Acorus gramineus is a wetland perennial that is also known as Japanese sweet flag, Japanese rush, grassy-leaved sweet flag and dwarf sweet flag. The source of calamus oil, this plant was used medicinally and in perfumes and soaps because of its cinnamon fragrance. It produces this smell when the leaves are bruised or crushed under foot.
Japanese sweet flag has semi-evergreen, long (up to 14"), narrow, grassy foliage. Its half inch glossy leaves are flat and look like thick, lush wet grass. Sweet flag is unusual in that it produces yellow horn shaped flowers on hollow stems in midsummer.
Japanese sweet flag is native to wetlands of eastern Asia, and can be grown in similar environments in Zones 6-9 in the United States. At the very least, it requires very moist soil. It thrives in wetlands, like along ponds or pools, and can even grow when submersed in up to 4" of water. If you grow it out of its natural aquatic element, it must be watered frequently. If the soil dries for even a day the leaves will suffer. Given this prerequisite, it also needs full sun to partial shade.
If you want to plant a year old sweet flag from a seed or rhizome, it is best to let it mature in a container for a month. Container grown plants can be set in their permanent positions any time you have a shovel handy, but if you plant in the summer, you must be comitted to watering as needed until cooler, rainy weather arrives. Mature plants should be divided every few years in early autumn or late spring. They’ll do fine through winter months without mulch.
Sweet flag may seem like the ideal plant for your Koi pond, but be aware that it spreads aggressively via rhizomes. Eventually it will establish a beautiful seamless turf that may be better for the front of a bog garden than an ornamental pool. If you don’t want yards of sweet flag, grow it in submerged containers so it can't spread. However, even container bound plants will need to be divided eventually.
Japanese sweet flag doesn’t attract any pests, but desperate deer might eat them. It is not prone to any diseases. It can become home to small wetland wildlife.
There are several varieties of Japanese sweet grass, but the two I’m most asked about are 'Ogon' and 'Variegatus'.
The cultivar 'Ogon' is also called Golden Japanese sweet flag because its green foliage is variegated with cream and bright light green stripes. Look for more golden colors if planted in the sun and greener colors in the shade. With a quarter of an inch wide and 10" long arching leaves, it is smaller than other cultivars. It is evergreen in warmer winter climates and produces unremarkable small yellow flowers and red fleshy berries. 'Ogon' will slowly spread about a foot and you should plant them at least 10" apart. Performs best in Zones 7-10.
'Variegatus' is also called variegated Japanese sweet flag. It is semi-evergreen, with green and white striped quarter inch wide leaf blades that can grow 12" long. Plant 'Variegatus' 12" apart. It produces small green blooms in early summer that later become tiny red berries and grows best in Zones 6-9.
In addition to being ideal in wet or submerged areas, Acorus gramineus delights the nose and the eyes. Its beautiful sweet smelling foliage provides year round color and blossoms and berries in the summer. It is easy on your schedule too, as it requires infrequent maintenance and has no common pests or diseases. This simple and rewarding plant will be perfect in or around your water feature.
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