Experts tackle readers’ garden questions.
Q: My two rosemary plants are the first things I planted at my new place. They are the same variety (Tuscan Blue) bought at the same nursery on the same day, and planted within minutes of each other. They are about 15 feet apart (on either end of a 10-foot window) and get watered on the same days. One flourished. The center of the other died, but peripheral branches continued growing, and I thought it might recover. Now another major segment of the sad rosemary is turning brown. Is this enough to identify the problem? What advice can you offer?
Louise Mehler, Sacramento
Sacramento County Master Gardener Sherrill Neidich: Rosemary, an excellent choice in the garden as a drought-tolerant plant, thrives in numerous climate zones, including the Sacramento Valley. In general, rosemary (Rosmarinus officinalis) is fine-leafed, aromatic and evergreen. The leaves of the plant are used fresh or dry as a seasoning in cooking and the bee-friendly flowers can be added as a garnish to salads, as well as the source of excellent honey.
The Tuscan Blue variety has relatively broad (up to a quarter-inch wide) leaves, deep violet-blue flowers, and an upright habit of 5 to 6 feet tall and 3 to 5 feet wide. In winter, Tuscan Blue can survive temperatures as low as 15 degrees.
Rosemary does best in areas of full sun and, once established, requires little to moderate amounts of water. However, inadequate irrigation also can damage landscape plants. Too little water will cause foliage to wilt, discolor and drop, while prolonged moisture and poor drainage will result in smaller leaves, dieback and/or limb drop.
Because excessive moisture can smother and kill roots, the dieback in this particular case could be attributed to poor drainage on the side of the window where the rosemary is failing. Poor drainage also makes plants more susceptible to mineral deficiencies, toxicities, wood-boring insects and other pests that eventually can kill plants.
If there is a slight grade in the garden bed or a layer of hardpan, water could be pooling at the base of the affected plant promoting the development of root and crown rot (Phytophthora). With root and crown rot, twigs and branches die back and the entire plant can be killed as the roots and vascular tissue die.
Since the damage to the plant is localized to the center, cutting out the affected area would probably ruin its appearance. This plant may need to be replaced. However, before planting a new rosemary bush, water and drainage issues should be addressed. Is the watering schedule appropriate for these plants? Is the coverage sufficient? Is too much or too little water getting into the root zone?
To determine if the plant is getting the correct amount of water, insert a garden trowel or screwdriver at least 8 inches into the soil near the root zone of the plant approximately one hour after scheduled watering. If the plant is standing in water or there is a rancid odor, the plant is drowning and not getting sufficient air circulation to the roots. If the soil is dry, the rosemary may not be getting enough irrigation.
In addition, the rosemary should be planted on a slight mound and soil should be kept from mounding around the crown to reduce the opportunity for Phytophthora to develop.
For more information on growing rosemary, visit the University of California (UC) Master Gardeners of Sacramento County website at http://sacmg.ucanr.edu/Grow_Herbs/ and the UC Integrated Pest Management website at http://ipm.ucanr.edu/PMG/GARDEN/PLANTS/rosemary.html.
Sherrill Neidich is a UC Cooperative Extension Master Gardener for Sacramento County.
Questions are answered by master gardeners at the UC Cooperative Extension services in Sacramento and Placer counties. Send questions to Garden Detective, P.O. Box 15779, Sacramento, CA 95852. Send email to firstname.lastname@example.org. Please put “Garden Detective” in the subject field and include your postal address. To contact UC Extension directly, call:
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