// SPECIES PROFILE · PERENNIAL · GRASS · PRAIRIE

Purpletop

Tridens flavus

Purpletop is the late-season fireworks of the tallgrass prairie — a sturdy, clump-forming warm-season bunchgrass that spends the spring and summer as an unassuming mound of blue-green foliage, then in August sends up slender, arching flowering culms crowned with an open, airy panicle of rich purple spikelets that catch the low-angle autumn light like embers. It is one of the most common and widespread native grasses of the eastern and central US, thriving on roadsides, in old fields, along woodland edges, and in prairie remnants across all of Oklahoma. Tridens flavus is a skipper butterfly host, a seed source for songbirds, and a fall color anchor that turns the entire plant reddish-purple after frost — a quiet workhorse of the native landscape that asks for nothing and delivers a long season of subtle beauty.

// QUICK FACTS

Family: Poaceae (grass family)
Life cycle: Warm-season (C4) perennial bunchgrass; long-lived
Native range: Eastern and central US — MA to FL, west to AZ, NM, NE; widespread in OK
USDA hardiness: Zones 4–9 (Tulsa = 7a/7b)
Mature size: 2–3 ft tall (foliage), 3–5 ft tall in flower; clump 1.5–2 ft wide
Bloom / seed display: August – October (NE OK)
Sun: Full sun to part shade; surprisingly shade-tolerant for a prairie grass
Soil: Adaptable; tolerates clay, loam, sandy, and disturbed soils; pH 5.5–8.0
Water: Dry to medium; drought-tolerant once established
Wildlife: Skipper butterfly larval host · seed for songbirds · cover for small wildlife · fall/winter structure

Purpletop (Tridens flavus) showing open, airy purple seed panicles above blue-green foliage in late summer — *Tridens flavus* in its September glory — the distinctive purple, open panicle and greasy-feeling spikelets give this grass its common name and the folk name "grease grass." Photo: Rooted Revival.

Identification

[ field key — clump · blade · ligule · panicle · spikelet · fall color ]

Habit & Clump Form

A dense, upright bunchgrass (cespitose) that forms a tight, rounded clump of basal foliage. The plant does not spread by rhizomes or stolons; it stays where you plant it and slowly expands the crown diameter over the years. The basal foliage mound reaches 2–3 ft tall and forms an attractive, fountain-like shape. Flowering culms rise well above the foliage to 3–5 ft, creating an open, layered silhouette. Purpletop is a C4 (warm-season) grass, breaking dormancy late in spring (May in Tulsa) and growing through the heat of summer when cool-season grasses have gone dormant.

Leaves & Ligule

Leaf blades are flat, 4–12 in long and ¼–½ in wide, with a blue-green to gray-green color that distinguishes them from the yellower green of many other grasses. The blade margins are rough (scabrous) to the touch when stroked from tip to base due to microscopic, forward-pointing barbs — a useful tactile ID feature. The ligule (the membranous or hairy structure where the leaf blade meets the sheath) is a fringe of short hairs, a key trait for distinguishing Tridens from similar-looking genera. No auricles are present. Foliage turns reddish-purple after frost, giving the entire plant a striking fall color.

Flowers & Spikelets

The inflorescence is a loose, open, spreading panicle (branched flower cluster), 8–14 in long and nearly as wide, held well above the foliage on slender, arching culms. The individual spikelets are 3–7-flowered, about ¼–⅜ in long, and are a distinctive rich purple to reddish-purple color (occasionally greenish-yellow in some individuals). The spikelets have a characteristic greasy or oily feel when rubbed between the fingers, the source of the folk names "grease grass" and "tallow grass." This greasy texture, combined with the purple color, makes the ripening seed heads unmistakable.

Distinction from Similar Grasses

From purple lovegrass (Eragrostis spectabilis): Purple lovegrass is shorter (1–2 ft), has a much finer, more diffuse, cloud-like panicle, and its spikelets are much smaller and lack the greasy texture. From switchgrass (Panicum virgatum): Switchgrass has a more compact, narrower panicle, green to reddish spikelets (not consistently purple), and a prominent hairy ligule (vs. the short-fringed ligule of Tridens). From little bluestem: Little bluestem has folded leaf blades, a very different inflorescence (raceme, not open panicle), and lacks the greasy spikelets.

Habitat & Range in NE Oklahoma

Tridens flavus has one of the broadest native ranges of any North American grass, extending from Massachusetts and New York south to Florida, and west to Arizona, New Mexico, Nebraska, and the Dakotas. It is native and common across all of Oklahoma, where it is one of the most frequently encountered native grasses in disturbed and semi-natural habitats. In NE Oklahoma it is ubiquitous on roadsides, highway medians, railroad rights-of-way, old fields, overgrazed pastures, prairie remnants, and open woodland edges. It is particularly common in the transition zone between Cross Timbers oak woodland and tallgrass prairie.

The species is notable for its tolerance of partial shade, which is unusual for a warm-season prairie grass. You will see Purpletop growing in the dappled light at the edges of post oak and blackjack oak woodlands, in culvert drainage swales, and along woodland paths where most other prairie grasses cannot persist. This shade tolerance, combined with its adaptability to disturbed and compacted soils, makes it one of the most versatile native grasses for the varied conditions of a NE Oklahoma property.

