PLACE · USDA ZONES
USDA Zones 8a and 8b.
The USDA plant-hardiness map is the most commonly cited environmental reference in residential horticulture and the most commonly misread. It is a thirty-year average of annual minimum temperatures, not a guarantee, not a worst-case envelope, and not a uniform value across the DFW service area. After February 2021, it has to be read with that in mind.
What 8a and 8b actually specify.
The USDA hardiness-zone system divides temperature ranges in ten-degree Fahrenheit increments, each further split into two sub-zones at the five-degree mark. Zone 8a carries an average annual minimum of 10°F to 15°F; zone 8b carries an average annual minimum of 15°F to 20°F. The numbers describe the average coldest day of the year across a thirty-year averaging window, not the absolute coldest temperature ever recorded and not the expected temperature in any given winter. On the long average, an 8b site will see a low in the 15°F-to-20°F band once per year. In practice, individual winters will go warmer than that, and individual winters will go colder.
The designation matters for long-lived plant selection because species carry documented minimum temperature thresholds. The live oak hybrid zone is a good illustration — Quercus fusiformis and Q. virginiana both perform reliably in zone 8 and carry enough tolerance to survive occasional excursions colder than the sub-zone average. Species with minimum temperatures at the 8a-8b boundary — certain Podocarpus cultivars, borderline Pittosporum, marginal citrus — behave differently on the two sides of the boundary when the averaging window drifts toward its edges. A garden designed as if 8a and 8b are interchangeable is a garden that will lose specimens on schedule whenever the average bumps toward the colder end of the range.
The 2023 revision and what it changed.
In November 2023, the USDA Agricultural Research Service published the first revision of the Plant Hardiness Zone Map since 2012. The new map uses the 1991–2020 averaging window, replacing the 1976–2005 window that the 2012 map had drawn from. The warming trend in that thirty-year window shifted a number of zone boundaries northward, and several DFW-area sites that had been designated 8a under the 2012 data moved to 8b under the 2023 data. Dallas city limits shifted from uniform 8a to uniform 8b in that revision — a citywide climate-trend upgrade, not a methodological artifact.
The reconciliation on the design side is straightforward: plant lists built against the 2012 map are due for a review, because species that were marginal at Dallas latitude under the old designation may be reliably in-zone under the new one. Alterra's working palette was reviewed against the 2023 revision during the S17 retroactive-zone-review cycle, and the resulting per-city designations are captured in the table below. That review did not conclude that the 2023 map retires the 2012 discipline around borderline specimens; the February 2021 Uri event produced single-digit temperatures well outside any 8a or 8b definition, and that event holds more weight in current design practice than any zone-map revision does.
By ZIP, across the service area.
The metro is not uniformly one sub-zone. The urban heat island across the denser core of Dallas and the older Collin County municipalities runs a measurable amount warmer than the outlying fringes, which is part of what pushed Dallas city limits to uniform 8b in the 2023 revision. Northern Collin County — parts of Frisco, parts of McKinney — still sits on the 8a side of the contemporary boundary. The per-city designations below reflect the S17 retroactive zone review with S15 amendments; operator reviews annually and updates within the calendar year of any USDA revision.
Uniform 8b per S17 review.
Uniform 8b — contradicted the initial S15 working hypothesis that Collin cities would show an 8a/8b split.
Uniform 8b. The 8a/8b boundary runs between Prosper and Frisco, not through both.
Split 8a/8b at the ZIP level.
Split 8a/8b at the ZIP level. Historic downtown (southern McKinney) tracks differently from northern MPC expansions.
Uniform 8b — breaks the split-zone pattern predicted for northern Collin cities.
Uniform 8b. Shifted from 8a to 8b between the 2012 and 2023 USDA maps. Flagship neighborhoods (Highland Park, University Park, Preston Hollow) are 8b, within Dallas city limits.
Designing within the zone, and against its edges.
For a project in Frisco or McKinney, the design-phase discipline includes a specific-lot ZIP check against the 8a/8b boundary, and if the lot sits on the 8a side, the palette gets trimmed accordingly. A north-facing, unprotected garden in north Frisco is an 8a site regardless of whether the address reads as “DFW metro” — the microclimate belongs to the northern Collin County cold pocket, and the specification has to reflect that. For projects in Richardson, Plano, Prosper, Lucas, and Dallas, the uniform 8b designation allows a slightly broader palette, though the practice continues to specify conservatively within the zone.
The 2023 revision opened a small window of species that had been borderline at 8a and are now safer at 8b — certain protected-exposure citrus cultivars, Texas mountain laurel (Sophora secundiflora) with appropriate placement, some evergreen shrubs from the 7b/8a margin that now carry a reasonable expectation of survival on DFW sites where the 2023 data applies. The practice uses that window cautiously. A zone-map designation is a thirty-year average, and a twenty-year planting horizon will outlive the next averaging-window update; every high-investment specimen decision is made against the Uri record rather than against the zone map alone.
The deeper working discipline — and the reason the native and adapted palette carries most of the specification work on any given project — is that a species firmly inside its range for the zone will cooperate with the site through the normal variance, and a species on the zone edge will cost a replacement cycle on every outlier winter. The native canopy species (Q. fusiformis, Q. virginiana, Q. stellata, Juniperus virginiana, cedar elm) sit firmly inside zone 8 and carry a demonstrable record on the other side of the 2021 event. Those species are doing the load-bearing work on Alterra plantings for a reason.
After Uri — advisory, not guarantee.
The February 2021 Uri event produced low temperatures in the single digits Fahrenheit across the DFW metro, with soil temperatures lower than anything in the region's recorded horticultural memory. Those readings are well outside any 8a or 8b definition and will not be represented in any averaging-window revision until enough similar events accumulate to shift the thirty-year numbers — a timeline the current planting horizon will not wait for. The working conclusion on the design side is that a zone designation is advisory rather than guarantee, and that Uri-scale events are the resilience constraint planting specs actually have to answer to regardless of what the map says.
What this means at the drawing board is a measured conservatism — specimen trees and high-investment plantings are selected from species that carry a documented record on the other side of Uri; borderline zone-8 imports are either declined or placed only on sheltered microclimates where exposure can be demonstrably established as 8b-plus; irrigation and hardscape details are specified to anticipate freeze events rather than survive them on favorable winter averages. Read the full discipline in the resilience section. For how this reconciles with the water-cycle side of the specification, see DFW water. For how it carries into the drawing set, see the landscape-design practice.
Where the data comes from.
Per-ZIP designations are drawn from the USDA Plant Hardiness Zone Map at planthardiness.ars.usda.gov. The map is reviewed annually by the practice, and this page is updated within the calendar year of any USDA revision. Where the USDA map and field observation conflict — as they occasionally do at microclimate scale, especially on heavily shaded or cold-pocket sites — field observation takes precedence in the project's design spec. The map describes a thirty-year average across a generalized area; the specification describes the garden that actually has to survive on the specific lot.