Revisiting Tandy Hills

View of downtown Fort Worth from Tandy Hills

Tarrant County, TX includes parts of both the Grand Prairie and the Western Cross Timbers ecoregions.  Thin clay soil over caliche supports prairies, while deeper, sandy soils can host scrubby oak-juniper forests. The difference is all in the water capacity. A tree can make a go of it if it spreads its root system in the sand, but the shallow clays just can't hold enough water. In this way, the soils structure the plant communities irrespective of their shared precipitation regime.

After returning to Tandy Hills last weekend, I looked at soil maps of Tarrant County to try and understand why it has stayed so open on the ridge top. Nothing struck me as different at the map resolution I could find- there were no unusual soil inclusions marking Tandy as different from the surrounding area. I do know that in savannas, hill tops are often left as prairie because their thinner soils hold less water. The main patch of prairie in Tandy is on top of the hill. Perhaps this is why.

It may be almost October, but the highs are still in the low 90s. The rain has returned, and a number of plants are putting on a second flush of flowers. Prickly pear fruits are bright red and ripe, reminding me of their alternate common name "cactus apple". Eryngo, snow-on-the-prairie, and false foxglove are all hitting their peak bloom. Others, like basket flower and white compass plant, have long since set their seed heads for the year.

Spiky, sticky white compass plant seed heads. 

Silphiums (i.e., compass plant, rosin weed etc) are unusual among composite flowers. Composite flowers, like sunflowers and asters, are distinguished by being composed of multiple smaller flowers bound together in a larger flower head. In a sunflower, the flowers that produce sunflower seeds are the 'disc' flowers, while the 'ray' flowers around the edges do not. This is the common arrangement for composites. However, silphiums set seeds from the 'rays'. If you harvest a seed head and break it apart in your hand it resembles an artichoke.

False foxglove, Agalinis spp. 

I also learned a new plant: false foxglove. False foxglove has flowers like a penstemon (or foxglove), but set along thin branching stalks with small leaves. I didn't notice the fringe of hairs along the edges of the petals until I looked at my pictures later. This widely distributed annual flower is in the Orobanchaceae family, and like other members of this family it is 'hemiparasitic' on the roots of other plants-- that is, it taps the roots of other plants to satisfy some of its nutritional needs. Perhaps this nutrient piracy could explain why the leaves seemed so undersized.

Eryngo, Eryngium leavenworthii, with small caterpillar

Eryngo is another flower whose details became more evident when I viewed the pictures later. I love the differences in tone between the indigo pollen and the royal purple flower. At first I mistook the late-season caterpillar for a bit of chaff or leaf matter.

Indeed, the insect life is still humming in this little prairie. On my way out, I noticed an oddly posed praying mantis near a clump of white fluff on a blazing star bloom. It didn't look like her egg sac to me- if anything, it looked like a small cocoon. I could be mistaken. I decided not to disturb her or the white mass and instead let them continue their preparations for the end of the growing season in peace.

Praying mantis, unIDd fluff, on blazing star