Acorus—Growing Japanese Sweet Flag Inside
The Acorus gramineus variegatus, sometimes called the Japanese sweet flag, is not the most obvious plant to keep indoors. It's used as an aquatic plant and frequently grown as a landscape plant around the margins of ponds or in very wet areas. It's even used sometimes as an aquarium plant for indoor aquascaping. Nevertheless, if the plant is kept sufficiently moist, it has attractive, grass-like foliage that forms a nice backdrop.
Because it prefers somewhat cooler, very moist conditions, it's an excellent plant to accompany certain ferns and even ivies. Overall, this is not a terribly difficult plant to grow—despite their water requirements, they are not affected by cold drafts or even low light—but it can be difficult to find a decent specimen in a typical garden center.
Light: Acorus do not like bright light, but do best in bright, non-direct light or partial shade. In the winter, if the plant is looking poor, a bit of morning sun is okay.
Water: These are sometimes grown as fully aquatic plants, and only plants grown in water will flower. Keep the soil continuously, even copiously, moist. They are highly susceptible to drought and their leaf tips will turn brown without enough water.
Soil: A rich, peat-based potting soil is fine. Drainage is not an issue because the plants are acclimated to growing in water.
Fertilizer: Feed year-round with a weak liquid fertilizer, and increase feeding during the summer months when it's actively growing.
Acorus readily repots by division. Older plants form clumps of spreading rhizomes, with tufts of leaves above the soil that, given enough room, will eventually form a durable turf. In the spring, when the plants are beginning to actively grow again, propagate by dividing the rhizome and planting the new plant in a pot. Keep the division warm and very wet for a few weeks, until new growth emerges, then move to its permanent location.
Repot in the spring when new growth emerges. Repotting is a good time to take a division of the rhizome (see Propagation above) and increase your plant stock. Because these are clumping grass-like plants, choose the pot size based on how big you want it to get, as opposed to the needs of the plant.
Acorus gramineus variegatus is the only species offered for sale in the trade. If you can't find one in an indoor garden center, try an outdoor garden center—they are frequently sold as landscape plants. The A. gramineus variegatus features pretty grass-like leaves with a creamy leaf-blade margin and a green stripe down the middle. They somewhat resemble an aquatic liriope plant.
The key to successfully growing Japanese sweet flag is to provide cooler temperatures and plenty of water. In warmer rooms, the plant will grow, but you're much more like to have pest problems, including mites and mealy bugs. These plants can also be used in terrariums, in the lower, wetter parts of the terrarium, where they can be grown fully submerged in 3" to 6" of water. Submerged plants will produce insignificant flowers on erect stems. They are also much less likely to send out rhizomes so tend to stay more compact. This is good news for terrarium gardeners who don't want to trim and divide their overgrown plants in small terrariums. Acorus is vulnerable to typical pests, including spider mites, aphids, and mealy bugs. Treat with insecticidal soap at the first signs of infestation. One nice thing about the plant is its fragrance: crushed leaves produce a mild, sweet cinnamon smell.