Growers might find an alternative in growing rosemary as a potted crop for December, linking Christmas tradition with one of the latest garden trends – herbs.
Rosemary is an herb that has multiple uses: the leaves, either fresh or dried are used for culinary flavoring; the aromatic oil distilled from fresh flowering shoots is used for perfume and in medicine; and in warmer climates the plant is grown as an ornamental evergreen shrub in landscapes. In addition, rosemary makes an ideal potted plant to be enjoyed year-round.
In Europe, rosemary boughs have been used for hundreds of years in Christmas decorations, along with the traditional mistletoe and holly. It is often used as a decorative centerpiece or as a tabletop tree. The narrow leathery leaves are aromatic, reminiscent of pine or fir needles. As a potted plant, it can be grown in a variety of shapes, from "Christmas trees" to topiary standards and living wreaths.
Seed: While rosemary can be grown from seed, the process is not recommended for pot plant production. Seed germination is sometimes slow and erratic, requiring 12 to 28 days or more. Optimum soil temperature for germination is 60 degrees F. Light is not necessary for germination. Crop time from seeding to sale in 4-inch pots is 16-18 weeks.
Cuttings: Asexually propagated cultivars are best for commercial pot plant production. Take tip cuttings that are approximately 3 to 4 inches long. One-fourth to one-third of the lower leaves are stripped from the cutting prior to sticking. A basal dip in a rooting compound, 0.1 percent indolebutyric acid, plus root zone heat enhances the rooting of cuttings and improves uniformity. A well-drained, soilless media (or a mixture of one part soilless median and one part coarse perlite) works well for rooting cuttings. About 200 cuttings can be stuck in a standard 10-inch by 20-inch propagation flat, or cell trays may be used. Rooting medium temperature should be maintained at 70 to 75 degrees F. Cuttings placed under intermittent mist will start to root in 10 days and are ready for transplanting in 17 to 21 days after sticking.
|Production stage||Date||Minimum temperature
|Take cuttings||June 8||70-75F||200 per 1020 flat|
|Transplant cuttings to 2.5" pots||June 29||65 night||Pot to pot|
|Soft pinch||June 29|
|Shift to 6-inch||August 10||65 night||Pot to pot|
|Soft pinch and shape||August 10|
|Soft pinch and shape||October 12|
|Final Spacing||October 12||65 night||10-inch centers|
|Ready for Sale||December 1|
Transplanting: At transplanting, select heavily rotted cuttings and discard weakly rooted cuttings (usually less than 5 percent). For 4-inch pot production, rooted cuttings may be transplanted directly into finishing pots. For 6-inch or gallon containers, cuttings are potted into 2 1/2-inch pots. Plants grow equally well in a soilless media or soil-based (one part each of perlite, peat moss and soil) growing medium, but will require more frequent watering in lighter media. Plants are soft-pinched by hand or scissors at transplanting to develop height uniformity, promote upright growth and to encourage branching. Plants grown in 4-inch pots will require about six weeks from transplanting to make a salable, branched plant.
Fertilization: A moderate rate of fertilizer is adequate. Research on rosemary conducted at the University of Massachusetts showed satisfactory growth in soilless media may be obtained by the following fertilizer regimes: constant liquid fertilization at 150 parts per million (ppm) nitrogen using 20-10-20; weekly liquid fertilization at 150 ppm nitrogen using 20-10-20, plus a top-dressing of 4.5 grams (three-fourths teaspoon) Sierra 12-12-15 per 6-inch pot as the sole source of nutrients. Additions of slow-release fertilizers may be reduced 25 percent when plants are grown in media having more than 20 percent soil.
Temperature and light: As cooler weather approaches, night temperatures should be maintained at 65 degrees F. Day temperatures may be reduced below 65 degrees F to produce- stockier plants. Plants will grow in full sun and can withstand greenhouse temperatures of at least 120 degrees F. However, a lightly shaded greenhouse may be used during summer. In spring and fall, plants should be grown in full sun.
Finishing: After six weeks, the plants in 2 1/2-inch pots are shifted to 6-inch or gallon containers, using one plant per pot. At this time, the plants are soft-pinched again. The grower now may begin shaping the plant to attain a conical "Christmas tree" form. Be sure that plants are transplanted upright in the pot and to the depth of the original rootball to avoid a mis-shaped form. After transplanting, space plants pot-to-pot until final spacing. Caution: Too much water may weaken or kill rosemary. It is usually several days before rosemary needs water after transplanting to larger pots.
After eight to 10 weeks, plants are ready for a last (third) shearing and pots are moved to final spacing on 10-inch centers. Five to seven weeks later the plants are ready for sale.
Insects and Diseases
Insects: The major pests on rosemary are whiteflies, spider mites and mint leafhoppers. Pesticide labels do not commonly include recommended rates for herbs, and since rosemary will likely be eaten growers should take care to treat plants accordingly. Soap sprays provide effective control of both pests. Growers should consult registered pesticides.
Diseases: Rosemary is subject to root and crown rot. Start with soilless growing media and avoid contaminating media with soiled hands, tools, or containers. Promptly remove diseased plants, avoid splashing water when irrigating and keep hose ends off the floor. Suspicious plants can be diagnosed through the UMass Extension Plant Diagnostic Laboratory.
Root and crown rot pathogens are aggravated by planting too deeply, growing in poorly drained media and by overwatering. Adequate air circulation will reduce incidence of foliar diseases.
Cultivars: Many asexually propagated cultivars are available, ranging from prostrate, hanging-basket types to upright forms. Scents vary with the cultivar and may be mild or strong. Rosemary flowers are small, clustering along the stem and typically are blue, pink and white cultivars are available.
Pest Management for Herb Bedding Plants Grown in Greenhouses (UMass Extension and UConn Extension)
Traven C. Rosemary Production, Grower Talks 2008.
Cox D. and L. Craker. Growing Herbs as Bedding Plants. Nov.Dec. Floral Notes 1993.
Adapted from an article originally written in 1988 by Thomas Boyle is Assistant Professor of Floriculture, University of Massachusetts, Amherst and Thomas DeBaggio Earthworks Herb Garden Nursery in Arlington, Virginia
Revised 2013 by Tina Smith, UMass Extension Greenhouse Crops and Floriculture Program