By Bill Ward
Published in The Boerne Star on February 17, 2006
As we discussed last month, many landscape plants in this part of the Hill Country must grow in soils which are developed on either Glen Rose or Edwards limestones. These soils are calcareous and alkaline.
Some yards near streams may have calcareous soils developed on old river terraces that are composed mostly of sediment eroded from Cretaceous limestones upstream.
Some of these old river terrace deposits may contain chert (flint) gravel and sand derived from the cherty limestones of the Edwards. This chert gives those soils a bit of silica and so probably lowers the alkalinity a little.
In Kendall County, it is only in the Guadalupe River Valley that stream erosion has cut all the way down to older formations, Hensel sandstone and Cow Creek limestone.
In those narrow areas along the Guadalupe River and its tributaries where Hensel is exposed, the soil may be sandy. However, the outcrop of the Glen Rose Formation is the most widespread in the eastern part of the Hill Country.
The younger Edwards crops out on some of the highest hills. Both of these formations are predominantly limestones.
So can we expect similar soils and similar vegetation across most of this wide limestone terrain? No, because there is a lot of heterogeneity in bedrock composition, in drainage, in degree of slope of the land and in exposure to the sun. All these determine which plants grow in a particular locality.
Just because a certain plant grows well in one area of the Glen Rose outcrop doesn’t mean that same plant will do well in another part of the Glen Rose. The Glen Rose outcrop is highly heterogeneous in soil type and drainage.
This is because during the deposition of the Glen Rose about 113 to 108 million years ago, the mainland was only a few tens of miles northward of Boerne. From time to time, mud and sand washed off the land and were carried out onto the shallow marine shelf.
Some parts of the Glen Rose, therefore, are clayey or marly. Where these layers crop out, the soils may be “tight” with poor drainage. These spots commonly are damp and support bushy bluestem, Lindheimer’s muhly and seep muhly grasses.
By contrast, parts of the Glen Rose outcrop that is underlain by pure limestone may be better-drained and support little bluestem, big bluestem and silver bluestem grasses.
By the time Edwards sediments were deposited over South Central Texas, the shoreline had advanced much farther inland and little or no material was shed from the mainland onto the shallow sea floor in this area.
Consequently, the Edwards is composed of non-clayey limestone. Dissolution holes and solution-enlarged fractures in the Edwards rocks make them well-drained.
Northwest of Boerne the high Edwards-capped hills support plants such as the smoke tree, Lacey oak and escarpment black cherry, which do not grow so well in poorly drained terrains.
Among the drought-resistant native plants commonly used in flower beds, blackfoot daisies and damianita do much better in well-drained soils. That is why, once they are established, these natives are better left unwatered in flower beds. The same goes for many native plants used in landscaping.
Healthier and longer-lived native plants survive in yards where they are not “loved to death” by too much watering and fertilizing. After all, one point of growing natives is to protect the quantity and quality of our ground water and surface water by limiting irrigation and application of fertilizers.
How does one decide what to plant with all this heterogeneity in growing conditions and soil types in our area? One way is to look around your area to see what natives grow best.
There is a large variety of native plants that are attractive landscape plants. Choose the ones that do well in your area. In other words, instead of choosing the plants, let the plants choose you. Chances are both you and the plant will be happier.
