Ornamental · Poaceae
Blue Oat Grass
Helictotrichon sempervirens
Even a first-timer can keep this steel-blue mound looking its best, as long as it gets full sun and lean, dry soil.
Full sun
Every 7–14 days
USDA 4–8
18–30 in · 46–76 cm
Overview
If you have ever worried about keeping a plant alive, blue oat grass is a kind one to start with. It settles into a soft, rounded clump of fine steel-blue blades that hold their color through the seasons, and as a cool-season grass it often stays evergreen where winters are mild. The mound grows 18 to 30 inches (46 to 76 cm) across and keeps its gentle, fountain shape on its own, so there is no staking to fret over.
When early summer arrives, thin stems lift above the leaves carrying oat-like, straw-colored panicles, the very feature behind the name. It takes its time growing and lives for years, hardy across USDA zones 4 to 8, which means a clump that has found its feet will reward you season after season with hardly any work.
Here is the reassuring part: the way to the best color is to do less, not more. Lean, dry, sharply drained soil and full sun bring you the tightest mound and the bluest leaves, and if it ever flops or fades you simply have it in soil that is too rich or a spot too shady. Poor soil is exactly what it wants, so the easy path is the right one.
Care
The short version: give it full sun and lean, sharply drained soil, skip the feeding almost entirely, and offer a drink only in a long dry spell once it has settled in. Trim it down near the ground in late winter, and the rest of the year you can happily leave it alone.
Light
Give blue oat grass full sun and it will thank you, the brighter the better. A touch of light shade is fine, but any more than that loosens the clump, makes it flop, and pulls the blue out of the leaves.
Water
Once it has settled in this is a very drought-tolerant grass that likes dry to medium soil and only wants a drink every 7 to 14 days during a real dry stretch. The one thing it cannot forgive is soggy roots, so when in doubt, keep it on the dry side and you will be fine.
Soil
Plant it in lean, sharply drained soil and it gives back the bluest color and the firmest shape. Poor, dry, even stony ground suits it, and any pH from 6.0 to 7.5 works. Rich soil is the one thing to avoid, since it just turns the clump soft and floppy.
Temperature
This is a hardy plant that copes with a wide swing, from -29°F to 100°F (-34°C to 38°C). Cold down to zone 4 will not faze it, and in the heat the thing to watch is the humidity that travels with it rather than the warmth itself.
Humidity
It prefers things on the dry side and is not fond of muggy air, which can bring rust to the leaves and rot to the base. More than any humidity number, good airflow around the clump is what keeps it healthy, so give it a little room to breathe.
Feeding
Feed it almost nothing and you will both be happier. A clump in lean soil holds its tidy shape and best blue, while feeding in spring or planting in rich ground only encourages the soft, green, floppy growth you would rather not see.
Pruning
The single yearly task is gentle and easy: cut the whole clump back close to the ground in late winter or early spring before new growth appears. During the season you can comb or rake out the tattered old blades by hand whenever you want it looking neat.
Repotting
Blue oat grass does not like being moved, and it seldom needs lifting at all. Divide it in spring only if the middle dies out or you would like another plant, and otherwise let a settled clump sit happily undisturbed for years.
Size & spacing
A grown clump reaches 18 to 30 inches (46 to 76 cm) tall and spreads roughly 24 to 30 inches (61 to 76 cm) wide. Space plants about that far apart so air can move between them and the leaves dry quickly, which keeps everything healthy.
When to plant
Blue oat grass goes in as a plant rather than being sown by the season like a vegetable, which keeps things simple for you. Set new plants out in spring once frost has passed, look forward to the oat-like flower stems in early summer around June, and give the clump its single yearly trim in late winter before the fresh growth begins.
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
A grass grown this lean rarely gives you trouble, and most of the time the cause is simply too much water or not enough sun. Try not to worry. Watch the blades and the base of the clump for early signs, and a small change in conditions usually puts things right before rot or rust can settle in.
