time and money.

Plant more native shrubs. I’ve come to appreciate native plants for their benefits to native wildlife. I’m no scientist but I’m in my garden a lot and the more natives I’ve added or let be, the greater variety of insects and birds I’ve observed. But frankly, the native plants are more carefree and thus bring me more joy.  (Granted I could really make a difference by getting rid of my lawn …)

Be bold about removing things that aren’t “right plant, right place” (apple tree in a shady valley, hostas in too much sun, hydrangeas in a cramped spot). They will struggle to flourish and you’ll be disappointed. Once something un-spectacular is gone from sight you will hardly remember that it was ever there.

Raise a few chickens. I had never lived anywhere that backyard chickens were allowed. So, it’s no surprise that it took me this long to consider raising them myself. There’s a perfect site in my yard (remember that shady valley where the apple tree struggled). And mine is an egg-eating household. Plus, chickens and gardens play well together.

Rejuvenate or replace the hedge sooner. Hedges are high maintenance. At least the really good-looking ones are. I’m always shy about making the first cut but have rarely regretted giving my hedge a confident trim or applying a rejuvenating prune to a shrub in need. Alternatively, plant a loose hedge; one that need not be squared off or rounded to look decent. Fragrant tea olive (Osmanthus fragrans) is a great choice as is the anise tree (Illicium floridanum).

Photo by A. Laine

Foundation plantings. Think twice before putting shrubbery up against the house. Mine were present when I moved in; But had I removed them a decade ago, they would not be the nuisance they sometimes are today. Vegetation up against the house is not necessary (in my opinion) and it’s a pain when it comes time to paint the exterior, power wash, or make a repair. It’s also a hassle to trim bushes placed so close to the house!

Focus, Focus, Focus. If I knew then what I know now, I would have heeded the advice to design and landscape one section of my yard at a time. Not strictly adhering to this rule haunts me on dry summer days as I traipse around the garden with a hose or watering can tending newly planted trees, shrubs or perennials.

There’s no time like the present to learn from our mistakes.  Ask yourself what you would do differently and then set out to do it.  

Extension Resources & Further Reading
Publications and factsheets from NC State Extension

A comprehensive look at soil compaction

A guide to maintaining quality turf in NC
https://content.ces.ncsu.edu/carolin a-lawns

Create your own native landscape, even in an urban landscape

Raising chickens

NC Extension Gardener Plant Toolbox

Pruning shrubs and trees

Learn With Us, week of October 13

Why We Study Soil – Durham Garden Forum
Tuesday, October 15⋅7:00 – 8:30pm

Sarah P. Duke Gardens
420 Anderson St, Durham, NC 27708
With Dr. David Crouse, Professor & Undergraduate Teaching Coordinator of Crop and Soil Science at North Carolina State University Soil is the basis of all garden success. Dr. David Crouse will outline some of the basic ecoservices provided by soil to give you the big picture of how soil impacts your garden and some specifics about how you can work to improve your soil.

The Durham Garden Forum is an informal group that meets once a month to enrich our gardening knowledge and skill, on Tuesdays, 7:00- 8:30 pm at Sarah P. Duke Gardens. Lectures free for members, $10 general public.
No pre-registration necessary.

Our Straw Bale Gardening Experiment

by Marty Fisher, EMGV

After three painful years of crop-destroying diseases on our beloved heirloom tomatoes, my husband and I decided to give straw bale gardening a try.

I had tried grafting—attaching an heirloom scion onto a hardy root stock—for two years with some success, but it was a long, painstaking process, and we still weren’t getting the yields we enjoyed in the days before the dreaded fungal, viral, and bacterial diseases took over in our garden. We also tried disease-resistant tomatoes like Early Girl, Better Boy, Celebrity, Sweet 100, etc., but we missed the complexity of flavors and variety of colors of our favorite heirlooms like Carbon, Amana Orange, Green Zebra, Pineapple, Lucky Tiger, and more. We also tried removing the lower branches of our plants to keep them away from disease spores in the soil as well as spraying with copper sulfate—it all seemed a lot of work for a limited return on investment!

I started researching straw bale gardening in January, thinking it offered the advantage of a sterile environment for the tomato plants as well as the effect of a raised be d. I found a book written by the man who pioneered the concept, Joel Karsten. He came up with the idea after observing healthy, vigorous weeds sprouting in old straw bales on his family farm. He began experimenting with vegetables, and over decades, developed the techniques for gardening with straw bales. Since then, he has published two editions of his book, Straw Bale Gardens, and has spoken all over the world about what is touted on the book jacket as “the hottest new method of veggie growing.” The book contains a wealth of useful information, plans, diagrams, and more.

Our first task was to find straw bales—they must be wheat straw bales, not hay. They are not cheap, especially given the size of our garden. The best price we found was about $6 a bale. We inquired about delivery and were told by several farm supply stores that they sell the bales at a loss or break-even and were not interested in delivering them. So, several pick-up truck trips later, we had lots of bales on our garden site. 

