Ornamental · Acoraceae
Sweet Flag
Acorus gramineus
Give it wet ground and a bit of sun, and sweet flag keeps a tidy grassy fan going all season.
Full sun
Every 1–3 days
USDA 5–11
0–4 ft · 0.2–1.2 m
Overview
Sweet flag is that aromatic, grassy-looking perennial you grow for its upright fans of leaves, not for any flowers worth mentioning. It's an Acorus, and here's the fun part: the genus is old enough to have a whole family to itself, Acoraceae, so it isn't really a grass or an iris no matter how much it looks like one. Crush a leaf and you'll get a warm, spicy smell, and that's exactly where the name comes from.
So there are actually two plants going by this name. The little Acorus gramineus stays short at 6 to 12 inches and gives you the gold and cream forms, while the bigger common sweet flag, Acorus calamus, gets up to a couple of feet with wide, iris-like blades. What do they have in common? Wet feet. That's why either one is right at home at a pond edge, in a bog, in a rain garden, or in a pot sitting in a saucer of water.
Honestly, all sweet flag wants is steady moisture and a little patience. It spreads out slowly on creeping rhizomes and won't ask much of you beyond water. Let the soil dry out, though, and the leaf tips scorch and brown faster than just about anything else, so that's the one thing to stay on top of.
Care
The short version: full sun to part shade, soil kept constantly moist to wet (or even a bit of shallow standing water), and never let it dry out. Feed it lightly in spring, shear off the tatty leaves in late winter, and that's really it.
Light
Sweet flag likes full sun best, but it's happy enough in partial sun and partial shade too, so don't stress about the perfect spot. The gold and variegated forms show off their color with more light, though where summers get hot, a bit of afternoon shade keeps them from scorching.
Water
This one's a marginal bog plant, so keep the soil constantly moist to wet all the time, or grow it in a few inches of standing water at a pond or stream edge. Acorus calamus doesn't mind deeper water than the little gramineus does. And whatever you do, don't let it dry out, because that's the number-one way to lose it and it browns the leaves in a hurry.
Soil
It grows in rich soil that stays moist to wet, and it's not fussy about texture at all, taking clay, loam, sand, or silt and shrugging off frequent standing water. Shoot for a pH between 5.5 and 7.5, which is comfortably on the mild side of acidic to neutral, so most soil will do.
Temperature
Sweet flag is a tough customer, hardy from about -20°F to 95°F (-29°C to 35°C). That's a big range, and it's the cold end that gets it through winter up in zone 5 without any drama.
Humidity
No need to fuss with humidity here, since ordinary outdoor air is plenty and there's no number to hit. What it actually cares about is wet roots, not damp air, so that's where to put your attention.
Feeding
This is a light feeder that gets by on next to nothing when the soil's rich and moist. A little balanced slow-release fertilizer in spring is all it really wants, and even that's optional, so skipping a year now and then is no big deal.
Pruning
Pruning couldn't be easier, because sweet flag is very low maintenance and doesn't need any routine trimming. Just shear or snip off the tatty, scorched, or winter-damaged leaves in late winter or early spring, and the clump tidies itself back up.
Repotting
If it's in a pot, plan to divide or move it up every 2 to 3 years. You'll know it's time when the roots start crowding the pot, the crown lifts up above the soil, or the middle of the clump looks like it's thinning out.
Size & spacing
Depending on which sweet flag you've got, plants run from 6 to 47 inches (15 to 120 cm) tall, from the teeny gramineus dwarfs to full-size calamus. Set them 6 to 24 inches (15 to 60 cm) apart, and go a little tighter if you want groundcover to fill in quicker.
When to plant
Good news on timing: sweet flag is hardy from zone 5 to 11, so there's not much to overthink. Pop your divisions in during spring once the frost's gone, into ground that stays wet, and the greenish flower spikes turn up in May and June. Since it's all about the foliage, you might not even notice them, and that's fine.
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
Here's the thing about sweet flag: nearly every problem comes down to water. Keep it wet and it pretty much runs itself, but let it dry and the leaves will tell on you fast. Have a look at what's below and match it up before the whole clump starts to struggle.
