Ornamental · Poaceae
Mexican Feather Grass
Nassella tenuissima
Plant it in lean soil and full sun and this airy grass shimmers all season, but keep its seed in check.
Full sun
Every 10–21 days
90–120 days
USDA 7–10
Overview
Mexican feather grass is the grass that moves. Its hair-thin blades catch the slightest breeze and ripple in soft waves, fading from green to a warm wheat color as the season runs on. It forms a tidy, fountain-shaped clump and works as well in a gravel garden as it does threaded through a border.
This is a tough, drought-hardy perennial from the dry country of the southwest and Mexico. Once the roots take hold it asks for next to nothing, shrugging off heat and lean ground that would stall fussier plants. It also goes quiet through the worst of summer, browning off before it greens up again when the weather cools.
The catch is the seed. A single clump sets thousands of wind-borne seeds, and in mild-winter regions like California it has jumped the garden and turned invasive in open grassland. Grown with a little care, snipping the seed heads before they ripen, it stays a graceful, easy plant. Left alone, it spreads where it isn't wanted.
Care
The short version: full sun and sharp drainage in lean, sandy soil. Water it sparingly once it's settled, skip the fertilizer, comb it out each late winter, and cut off the seed heads before they ripen to keep it in bounds.
Light
Give it full sun for the best color and the densest, most upright clump. It will take a little afternoon shade, but too much and the plant flops open and loses its tidy fountain shape.
Water
Once it's established this grass is very drought tolerant and needs water only every 10 to 21 days, and even less in cooler stretches. Soak it occasionally during a long dry spell, but never let it sit in wet ground, which rots the roots.
Soil
Plant it in lean, sharply drained sandy loam and it thrives, and it copes with rocky or clay ground too as long as that ground stays dry. It handles a wide pH range from 5.5 to 8.0, so the soil's acidity is rarely the issue, but its wetness always is.
Temperature
It's hardy across a broad swing, from about 0°F to 100°F (-18°C to 38°C), which covers USDA zones 7 through 10. Hard winters below that range will kill it, so colder gardens grow it as a short-lived seasonal plant.
Humidity
Dry air suits it fine, since this is a plant of arid country. High humidity is no help and can hurt by keeping the crown damp, so give it open space and moving air rather than a sheltered, muggy corner.
Feeding
Skip the fertilizer almost entirely. It thrives in poor soil and a light spring feed is all it ever needs, if that, since rich ground only makes it floppy and pushes even more seed.
Pruning
Comb or cut back the strawy old blades in late winter or early spring before fresh growth starts, and expect it to look ragged through summer dormancy. The one job that matters most is shearing off the seed heads before they ripen to keep the plant from spreading.
Repotting
In a container it rarely needs moving, but lift and divide it when the roots circle the pot, the center dies out, or it dries out fast after every watering. Replant the healthy outer pieces in fresh, gritty mix and discard the spent middle.
Size & spacing
A mature clump reaches about 12 to 24 inches (30 to 61 cm) tall and just as wide. Set plants the same 12 to 24 inches (30 to 61 cm) apart so each fountain has room to arch without crowding its neighbor.
When to plant
Sow the seed indoors in late winter, six to eight weeks before your last frost, and set young plants out after the cold has passed, from spring into early summer. The feathery flower heads arrive from June through September. Cut them off before they ripen if you want to hold the spread down.
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, 6-8 weeks before last frost
- Plant: After last frost, spring through early summer
- Harvest: June to September
Hardiness
USDA hardiness zones: 7–10
Temperature: down to -18°C
Troubleshooting
Most trouble with this grass is either the seeding getting out of hand or the roots sitting too wet. The browning that worries people in midsummer is usually just the plant resting. Match what you see below and you'll know whether to act or leave it be.
Seedlings popping up everywhere, even past the garden edge
This is self-seeding, and it's the plant's biggest drawback. One clump throws thousands of wind-borne seeds that build a long-lived seed bank and drift into lawns, paths, and wild ground. Cut and bag the feathery seed heads before they ripen, dig out young volunteers roots and all, and bin spent plants rather than composting them where the grass runs wild.
