Ornamental · Acoraceae
Sweet Flag
Acorus gramineus
Keep the ground reliably wet and give it a little sun, and sweet flag rewards you with a neat grassy fan all season long.
Full sun
Every 1–3 days
USDA 5–11
0–4 ft · 0.2–1.2 m
Overview
Sweet flag is a gentle, grass-like perennial you grow for its upright fans of aromatic leaves, not for any big show of flowers, so there is nothing tricky to coax into bloom. It belongs to Acorus, a genus old enough to keep a family all its own, Acoraceae, with neither the true grasses nor the irises among its close kin. Crush a leaf between your fingers and a warm, spicy scent lifts off it, and that welcoming fragrance is just how the plant came by its name.
Two different plants answer to this name, and both are easygoing. The dwarf Acorus gramineus keeps to a modest 6 to 12 inches and gives you the pretty gold and cream forms, while the larger common sweet flag, Acorus calamus, climbs to a couple of feet with broad, iris-like blades. What they share is a love of wet feet, which makes either one a happy fit for a pond edge, a bog, a rain garden, or a pot left to stand in a saucer of water.
All sweet flag really wants from you is steady moisture and a bit of patience. It widens slowly by creeping rhizomes and asks for little beyond regular water, so it is a forgiving plant to start with. The one thing to watch is dryness, since letting the soil go dry makes the leaf tips scorch and brown quicker than almost anything else you might put right.
Care
The short version: give it full sun to part shade in soil kept constantly moist to wet, or even a little shallow standing water, and never let it dry out. A light feed in spring and a shear of tatty leaves in late winter are all the upkeep it needs, and both are easy to get right.
Light
Sweet flag is happiest in full sun, though it takes partial sun and partial shade in good stride, so a less-than-perfect spot is rarely a problem. The gold and variegated forms keep their color brightest with more light, and a little afternoon shade helps them avoid scorching where summers turn hot.
Water
Think of this as a marginal bog plant and you will not go wrong: keep the soil constantly moist to wet every day, or grow it in a few inches of standing water at a pond or stream edge. The larger Acorus calamus is glad of deeper water than the dwarf gramineus. Drying out is the one mistake that really sets it back, browning the foliage in a hurry, so lean toward too wet rather than too dry.
Soil
Sweet flag grows in rich soil that stays consistently moist to wet, and it is not at all picky about texture, settling happily into clay, loam, sand, or silt and putting up with frequent standing water. Aim for a pH between 5.5 and 7.5, which sits comfortably from mildly acidic to neutral and is easy to hit.
Temperature
This is a tough plant across a wide span, hardy from about -20°F to 95°F (-29°C to 35°C). That deep cold-hardiness is exactly what carries it safely through winter as far north as zone 5, so a hard freeze is nothing to fear.
Humidity
Ordinary outdoor humidity is plenty here, so there is no target number to chase and nothing to measure. What sweet flag truly cares about is wet roots, not damp air, which makes it a relaxing plant to keep.
Feeding
Sweet flag is a light feeder that gets along on very little when the soil is rich and moist. A single gentle dose of balanced slow-release fertilizer in spring is all it asks for, and even that is optional, so skipping it now and then does no harm.
Pruning
Pruning could hardly be simpler, since sweet flag is very low maintenance and needs no routine cutting back. Shear or trim away any tatty, scorched, or winter-worn leaves in late winter or early spring, and the clump freshens itself up on its own.
Repotting
Container plants like a division or a move up every 2 to 3 years, which is an easy rhythm to keep. Lift and split the clump when you notice roots crowding the pot, the crown lifting above the soil, or the middle of the clump starting to thin.
Size & spacing
Depending on which sweet flag you grow, plants stand anywhere from 6 to 47 inches (15 to 120 cm) tall, from the tiny gramineus dwarfs up to full-size calamus. Space them 6 to 24 inches (15 to 60 cm) apart, tucking them a touch closer if you want quicker groundcover.
When to plant
Sweet flag is hardy all the way from zone 5 to 11, so timing is nothing to lose sleep over. Set your divisions out in spring once the frost has passed, into ground that stays wet, and the small greenish flower spikes will follow in May and June. Since the plant is grown for its foliage, it is perfectly fine if you never even notice those blooms.
Northern hemisphere
Southern hemisphere
Typical months for a temperate climate: start earlier in warm zones, later where spring frosts linger.
- Sow: not typically grown from seed; propagate by division instead
- Plant: spring after frost, into constantly moist or wet ground or shallow water
- Harvest: spring to early summer (May-June); flowers are inconspicuous greenish-yellow spadixes and the plant is grown for foliage, not bloom
Hardiness
USDA hardiness zones: 5–11
Temperature: down to -29°C
Troubleshooting
Almost everything that troubles sweet flag comes back to a single thing, and that is water. Keep it wet and the plant largely takes care of itself, but let it dry and the leaves will let you know quickly. Match what you are seeing to the cases below and you can step in before the whole clump is affected.
Leaf tips turning brown as the clump scorches out
Sweet flag is a bog and waterside plant at heart, so dry soil is nearly always why a clump browns and withers back. The gold 'Ogon' and the variegated forms are the first to scorch in a hot, open spot. Move it to where the ground stays wet, or set the pot in a saucer of water, and trim off the worst leaves so fresh new growth can come through, since recovery is very likely.
