The Blackland Prairie.

The Blackland Prairie is the Texas extension of the broader tallgrass prairie ecoregion — a north-south band running from the Red River south through McKinney, east Dallas, Corsicana, Waco, and down past Austin. The metro sits on its western edge. Collin County lies almost entirely within it; east and central Dallas sit on it east of the Trinity River; Lake Highlands, Lakewood, Old East Dallas, and the neighborhoods that follow White Rock Creek all read as Blackland at the level of soil and vegetation. The ecoregion is EPA Level III 26 in the federal classification, and on the ground it is the substrate that Jeffery and most of the practice's Collin-County work sit inside.

The geology is Cretaceous. Roughly 80 to 100 million years ago the interior of North America was a shallow sea that deposited calcium-rich sediments across much of what is now Texas — Austin Chalk, Eagle Ford Shale, and the Woodbine Formation at the western edge. As those layers weathered across the subsequent geologic time, the chalk and marl became the parent material for a family of vertisols the NRCS names Houston Black, Heiden, and Austin silty clay. Those soils run 60–80% of residential coverage across McKinney and sit under most of east Dallas. They carry 20–30% shrink-swell potential and a pH between about 7.4 and 8.5 — a narrow but consistent range that decides which species will cooperate with the ground and which will not.

Calcareous clay does not behave like the deep humic loams of the American Midwest that share the “tallgrass prairie” label. In August, Blackland clay cracks open in wide vertical seams as it loses moisture. In November, after autumn rains, it closes back shut and lifts whatever was sitting on it a half-inch in the process. A foundation pier on a Blackland lot is specified for this; a terrace detail has to be specified for it too.

The pre-settlement Blackland was a tallgrass matrix punctuated by scattered motts of canopy where the soil and hydrology allowed it. The working grass species were big bluestem (Andropogon gerardii) at the shoulder-high upper end, Indiangrass (Sorghastrum nutans) and switchgrass (Panicum virgatum) through the midstory, and little bluestem (Schizachyrium scoparium) and sideoats grama (Bouteloua curtipendula, the Texas state grass) carrying the shorter cover. Forbs — prairie clover, prairie coneflower, blackfoot daisy, and dozens of Asteraceae — filled the intergrass canopy. What held this mix in place was a specific fire regime: low-intensity grassland fires at a roughly three-to-five-year interval, set by lightning and by Indigenous land practice, that prevented woody species from converting grassland to woodland in the middle of the prairie.

Where canopy did establish, it concentrated along stream bottoms, on north-facing protected slopes, and in the occasional upland mott on soils that drained more sharply. The consistent canopy species for those pockets — and the consistent native canopy species in any Alterra Blackland planting plan today — are escarpment live oak (Quercus fusiformis) on drier westward-facing Blackland-side sites, southern live oak (Q. virginiana) on moister eastern sites with the two species hybridizing freely through the metro, and eastern red cedar (Juniperus virginiana) as the reliable native evergreen structure. Cedar elm (Ulmus crassifolia) and sugar hackberry (Celtis laevigata) round out the canopy palette for stream-bottom and transitional situations.

Settler agriculture reached the Blackland in the 1840s and read the substrate as one of the most fertile cotton soils in North America. Within a generation the region was among the most heavily plowed ecosystems on the continent, and the tallgrass matrix that had taken thousands of years to compose was converted more completely than almost any other North American prairie. The commonly cited figure — that less than one percent of the original Blackland Prairie remains intact — is the right order of magnitude. The remnants that survive are pocket-scale: rail corridors that never got broken to plow, a few cemetery patches in Collin and Dallas counties, the Blackland Prairie Raptor Center tract outside Lucas, scattered easement margins.

What survived in residential ornamental cultivation was the subset of the native canopy that proved willing to coexist with lawn, irrigation, and compacted clay fill. Cedar elm, sugar hackberry, Mexican plum (Prunus mexicana), Texas redbud (Cercis canadensis var. texensis), and the live oaks carried the canopy on Blackland residential lots from the mid-twentieth century forward. The grasses largely did not — until the native revival turn of the 1990s and 2000s began restoring sideoats grama, little bluestem, and Mexican feathergrass (Nassella tenuissima) to the ornamental palette. Today the grasses are back, in designed form, on residential projects across the service area. The canopy selections sit on a continuous line of horticultural use across the same century.

The first design decision on a Blackland lot is species selection against the alkaline clay. Acid-loving imports — azalea, rhododendron, blueberry, pin oak (Quercus palustris), red maple (Acer rubrum), sweetgum (Liquidambar styraciflua) — fail chronically on Houston Black and Heiden clay soils through iron chlorosis and root stress. The species list that does cooperate is narrow but entirely workable: escarpment and southern live oak for the canopy; eastern red cedar for evergreen structure; cedar elm, Mexican plum, and Texas redbud for understory; the sideoats-and-little-bluestem grass palette for meadow and edge plantings; clay-adapted perennials including salvias, blackfoot daisy, Texas sage (Leucophyllum frutescens), and Gulf muhly (Muhlenbergia capillaris). Every species named here tolerates calcareous alkalinity and survives the 2021 freeze record; those two filters narrow the palette in the same direction.

The second decision is drainage. Shrink-swell clay moves water slowly in the top foot and holds it in the middle column, which means a planting bed has to be graded and detailed to shed water in the first few inches rather than collect it. Foundations on Blackland lots are on drilled piers and post-tension slabs for the same reason the garden has to be graded away from the house — the ground under both will move through the calendar year, and the drawing has to assume the motion rather than wish it away. Hardscape terraces are specified with mortar chemistry called out for hard alkaline water and with expansion joints placed to accommodate shrink-swell rather than resist it.

The third decision is turf. Front-yard lawn expectations on Blackland lots are cultural rather than ecological; the substrate supports bermuda and zoysia sufficiently, but the ecological honesty of a prairie site would argue for more grass bed and less turf. Most Alterra projects on this substrate land on a middle register — enough turf to read as residential at the front setback, native grass beds and prairie-derived planting through the rest of the lot. See the planting practice for how that reconciliation gets executed on site.