The center of the clump rots and falls apart
When wet, heavy soil presses against the crown through winter, crown and root rot can set in, and sadly a clump cannot be saved once the base goes soft and brown. The answer is drainage. Move the plant into a raised, gritty bed, pull back any wet mulch heaped against the crown, and never let it stand in water, and a healthy plant will stay healthy.
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
Those orange, powdery flecks are foliar rust, a fungus that moves in during warm, muggy weather and where plants sit too close together. Pick off the spotted blades, loosen the clump so air can pass through, and water low at the base rather than over the leaves. If your summers run humid, the Sapphire form handles rust much more easily than the plain species, so do not feel discouraged.
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
If the mound splays open and shifts from steel-blue toward green, it is simply asking for lean soil and full sun. Shade and rich, well-watered ground coax out soft, lazy growth. Shift it to a sunnier spot, ease off the feed and water, and shear it down so it comes back tighter and bluer, an easy fix that almost always works.
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
You can relax about this one: blue oat grass is safe around pets, livestock, and children. The ASPCA does not list it as poisonous, and there is no toxin in its leaves or roots. The only small note is grass pollen, which may bother anyone prone to hay fever.
Safe for
People · Cats · Dogs · Horses · Rabbits · Livestock · Birds
Cats Safe
Blue oat grass is not poisonous to cats, so a curious nibble does no harm at all. A cat that gulps down a big mouthful of blades might bring it back up, just as with any grass, and that is nothing to worry about.
Dogs Safe
Nothing in this grass will poison a dog. Chewing a lot of it can lead to a little vomiting, but that is simply the roughage doing its thing rather than any toxin.
Horses Safe
It is safe to have around horses, who usually leave the wiry blue blades alone and reach for softer pasture grass instead.
Rabbits Safe
Rabbits can graze it without any risk, though most would rather have tender greens than these stiff, coarse leaves.
Livestock Safe
Cattle, goats, and sheep are in no danger of poisoning from blue oat grass. It is an ornamental clump rather than a forage crop, so they get very little real feed from it.
Birds Safe
It is harmless to birds, which often tug thin strands from the clump to help build their nests in spring.
People Safe
The plant is non-poisonous and safe to handle, so you can work around it freely. The only thing to keep in mind is that grass pollen can stir up hay fever in anyone 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
Blue oat grass comes in only a couple of named forms, but if your summers turn humid, knowing the difference between the species and its best selection is genuinely worth it.
Saphirsprudel (Sapphire)
A German selection that holds a deeper blue than the species and copes far better with rust, heat, and humid summers, which makes it a comforting choice if your garden gets sticky.
Species (Helictotrichon sempervirens)
The plain species builds a steel-blue mound of 18 to 30 inches (46 to 76 cm) crowned in summer by oat-like, straw-colored panicles that arch gracefully over the leaves.
How to propagate
Want a second plant? You can split an old clump or sow seed, and either way is doable, though the grass is slow to bulk up and would rather not be disturbed.
DivisionModerate
Spring · One growing season
Since blue oat grass would rather not be dug up, only divide when an old clump begins to thin or you simply want a second plant. Do it in spring, and the splits will settle in and fill out over a single growing season, so be patient and they will come along nicely.
SeedModerate
Late winter to spring · One to two growing seasons
You can also raise the species from seed sown in late winter or spring, with a young plant taking one to two seasons to reach full size. Named selections like Saphirsprudel will not come true from seed, so if you want a faithful copy, dividing is the kinder, surer route.
Myths
Blue oat grass and blue fescue are the same plant.
It is easy to mix them up, but they share only the blue color. Blue oat grass is the larger and longer-lived of the pair, building an 18 to 30 inch (46 to 76 cm) clump topped by arching, oat-like flower stems. Blue fescue stays a little 8 to 12 inch (20 to 30 cm) tuft that hollows out in the middle and needs splitting every couple of years to look good, so do not expect the two to behave alike.