Ornamental · Poaceae
Blue Oat Grass
Helictotrichon sempervirens
Fine steel-blue blades rise in a rounded fountain, holding their color all year in full sun and lean, dry soil.
Full sun
Every 7–14 days
USDA 4–8
18–30 in · 46–76 cm
Overview
Blue oat grass is all about the blades. Each one is fine and narrow, stiff to the touch, and washed in a cool steel-blue that holds through the seasons, the whole bunch radiating from a single crown into a rounded, fountain-like spray. As a cool-season grass it keeps that color in mild winters. The clump fans 18 to 30 inches (46 to 76 cm) across, every blade arching out and back so the mound stays upright and even without any staking.
Come early summer, slender stems push up well clear of the foliage and unfurl oat-like, straw-colored panicles, loose little spikelets that nod and catch the light against the blue beneath, and they are exactly what earns the grass its name. The blades thicken into the clump slowly, but the plant lives for years, hardy across USDA zones 4 to 8.
That blue is at its most intense when the plant is grown hard. On lean, dry, sharply drained soil in full sun the blades stay tight, narrow, and steel-toned, while rich ground and shade leave them soft, splayed, and faded toward plain green. The harder the conditions, the bluer the blades, right down to the fine waxy sheen on each leaf.
Care
The short version: full sun and lean, sharply drained soil, almost no feeding, and a drink only through a long dry spell once it is established. A single cut close to the ground in late winter clears the old blades, and otherwise it is left alone.
Light
Blue oat grass needs full sun to keep its color, the more the better. A little light shade is tolerated, but under deeper shade the blades soften, the mound opens and flops, and the steel-blue thins toward green.
Water
Once established the clump is very drought-tolerant, content on dry to medium soil and wanting water only every 7 to 14 days through a real dry spell. Standing wet is what it can't take. Soggy, heavy, or humid ground rots the crown, so the soil is kept on the dry side.
Soil
Set in lean, sharply drained soil, the blades come through at their bluest and the clump at its firmest. Poor, dry, even rocky ground suits it, across a pH from 6.0 to 7.5. Rich soil only loosens the blades into a soft, floppy spray.
Temperature
The plant takes a wide range, from -29°F to 100°F (-34°C to 38°C). Cold to zone 4 leaves the blades unharmed, and in the heat it is the humidity riding along that threatens the foliage rather than the warmth itself.
Humidity
Blue oat grass prefers the dry side and dislikes muggy air, which dusts the blades with rust and softens the crown into rot. Air moving freely around and through the clump matters far more than any humidity figure, so the blades are not packed tight.
Feeding
It asks for little to no feed. Grown lean, the clump holds its tight form and the blades keep their deepest blue, while a spring feed or rich ground pushes the leaves soft, green, and floppy.
Pruning
The single yearly job is to cut the whole clump back close to the ground in late winter or early spring before the new blades emerge. Through the season, the tattered, faded old blades are combed or raked out by hand to leave the fresh blue showing.
Repotting
Blue oat grass resents disturbance and rarely needs lifting. The crown is divided in spring only if the center dies out or another plant is wanted, and otherwise the clump is left to build its blades undisturbed for years.
Size & spacing
A mature clump rises 18 to 30 inches (46 to 76 cm) tall and the blades fan about 24 to 30 inches (61 to 76 cm) wide. Plants are set to that width so air moves between the blades and the foliage dries quickly.
When to plant
Blue oat grass is set out as a plant rather than sown by season like a vegetable. New plants go in during spring once frost has passed, the oat-like flower stems lift above the blades in early summer around June, and the whole clump gets its single yearly cut in late winter before the fresh, bright blades push through.
Northern hemisphere
Southern hemisphere
Typical months for a temperate climate: start earlier in warm zones, later where spring frosts linger.
- Sow: Late winter to early spring
- Plant: Spring after last frost
- Harvest: Early summer (June); oat-like straw-colored panicles rise above the foliage
Hardiness
USDA hardiness zones: 4–8
Temperature: down to -34°C
Troubleshooting
Grown lean and dry, this grass rarely shows trouble, and what little it gets reads plainly on the blades and at the crown. Watch for color shifts and powder on the leaves and softness at the base, then correct the conditions before rot or rust marks the foliage.
The center of the clump rots and falls apart
When wet, heavy soil packs against the crown through winter, crown and root rot sets in, turning the base soft and brown until the blades collapse and brown out with no way back. Drainage is the cure. The plant is reset into a raised, gritty bed, sodden mulch is pulled away from the crown, and it is never left standing in water.