Fix:
- Move the plant to constantly moist or wet ground, a bog, pond/stream edge, or stand the container in a saucer of water so the soil never dries
- Water deeply and keep the soil saturated; mulch to retain moisture
- Trim off badly scorched foliage; new growth recovers once moisture is restored
- In hot climates give gold and variegated forms some afternoon shade or filtered sun to reduce scorch
A clump that stays small and fills in slowly
More often than not this is simply the plant's unhurried nature and not a sign that anything is wrong. Sweet flag widens by rhizomes over several seasons, and the dwarf gold cultivars are the slowest of the bunch. Keep the soil wet and the light bright to encourage what growth it will make, and set divisions about 8 to 12 inches apart if you would like cover to come sooner.
Fix:
- Be patient: allow clumps to expand over several seasons
- Keep soil consistently wet and provide adequate light to maximize growth
- Plant closer together (about 8-12 in apart) for faster groundcover coverage
Creeping rhizomes wandering into the wet ground nearby
In soil that stays wet, the bigger common sweet flag (Acorus calamus) creeps steadily outward and can build up sizable colonies. The dwarf Acorus gramineus is far better behaved and stays put. Lift and divide the edges in spring to keep the clump within bounds, or grow the vigorous calamus in a submerged pot so its rhizomes have nowhere to run.
Fix:
- Divide and lift edges in spring to keep the clump in bounds
- Grow in a submerged container or pot to contain the rhizomes, particularly for Acorus calamus
- Install an edging barrier in bog or pond plantings
Toxicity
Sweet flag is safe to grow and handle around both pets and people, and it does not appear on the toxic-plant lists. The single thing to be mindful of is the rhizome, which holds beta-asarone, a mild toxin and possible carcinogen. The leaves are no cause for worry, so the simple rule is to leave the roots in the ground rather than let anyone eat them.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Mild
The foliage is not a real worry, but the aromatic rhizome carries beta-asarone, and a cat that chews into it might drool a little or have a brief stomach upset. Keeping the roots out of reach is an easy way to stay on the safe side.
Dogs Mild
A dog sniffing at the leaves is perfectly fine, though chewing the pungent rhizome can bring on mild drooling or loose stools. The foliage itself gives no cause for concern.
Horses Mild
Grazing the leaves poses little risk, but the beta-asarone in the rhizome makes the roots worth fencing off from a curious horse.
Rabbits Mild
Rabbits can nibble the grassy foliage without trouble, yet the aromatic rhizome holds beta-asarone and is best kept out of their reach.
Livestock Mild
Cattle, sheep, and goats usually leave this bitter, aromatic plant alone, and the only real caution is the rhizome rather than the leaves.
Birds Mild
Birds seldom take any interest in sweet flag, but the beta-asarone in the roots means the rhizome is not something they should be eating.
People Caution
The plant is safe to grow and to handle, but do not eat the rhizome. For all its old use as calamus flavoring, it holds beta-asarone, which the US FDA bars from food as a possible carcinogen.
Safe as an ornamental around pets. Note, however, that the rhizome of the larger common/Eurasian sweet flag (Acorus calamus), especially diploid/triploid forms, contains beta-asarone, a compound the US FDA has banned as a food or flavoring additive (21 CFR 189.110) because of potential carcinogenicity. Although the rhizome was used historically as a medicine and flavoring (calamus), it should NOT be eaten or used as a calamus herbal remedy. This does not affect the plant's safety as a non-toxic ornamental for pets.
Helpline numbers
United States
ASPCA Animal Poison Control: (888) 426-4435 (fee applies)
Poison Control: (800) 222-1222
Canada
Poison Centres: 1-844-764-7669
Varieties
Most of the Acorus you are likely to find are dwarf cultivars picked for their leaf color, along with the bigger common sweet flag. These are the few worth learning by name.
Acorus gramineus 'Ogon'
A dwarf Japanese sweet flag whose bright gold, yellow-and-green striped fans reach about 6 to 12 inches tall, showing off their richest color in more sun while still welcoming a little afternoon shade where summers run hot.
Acorus gramineus 'Variegatus'
A dwarf form striped in cream and green that gardeners love as a fine-textured groundcover and as a gentle accent at a pond edge or in a container.
Acorus gramineus 'Minimus Aureus'
A tiny golden dwarf just a few inches tall, perfectly suited to small containers and to miniature or fairy plantings.
Acorus calamus 'Variegatus'
The large common sweet flag, standing around 2 to 2.5 feet tall, with broad iris-like leaves boldly striped cream and green on a sterile triploid plant.
How to propagate
You make more sweet flag by dividing the clump, and that is the only dependable way to keep the named color forms coming back true.
DivisionEasy
Spring (or early summer) · A few weeks to a season
Dividing the clump is the standard, dependable way to make more sweet flag, and it is a genuinely easy job to take on. In spring or early summer, lift the rhizomatous clumps, gently pull them apart, and keep the pieces constantly moist or sitting in shallow water while they settle in over the next few weeks to a season. Named forms like 'Ogon' and 'Variegatus' have to be divided rather than raised from seed if you want them to come back true.
Myths
Sweet flag is really a grass or some kind of iris.
It is honestly neither one. Sweet flag is Acorus, standing on its own in the family Acoraceae, and it is grown for those aromatic, sword-shaped fans of leaves. The grassy, iris-like look is only a resemblance, not a family tie.