Fix:
- Cut off and bag the feathery seed heads before they ripen and shed seed
- Pull or dig volunteer seedlings while young, including the roots
- Remove and dispose of spent plants in the trash, not compost, where invasive
The middle of the clump going brown and mushy
Soggy soil brings on root rot, and this grass has no patience for wet feet. The crown and roots turn soft and dark when drainage is poor or watering runs too heavy. Cut back the water and improve drainage, then lift the clump, save the healthy outer growth, and toss the dead center. An old plant can also just be reaching the end of its short life.
Fix:
- Improve drainage and reduce watering
- Lift and divide healthy outer portions, discarding the dead center
- Replant in sharply drained, lean soil
Strawy, brown foliage that looks dead by midsummer
This is usually just summer semi-dormancy, the plant's normal response to heat, not a sign of trouble. The crown stays alive and fresh green growth returns once the weather cools or you cut it back. Old dead blades left to pile up make it look worse, so comb them out by hand or rake and shear the clump in late winter.
Fix:
- Comb out dead blades by hand or rake
- Cut foliage back in late winter to early spring to refresh growth
Sticky blades, twisted growth, or small leaf spots
Sticky, distorted foliage points to aphids feeding on the blades, while spotting is a sign of leaf spot disease. Blast the aphids off with a strong jet of water and they rarely come back in any number. Pull off badly spotted leaves and open the clump up so air moves through it.
Fix:
- Rinse aphids off with a strong jet of water
- Remove badly spotted foliage and improve air circulation
Toxicity
Good news for pet owners: Mexican feather grass is not toxic to cats, dogs, horses, or any other animal, and it isn't poisonous to people either. The only real nuisances are the bristly seeds, which can tangle in fur or catch in a dog's paws like a foxtail, and the pollen, which can bother hay fever sufferers.
Safe for
People · Cats · Dogs · Horses · Rabbits · Livestock · Birds
Cats Safe
No toxins here, so a curious cat is in no danger. A cat that chews the blades might bring them back up, and the fine seeds can catch in a long coat.
Dogs Safe
Safe for dogs to be around and nibble. The bristly awns can tangle in fur and burrow into paws or ears like a foxtail, so brush them out after a walk through the clump.
Horses Safe
Not poisonous to horses and grazed without harm.
Rabbits Safe
A rabbit can nibble the foliage without any risk of poisoning.
Livestock Safe
Cattle, goats, and sheep can graze it freely, though it offers little real feed value.
Birds Safe
Harmless to birds, which often pick at the ripe seed and help carry it well beyond the garden.
People Safe
No part of the plant is poisonous to people. The grass pollen can set off symptoms in anyone prone to hay fever.
The fine, bristly seeds and awns can cling to and tangle pets' coats (a minor foxtail-type mechanical nuisance), and pets that nibble grass may vomit. Grass pollen may bother allergy-prone people.
Varieties
There isn't much to choose from beyond the straight species, but a couple of names turn up at the nursery, and one group of look-alikes is worth knowing if you want the look without the spread.
Pony Tails
The most widely sold selection, prized for unusually soft, fine foliage that flows in the lightest breeze, though it self-seeds and runs just as freely as the species.
Stipa tenuissima (synonym)
The same plant under its older botanical name, which many nurseries still use, so you will often find it labeled Stipa rather than Nassella.
Native needlegrasses (Nassella / Stipa species)
Native look-alikes such as purple needlegrass give you the same airy texture without the invasive habit, which makes them the better choice where the Mexican species is a problem.
How to propagate
You can multiply this grass two ways, and the choice mostly comes down to whether you mind it seeding. Both are easy.
SeedEasy
Sow indoors late winter to early spring, or it self-sows readily outdoors · One growing season
Seed is the easiest route and the plant does most of the work itself, sprouting all over once it sets seed. Start it indoors in late winter to early spring, or let it self-sow outside, and a seedling fills out into a proper clump within one growing season. That same eagerness is exactly why it spreads where you don't want it, so keep the seeding in check where it's a problem.
DivisionEasy
Spring · One growing season
Division is the clean way to make more without scattering seed. Lift an established clump in spring, pull it apart, and throw out any dead center, then replant the living outer pieces and they settle in over a single season.
Myths
Mexican feather grass is a harmless, well-behaved ornamental everywhere.
It self-seeds hard and is listed as invasive in California and other mild-winter regions, where it jumps the garden fence and crowds out native grass in open country. Some areas now discourage or outright restrict planting it. Cut the seed heads off before they ripen, and where it runs wild, reach for a native bunchgrass instead.