Ecology & Wildlife Value

[ skipper host · seed for birds · cover · soil stabilization ]

Skipper Butterfly Host

Purpletop is a larval host plant for several grass-feeding skipper butterflies (subfamily Hesperiinae), whose caterpillars feed on the foliage of native grasses. Documented species include the crossline skipper (Polites origenes), the little glassywing (Pompeius verna), and the broad-winged skipper (Poanes viator). The caterpillars construct shelters by folding or rolling grass blades and tying them with silk, feeding at night and hiding during the day. Planting a diversity of native grasses including Purpletop is one of the most effective ways to support the full life cycle of these subtle, often-overlooked butterflies.

Seeds & Songbirds

The abundant, small seeds produced by the purple panicles are consumed by a wide variety of ground-feeding and grass-foraging songbirds, including several species of native sparrows (field sparrow, chipping sparrow, song sparrow), dark-eyed juncos, and mourning doves. The seeds are retained on the panicles well into winter, providing a reliable cold-season food source when insect prey is scarce. The standing dried culms also provide winter cover for small mammals and overwintering insects.

Insect Foraging Substrate

The tall, sturdy flowering culms serve as perching and foraging structures for insectivorous birds, including eastern bluebirds, which use elevated grass stems to scan for insects in the lower vegetation. The dense basal clumps provide overwintering habitat for beneficial predatory insects, including ground beetles and spiders that are critical to garden pest control the following spring.

Soil & Erosion Function

The deep, fibrous root system of Purpletop penetrates 3–5 ft into the soil, binding soil particles, improving infiltration, and contributing substantial organic matter to the soil profile as roots die and regenerate. On slopes, roadsides, and disturbed sites, it functions as an effective erosion-control grass while also providing the wildlife and aesthetic benefits that non-native erosion-control grasses (such as tall fescue) lack.

Design note — fall and winter value: Purpletop's late-season display is among the best of any native grass. The purple panicles glow in backlit autumn sun, and after frost the entire plant takes on reddish-purple tones. Leave the plants standing through winter for structural interest and wildlife value; cut back in late February before new growth begins.

Horticulture & Care

[ seed starting · site · maintenance · design · companions ]

Site selection & sowing

Purpletop is easy from seed and can be direct-sown in fall (for natural cold stratification) or spring. Seed is small and should be broadcast onto a firm, weed-free seedbed and pressed into the surface; do not bury deeply (¼ in maximum). Keep the seedbed consistently moist until germination (10–21 days at warm soil temperatures). As a warm-season grass, it germinates best when soil temperatures reach 65–70°F (typically May in Tulsa). First-year plants focus on root development and may not flower until year 2.

Sun: Full sun to part shade. Tolerates more shade than most prairie grasses.
Soil: Adaptable; grows in sand, loam, clay, and rocky soils. Tolerates compacted and disturbed soils.
Spacing: 18–24 in apart for specimen clumps; 12–18 in for a denser grass matrix.
pH: Tolerates a wide range, from acidic to slightly alkaline.

Maintenance & management

Once established, Purpletop requires virtually no maintenance. No fertilizer is needed, and supplemental irrigation is only necessary during severe prolonged drought. The clump expands slowly over the years but does not spread aggressively by rhizomes. It can be divided in early spring every 4–5 years to propagate or rejuvenate older clumps (cut the crown into sections with a sharp spade or knife). Cut dead tops to 6–8 in in late winter before new growth begins.

Pests & diseases

Essentially pest-free. Native grasses in their home range have few significant pest issues.
Rust fungi — occasional orange pustules on leaves, especially in humid, crowded conditions. Cosmetic only.
Leaf spot — minor fungal spotting possible in wet years. No treatment needed.
Deer & rabbits — foliage is not preferred browse; deer may nibble young shoots but leave mature clumps alone.

Design uses & companions

Purpletop shines in several garden roles:

Prairie / meadow planting: Mix with little bluestem, switchgrass, and Indian grass for a diverse native grass matrix. Underplant with wildflowers like purple coneflower, black-eyed susan, and rough blazing-star.
Woodland edge transition: Plant where open lawn or meadow meets woodland, taking advantage of its shade tolerance. Pair with roundhead lespedeza, Maryland senna, and goldenrods.
Culvert / drainage swale: Excellent for stabilizing drainage features that are too wet for other prairie grasses. Combine with rose mallow and cardinal flower.
Specimen or small group: The fountain-like form and late-season purple display make it a striking specimen plant. Plant 3–5 clumps together for a mass effect against a dark background.
Living mulch: The dense basal clumps suppress weeds effectively and can be used as a low-maintenance ground layer beneath taller perennials.

Sources & Further Reading

USDA NRCS PLANTS Database — Tridens flavus: plants.usda.gov/plant-profile/TRFL2
Lady Bird Johnson Wildflower Center — Native Plant Database: wildflower.org — TRFL2
Wikipedia — Tridens flavus: en.wikipedia.org/wiki/Tridens_flavus (CC BY-SA 4.0; portions of description and ecology summarize Wikipedia content).
Hitchcock, A.S. (rev. Chase, A.) (1950). Manual of the Grasses of the United States. USDA Miscellaneous Publication No. 200.
Dark, J. (2010). The American Meadow Garden: Creating a Natural Alternative to the Traditional Lawn. Timber Press.
Cullina, W. (2000). Wildflowers: A Guide to Growing and Propagating Native Flowers of North America. New England Wild Flower Society.

Hero photo: Rooted Revival.