Brown leaf tips and a clump that's scorching out
Since sweet flag is really a bog and waterside plant, dry soil is almost always the culprit when a clump browns and withers. The gold 'Ogon' and the variegated forms are the first to scorch in a hot, open spot. Move it somewhere the ground stays wet, or stand the pot in a saucer of water, then trim off the worst leaves so fresh growth can come back.
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
Most of the time this is just the plant's slow nature, not anything going wrong. Sweet flag creeps out on rhizomes over several seasons, and the dwarf gold cultivars are the slowest of the lot. Keep the soil wet and the light bright to nudge it along, and if you want cover sooner, set divisions about 8 to 12 inches apart.
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 wet ground nearby
In soil that stays wet, the bigger common sweet flag (Acorus calamus) creeps outward and can turn into big colonies. The dwarf Acorus gramineus is way more polite about it. Lift and divide the edges in spring to keep things in bounds, or grow that pushy calamus in a submerged pot so the rhizomes can't take off.
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
Good news for pet and kid households: sweet flag is safe to grow and handle, and it isn't on the toxic-plant lists. The one thing to keep an eye on is the rhizome, which has beta-asarone in it, a mild toxin and possible carcinogen. The leaves aren't a worry, so just leave the roots in the ground instead of letting anyone snack on them.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Mild
The foliage isn't really a worry, but the aromatic rhizome has beta-asarone in it, so a cat that chews into the root might drool or get a quick stomach upset. Keeping the roots out of reach is an easy call.
Dogs Mild
A dog sniffing around the leaves? Totally fine. Chewing the pungent rhizome is another story, since it can bring on mild drooling or loose stools, though the foliage itself is nothing to worry about.
Horses Mild
Grazing the leaves is low risk, but the beta-asarone in the rhizome makes the roots worth fencing off from a nosy horse.
Rabbits Mild
Rabbits can nibble the grassy leaves no problem, but the aromatic rhizome has beta-asarone in it, so it's best kept away from them.
Livestock Mild
Cattle, sheep, and goats usually steer clear of this bitter, aromatic plant on their own, and the only real thing to watch is the rhizome, not the leaves.
Birds Mild
Birds hardly ever mess with sweet flag, but the beta-asarone in the roots means the rhizome shouldn't be on the menu.
People Caution
The plant's safe to grow and handle, but don't eat the rhizome. Even though it was once used as calamus flavoring, it's got beta-asarone in it, which the US FDA keeps out of 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'll run across are dwarf cultivars grown for their leaf color, plus the bigger common sweet flag. Here are the ones worth knowing by name.
Acorus gramineus 'Ogon'
A dwarf Japanese sweet flag with bright gold, yellow-and-green striped fans about 6 to 12 inches tall, and it shows its best color in more sun while still appreciating a little afternoon shade where summers run hot.
Acorus gramineus 'Variegatus'
A dwarf form striped cream and green that people love as a fine-textured groundcover and as an accent at a pond edge or in a container.
Acorus gramineus 'Minimus Aureus'
A tiny golden dwarf just a few inches tall, and it's made for small containers and miniature or fairy plantings.
Acorus calamus 'Variegatus'
The big common sweet flag at 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
Want more sweet flag? Divide the clump. It's the only dependable way to keep those named color forms coming back true.
DivisionEasy
Spring (or early summer) · A few weeks to a season
Dividing the clump is the go-to way to make more sweet flag, and it's an easy job to boot. In spring or early summer, lift the rhizomatous clumps, pull them apart, and keep the pieces constantly moist or in shallow water while they settle in over the next few weeks to a season. Just remember that named forms like 'Ogon' and 'Variegatus' have to be divided rather than grown from seed if you want them to come back true.
Myths
Sweet flag is a real grass or some kind of iris.
Nope, it's neither. Sweet flag is Acorus, off in its own family Acoraceae, and it's grown for those aromatic, sword-like fans of leaves. The grassy, iris-like look is just a resemblance, not a family connection.