In 2000, rosemary was selected as Herb of the Year by the International Herb Association, and it's easy to see why. This aromatic evergreen is an indispensable kitchen herb, it's used as an ornamental element in the garden, and it is used in aromatherapy.
Rosemary is a member of the Labiatae or mint family, and it grows as an evergreen perennial shrub in mild-wintered regions of the world. Its Latin name, Rosmarinus officinalis, means "dew of the sea," a reference to its Mediterranean roots.
But I don't live in anything like a Mediterranean climate. Here in Minnesota, conditions are less than ideal for growing rosemary, but I don't let this deter me. Still, growing rosemary indoors in the winter can be a challenge. It is easy to nurture and care for indoor rosemary too much. Excess water will damage the roots and cause the plant to die, so I let the soil dry, then water thoroughly. Rosemary needs a southern exposure, and my kitchen window is perfect for this.
When I was first learning about rosemary, it struck me that there is a great deal of variability within the genus Rosmarinus. The many cultivars offer diverse plant shapes and flower color, as well as a range of foliage color and subtly different flavors (both leaves and flowers are edible).
Propagate by cuttings
Most rosemaries are cultivars or clones, which are propagated by cuttings. Growing rosemary from seed typically results in low germination and excessive plant variation. With cuttings, the plants are always identical to the stock plant. Rosemary is easy to propagate, and sometimes roots will develop even in a glass of water on a sunny windowsill. I have found that the best time of year to take cuttings is in the late fall and early winter.
To take cuttings, clip 2-1/2-inch stems from new growth on an established plant (see Propagating rosemary). Snip off the bottom leaves (rather than pulling them off) and dip the bottom 1/4 inch into a hormone rooting powder. Place the cuttings in a container with equal amounts of peat moss and perlite. Spray the cuttings with a light mist on sunny days.
Cuttings usually root in 14 to 21 days, though bottom heat will speed the rooting process. Once the cuttings have rooted, you can transplant them into 3-1/2-inch pots. Pinch the top terminal bud to encourage branching.
Good drainage and full sun keep rosemary thriving
Rosemary is easily grown in a garden with full sunlight (six to eight hours a day) and good drainage; these are the essential requirements. Well-rotted manure added to the garden soil will encourage new growth, but it's not usually necessary.
Because good drainage is so important, lighten up heavy soils prior to planting. When growing rosemary in the ground in southern climates, mulch around the plant to keep the roots cool. Opt for a fast-draining mulch like white sand. If you have mature rosemary plants that have been in the ground for many seasons, you can give them a good feeding in the springtime with a kelp-based liquid fertilizer.
Here in the North, I recommend growing rosemary in a clay pot during the summer and bringing it indoors for the winter. As a matter of fact, in Zones 6 or colder, you must overwinter rosemary indoors. In Minnesota, I have seen rosemary survive to about 25ºF without any sign of damage, but I would bring plants in before temperatures get that low.
Bringing rosemary indoors
In the fall, when the temperature dips to 30ºF, it's time to bring rosemary indoors. Successfully growing rosemary indoors requires good sunlight—the more the better — and ideally a southern exposure. If the plant is large, rotate it weekly so all sides of the plant receive sunlight. Wiry growth often indicates inadequate light, and if you can't increase natural light, consider using artificial light. You can also prune plants to encourage bushiness. Indoor plants sometimes develop powdery mildew because of lack of air circulation. If this occurs, run a small (3-inch) fan for three to four hours a day. I've noticed that the creeping varieties grow and tolerate dry interior air better than the upright varieties.
When rosemary is planted outdoors, insects usually aren't a problem. But in the house, aphids and spider mites are more likely to cause trouble. When this happens, spray rosemary regularly with an insecticidal soap until the plant is again healthy.
Rosemary grows best indoors at cool temperatures, preferably around 60ºF. Increased humidity is not recommended because it promotes powdery mildew.
Container gardening is a cinch
Rosemary thrives outdoors in pots, especially in an easterly spot where it gets full sun until midday. If you have all-day sun, make sure the container you're using is large enough that it won't dry out during the day. A good potting soil consists of one part sterilized soil, one part peat moss, and one part perlite. This mix should provide ample drainage. A thin layer of gravel in the bottom of the pot also improves drainage, keeping the roots a little drier during rainy spells.
Because rosemary can be a sculptural plant with an interesting form, I enjoy using it as a specimen. But it also works well planted with other herbs and flowers in containers. I pot it up with curry, society garlic, 'Red Rubin' basil, bay, garden sage, curly parsley, and thyme.
Large pots of rosemary should be transplanted twice a year. When the plant finally gets too big to move to a larger pot, remove it from the pot and shave off about 2 inches of roots and soil from both the outside edge and bottom. When doing this sort of invasive pruning, make sure to cut part of the top of the plant back to compensate for the root pruning. You can then put the plant back into the same pot. Transplanting will stimulate new growth, as will use of an organic fertilizer twice a month.
A sample of rosemary cultivars
My husband and I have been growing rosemaries in both greenhouses and gardens for years, looking for plants with good upright growth, dark-green leaf color, and good flavor and aroma. In the summer of 1999, we introduced 'Shady Acres', named for our herb farm. This rosemary has the characteristics we were looking for. It is easy to propagate from cuttings and grows relatively fast. When the plants are growing on in 3-1/2-inch pots, I pinch the top growth to encourage branching. Soon a sturdy, bushy plant develops. 'Shady Acres' seldom flowers, and the dark green leaves grow up to 1 inch long while keeping close to the branch. It's a great culinary herb with a lot of flavor and a heavy aroma.
Of course, there are endless cultivars of rosemaries, and many can be purchased by mail order. One of the best pink-flowered varieties is 'Majorca Pink'. It has a unique growing habit with long branches that arch downward as they get older.
'Nancy Howard' grows to be a large, airy plant with nearly white flowers appearing at the top of the stems. 'Pinkie' is a dwarf bush rosemary named for its pink flowers. 'Pinkie' has small leaves and tight, compact growth. 'Miss Jessopp's Upright' is a beautiful specimen with light-blue flowers on a multi-stemmed plant. This plant can reach a height of 6 feet or more.
'Huntington Blue' is upright, with open growth and pale-blue flowers. Leaves grow up to 1 inch long on long, arching stems. Last year I acquired a 'Silver Spires' rosemary plant with variegated leaves that range in color from pale yellow to white and green. Occasionally, green shoots emerge, though these should be cut off.
If you're looking for a good candidate for topiaries, try 'Shimmering Stars'. It's a trailing rosemary with pink flower buds that open medium-blue to lavender.
For a rosemary that's ideal to grow in a pot or a planter with other herbs, look for 'Blue Boy'. It grows as a small bush, with clusters of leaves forming at the end of the stems. Its flowers are pale blue on small, 1/2-inch leaves.
Rosmarinus officinalis 'Joyce De Baggio' (also known as 'Golden Rain') grows compactly and sports blue flowers. Its new leaf growth is streaked light yellow, which fades to green in summer. 'Dancing Waters' has dark-blue flowers on a tight, compact plant with rather erratic growth. Leaves are up to 3/4 inch long and flowers are medium blue-violet.
'Herb Cottage' originated at the Cathedral Herb Garden in Washington, D.C. It has dark blue flowers, and its compact, upright growth makes it a very handsome plant. It has also been called 'Foresteri'.
'Severn Sea' grows to be an attractive spreading shrub with an upright habit and branches that arch as they get taller. Its leaves are light green and the flowers a deep blue. It was brought from Somerset, England, by Norman Hadden.
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