Sweet flag is a genus of grass-like, low-maintenance perennials. It grows well in moist soil or several inches of standing water, making it an excellent choice when used as an accent plant in water gardens or moist, marshy areas along shorelines. Sweet flag spreads slowly over time via rhizomes and forms a dense groundcover. The foliage of sweet flag has a light, sweet scent when crushed.
The most widely known sweet flag species is Acorus calamus, which is native to Asia and Europe and was introduced into North America by settlers in the 1600s. It has naturalized over time throughout much of the eastern and central United States, but it is not considered invasive.
Sweet Flag Overview
Where to Plant Sweet Flag
Sweet flag can be planted in full sun to partial shade where the soil is consistently moist to wet. At the edge of a pond or creek or boggy conditions with shallow standing water (up to 4 inches) are ideal. Sweet flag is happy with slightly acidic to neutral soil. You can also plant it directly in a pond or other standing water but place it in a container, which confines the root growth.
Other good landscape uses of sweet flag are damp or wet hillsides, streambanks, or slopes, which can be stabilized with mass plantings of sweet flag and thus help with erosion control.
How and When to Plant Sweet Flag
Sweet flag can be planted in the spring or in the summer. Dig a hole that is about 1.5 times as wide as the root ball and about as deep. Set the plant in the hole and backfill it with original soil. Water it well after planting.
Space plants 1 foot apart.
Sweet Flag Care Tips
Sweet flag requires little care if it’s planted in a location where it naturally gets a lot of moisture. In drier locations, you’ll need to keep up with watering.
Light
While sweet flag can grow in full sun, this also means that it needs more water because the soil dries out faster. If you planted it in a very wet location, or in standing water, full sun is fine.
Soil and Water
As long as the soil is consistently moist, sweet flag is not choosy in terms of soil; it can even grow in heavy clay but prefers fertile soil with a pH between 6.5 and 7.5.
Temperature and Humidity
The hardiness of Acorus varies slightly with the species. American sweet flag is the hardiest, it can be grown down to Zone 3. When grown in a warm to hot climate, a location in partial shade is recommended to protect it from the scorching afternoon sun.
High humidity is not an issue for the plant, but dry conditions are not suitable.
Fertilizer
Add a slow-release fertilizer in the spring at the beginning of the growing season and again a couple of times in the summer. Stop fertilizing in the late summer because new leaf growth is especially vulnerable to frost.
Pruning
There is usually no need to prune sweet flag. If it looks untidy, cut it back slightly just like ornamental grass.
Potting and Repotting Sweet Flag
Planting sweet flag in pots is a good idea to keep it in check but the container needs to be immersed in standing water or boggy soil to provide the consistent moisture the plant needs.
Plant it in a container that is about 4 inches larger than the current root ball and has large drainage holes. Fill the container with a layer of gravel, followed by rich, fertile soil or potting mix. Sink the container into its permanent location.
Repot the plant in a larger pot when the roots have reached the sides of the pot or grow out of the drainage holes.
Pests and Problems
Sweet flag has no serious insect or disease issues and it is deer-resistant.
Leaf scorch is a sign that the plant is not getting sufficient moisture. The edges of the leaves can turn brown in the winter, but the plants usually recover in the spring.
How to Propagate Sweet Flag
Sweet flag is best propagated by division of a large mature clump that is at least 2 to 4 years old. In the early fall or early spring before the new growth starts, dig up the entire clump with a shovel. Separate it into sections of 2 to 4 inches each and plant them in suitable moist to wet locations about 4 to 6 inches deep and 1 foot apart.
Types of Sweet Flag
Variegated Sweet Flag
Acorus calamus ‘Variegatus’ is a cultivar with variegated leaves that have white and green stripes. The plant grows 2 to 2.5 feet tall and spreads 1.5 to 2 feet. Zone 4-10
Grassy-Leaved Sweet Flag
Acorus gramineus is a semi-evergreen sweet flag species native to Asia. It has narrow, grass-like leaf blades and grows only 6 to 12 inches tall. ‘Ogon’ is a popular cultivar. Zone 5-9
American Sweet Flag
Acorus americanus, the American sweet flag, is a hardy wetland plant that is native to the northern United States and Canada where it grows in swamps and bogs. It grows 2 to 3 feet tall and wide. Zone 3-6
Sweet Flag Companion Plants
Louisiana Iris
Louisiana irises are native to areas of North America. Many hybrids have been developed, but one of the most elegant is the wild, red-flowered species that flourishes beside ponds. Zone 5-9
Pitcher Plant
Pitcher plant is a carnivorous, waterside plant with tube-shaped leaves and fascinating springtime red, purple, yellow, or green flowers. Zone 2-11, depending on type
Cardinal Flower
Cardinal flower, an excellent late-summer bloomer, is a perennial that will thrive at the edge of a water garden or fountain. The bright red flowers attract hummingbirds. Zone 3-9
Frequently Asked Questions
- Does sweet flag have flowers?It does flower from spring to early summer, but the yellow-green or greenish-yellow flower spikes are tiny and insignificant, and they typically are only developed when the plant is grown in water.
It does flower from spring to early summer, but the yellow-green or greenish-yellow flower spikes are tiny and insignificant, and they typically are only developed when the plant is grown in water.
- What does sweet flag smell like?When crushed. sweet flag leaves smell spicy or with citrusy overtones of lemon or tangerine. The roots of Acorus calamus smell of cinnamon when crushed.
When crushed. sweet flag leaves smell spicy or with citrusy overtones of lemon or tangerine. The roots of Acorus calamus smell of cinnamon when crushed.