Fix:
- Improve drainage or replant in a raised, gritty, sharply drained bed
- Reduce watering and cut off wet, mulch-heavy material at the crown
- Remove severely rotted plants
Rusty orange spots that rub off as powder
Raised orange-brown specks that smear into a rusty dust on your fingers are foliar rust, a fungus that settles onto the blades in warm, humid weather and where the clump stands too close to its neighbors. The marked blades are stripped out, the clump is opened so air moves through the foliage, and water goes at the base to keep the leaves dry. In muggy regions the Sapphire selection carries far cleaner blades than the plain species.
Fix:
- Remove and discard affected foliage
- Improve air circulation by spacing and thinning
- Avoid wetting the foliage; water at the base
- Grow rust-resistant cultivars such as Saphirsprudel/Sapphire in humid regions
Floppy growth and a fading blue color
Blades that splay open and drift from steel-blue toward plain green are signaling for lean soil and full sun. Shade and rich, watered ground draw the leaves out soft and lax. The clump is moved somewhere sunnier, the feed and water are cut back, and it is sheared down so the new blades come in tighter, stiffer, and bluer.
Fix:
- Move or replant in full sun
- Reduce fertilizer and water; grow in leaner, drier soil
- Cut back to tidy the clump and let it regrow firmer
Toxicity
Blue oat grass is safe to grow around pets, livestock, and children. The ASPCA does not list it as poisonous, and there is no toxin in the wiry blades or the roots. The only thing worth a mention is grass pollen, which can bother people prone to hay fever.
Safe for
People · Cats · Dogs · Horses · Rabbits · Livestock · Birds
Cats Safe
Blue oat grass holds no toxin harmful to cats, so a curious nibble at the blades does no harm. A cat that swallows a big mouthful of the stiff leaves may bring them back up, the same as with any grass.
Dogs Safe
Nothing in these blades will poison a dog. Chewing a lot of the coarse leaves can bring on a little vomiting, but that is the roughage at work rather than a toxin.
Horses Safe
It is safe around horses, which usually pass over the wiry blue blades for softer pasture grass.
Rabbits Safe
Rabbits can graze the clump without any risk, though most prefer tender greens to these stiff, coarse leaves.
Livestock Safe
Cattle, goats, and sheep face no poisoning from blue oat grass. The wiry clump is ornamental rather than a forage crop, so they get little real feed from the blades.
Birds Safe
It is harmless to birds, which often pull thin strands from the clump for nest building in spring.
People Safe
The plant is non-poisonous and safe to handle, blades and all. The one thing to note is that grass pollen can set off hay fever in people prone to seasonal allergies.
Grass pollen can trigger seasonal allergies in sensitive people; pets that nibble grass blades may vomit, but the plant is not poisonous.
Varieties
Named forms of blue oat grass are few, but the difference between the species and its best selection shows up in both the depth of blue and how cleanly the blades come through a humid summer.
Saphirsprudel (Sapphire)
A German selection whose blades take on a deeper blue than the species and stay far cleaner through rust, heat, and humid summers.
Species (Helictotrichon sempervirens)
The plain species builds a steel-blue mound of 18 to 30 inches (46 to 76 cm) whose blades are topped in summer by oat-like, straw-colored panicles arching above the foliage.
How to propagate
More blue oat grass comes from splitting an old crown or sowing seed, though the blades bulk up into a full clump slowly and the plant resents having its roots disturbed.
DivisionModerate
Spring · One growing season
Because its roots resent the spade, blue oat grass is divided only when an old crown thins at the center or a second plant is wanted. The crown is split in spring, and the divisions root and fill back into a full clump of blades over a single growing season.
SeedModerate
Late winter to spring · One to two growing seasons
The species grows from seed sown in late winter or spring, with a seedling taking one to two seasons to build its blades to full size. Named selections like Saphirsprudel won't come true from seed, so the crown is divided to keep the same deep-blue blades.
Myths
Blue oat grass and blue fescue are the same plant.
Both wear a blue cast, and that is nearly all they have in common. Blue oat grass is the larger and longer-lived of the two, its blades building an 18 to 30 inch (46 to 76 cm) clump topped with arching, oat-like flower stems. Blue fescue stays a small 8 to 12 inch (20 to 30 cm) tuft of finer blades that dies out in the middle and needs splitting every couple of years to keep its shape.