The Mediterranean herb rosemary (Rosmarinus officinalis) brings a spicy, piney flavor and aroma to roasted meats and many other dishes. Generally hardy in U.S. Department of Agriculture plant hardiness zones 6 through 9, with some varieties hardy into zone 10, rosemary can be propagated from cuttings. Growers differ over whether water or soil makes the best rooting medium, and estimates of the time needed for rooting vary.
Estimates from experienced gardeners and professional growers of the time it takes rosemary cuttings to root range from less than two weeks to a month or more. The method you use to root the cuttings -- in plain water, in water and rooting hormone or in potting soil -- seems to be only one factor in estimating time. The growing cycle of the source plant, available light, humidity and indoor temperature can all have an impact on rooting. In general, allow a maximum period of about four weeks to determine which cuttings will grow roots and which will not. More time may be needed for rooted cuttings to establish the strong and extended root system necessary to support new plants. In areas where winters are severe -- USDA zone 5 and colder -- bring spring cuttings in for the winter and transplant them to large containers for outdoor growth the following spring.
Using the Growth Cycle
Cuttings root most successfully when taken from plants during a period of vigorous growth. Take spring cuttings when the plant is completely out of possible winter dormancy and is actively sending out new growth. Make cuttings 3 to 4 inches long, cutting the stem just below a leaf node. Remove leaves from the bottom half of the stem. Removing leaves and the resulting scarring of the outer stem surface can redirect residual growing energy into root formation. Herb grower Adam Gilpin says cuttings can be made in summer and fall as well. Summer and fall stems are more mature, fall stems to the point of woodiness, so you should take longer cuttings and expect to wait longer for rooting results.
Placing new cuttings in a plastic bag is not just a way to keep track of them until you're ready to plant. Live tissues need to be kept moist, although not soaking wet, until they are placed in planting media. Consistent moisture throughout the rooting process is critical to healthy root formation. In very dry climates, you may find it useful to spray cuttings daily with a mister. You can also place several short sticks in the rooting cup or pot and slip a plastic bag over the sticks to create a humidity-enhancing mini-greenhouse.
Although some gardeners report good results with rosemary cuttings on sunny windowsills, excessive heat can stress plants and strong sun can burn delicate new sprouts. Your cuttings will get adequate light to grow if placed in less stressful bright, indirect light or in filtered sunlight.
Although rosemary flourishes in warm, sunny Mediterranean conditions, tender cuttings do best in the moderate mid-60s to mid-70s Fahrenheit characteristic of indoor spaces.