The next step was placing and conditioning the bales, a process that takes 10-12 days. Conditioning should be started two weeks before the average last frost date. 

Although I had reservations about the fertilizer recipe given in the book, we decided to just follow the instructions and evaluate the results. Over the conditioning period, fertilizer and water are applied to the bales according to a day-by-day schedule. My reservation concerned the recommendation of nitrogen-rich “traditional refined lawn fertilizer.” Instructions are also given for organic nitrogen sources, but this seemed more complex, and we followed the conventional method. Fertilizer is sprinkled over the top of each bale, and water carries it deep into the bale, starting the bacterial process of breaking down the bales to release nitrogen.

On Day 10, the instructions call for one cup per bale of 10-10-10 general garden fertilizer, watered in. 

Planting can begin on Day 12. A little sterile potting soil can be used to anchor the plant in the straw bale, but all the nutrients and growing medium the plants will need are contained within the conditioned bales. Adding garden soil or compost could introduce fungal spores to the sterile bales.


Our concerns about using lawn fertilizer were confirmed over the spring and early summer: TOO MUCH NITROGEN! Our plants quickly became enormous, with fewer blooms than normal. They also began to shade out the eggplants and herbs that we had planted beside the tomato plants. We added a phosphorous- and potassium-rich fertilizer and gradually the plants started to have more blossoms and bear fruit. We also noticed that plants growing in straw bales require more diligent watering. Karsten recommends placing soaker hoses on top of the bales, controlled by a timer. 

In researching this article, I found an N.C. Cooperative Extension Service article (see “Hay” Bale Gardening link below) that recommends a different conditioning method. It calls for simply watering the bales for three days. On Day 4, you add 2 cups of dolomitic limestone and a half cup of ammonium sulfate, and water it in. On Days 5-9, you add more ammonium sulfate, followed by a cup of 10-10-10 or 8-8-8 on Day 10. Planting commences on Day 11.

This year we did lose a few plants to “the *%#* fungus,” our common term for fungal, viral, and bacterial diseases including early blight, late blight, fusarium wilt, tomato spotted wilt, various cankers—you name it. My husband also sprayed the plants with copper sulfate as a preventative. 

Overall, we deemed the straw bale gardening experiment a success. We canned, made salsa, roasted, sliced, and froze plenty of colorful tomatoes to get us through the winter. We plan to try it again next year using the Extension Service-recommended fertilizer recipe.

I also plan to graft tomatoes again next year after taking this year off. And, I’ll give some of the newer disease-resistant varieties a try. Some I’m particularly interested in include Iron Lady, described as “super resistant,” and Mountain Magic, which was developed by Dr. Randy Gardiner, professor emeritus at North Carolina State University. Apparently, some breeders are also developing disease-resistant heirlooms, including two old favorites of mine, Lemon Drop and Mr. Stripey.

This was our first harvest this year–it got better after we adjusted the fertilizer.
Good old fashioned canned tomatoes! Heirlooms make the jars so colorful.
Tomatoes growing in straw bales in our garden. In addition to diseases, we also battle squirrels, hence the large cage under construction.

All photos taken by Marty Fisher.


Straw Bale Gardens Complete, by Joel Karsten

Understanding Tomato Varieties:

How to Grow Better Tomatoes

Grafting for Disease Resistance

Blight Resistant Tomato Varieties Worth Growing

“Hay” Bale Gardening

Core Aeration of Lawns

by Carl J. Boxenberger, EMGV

Core aeration is a process by which cores or plugs of soil and thatch are removed from the lawn. Core aeration is done by a machine with hollow tines.

Soils that are prone to heavy traffic are subject to compaction. Core aeration reduces soil compaction by removing plugs of soil which opens up a channel in the lawn and allows water, oxygen and nutrients to penetrate down in the soil.

Core aeration should be done when the grass is actively growing. Fall is the time to core aerate cool season lawns such as tall fescue. Spring and early summer is the time to core aerate warm season grasses such as Bermudagrass.

Gardener operating soil aeration machine on grass lawn. Stock Photo c Mikhail Pavlenko

Core aeration equipment with hollow tines can be rented at local equipment rental companies.  There are also professional turf maintenance companies that will aerate your lawn if you do not want to tackle this by yourself.

Run the aeration equipment over your lawn to remove soil cores. Chop up the cores by running a lawn mower over them. If you have a large lawn and a tractor, you can distribute the cores by dragging a piece of chain link fence or mat over them.

Core aerate a few days after a rain. This will have allowed the soil to drain. If you pull plugs when the soil is wet they will form wet clods of soil alongside the aeration holes and actually inhibit air infiltration into the soil, defeating the purpose of aeration.

Further Reading

NC State Extension Turf Files: https://www.turffiles.ncsu.edu/

News alert about zoysiagrass mite damage