Left: Rosemary (Rosmarinus officinalis) in the arcade of Cuxa Cloister; right: a rosemary plant in flower.
Although rosemary (Rosmarinus officinalis) is one of the most popular plants at The Cloisters for its great ornamental value and colorful history, this plant does have its problems, which become especially evident when we bring it indoors during the winter months.
As a native to the Mediterranean, rosemary is extremely sensitive to low temperatures and frost, which I know from a past experience in which a brief exposure to a low temperature killed my plant instantly; it turned black and defoliated soon after I brought it back inside. Now I know that rosemary should not be exposed to anything below 10 degrees Fahrenheit. Some cultivars, such as ‘Arp,’ are able to tolerate cooler conditions, but they are much slower growers and are not nearly as attractive.
When the temperature drops at The Cloisters, we bring our plants into the greenhouse. Once inside, the risk of problems most often associated with rosemary increases intensely. Our two most common problems are powdery mildew, or Erysiphe graminis, and whitefly, or Trialeurodes vaporariorum. Both mildew and whitefly thrive in warm, moist conditions, and the most effective means of controlling each is prevention. Good air circulation and ventilation is a must. At The Cloisters, we use fans and make sure there is adequate space between plants. Cleanliness is also extremely important. Never leave piles of plant debris or unsterilized soil—obvious breeding grounds for fungi and insects—lying around. Also, infected plant material should be placed immediately in a sealed bag; infected material left lying in an open garbage can may continue to produce pests that migrate back to host plants.
Whitefly often remain on the underside of foliage and may be difficult to see on your rosemary, especially when they’re still. However, once you shake the plant to stir up the population, they become extremely visible. I find this the best way to scout for the adult whitefly, and to make a rough estimate as to the size of the population. Yellow sticky cards are also a good resource to monitor this pest.
The best time to treat for whitefly is when the insects are in the egg or nymph stage, but unfortunately that’s also when they’re most difficult to recognize. Many beneficial insects, including the ladybug and the praying mantis, feed on whitefly during these stages. Research shows the best biological control for this pest is the whitefly parasite, Encarsia Formosa. We spotted several praying mantises and ladybugs at The Cloisters this year, which we hope helped control our whitefly population.
Typically we’ve used traditional horticultural soap to treat for the adult whitefly, but now we’re experimenting with neem oil, which has been a tremendous success—the oil is extracted from the seeds of the neem tree, (Azadirachta indica)—and Neem has become our top defense against the many problems that plague rosemary, including powdery mildew.
I’m sure many of you are dealing with these problems right now. So what do you do to keep your rosemary happy in the winter?
Arthur Tucker and Thomas Debaggio, The Big Book of Herbs (Loveland, CO: Interweave Press, 2000).
Beverly Sparks, Will Hudson, and Ron Oetting, “Whitefly Control in Greenhouses and Interior Plantscapes,” (University of Georgia College of Agricultural & Environmental Sciences Cooperative Extension, Bulletin 1077, 2002), http://pubs.caes.uga.edu/caespubs/pubcd/b1077-w.html (January 30, 2009).
“Pest and Disease Control in Your Greenhouse,” (Greenhouse Growing), http://greenhousegrowing.co.uk/PestDiseaseControlGreenhouse.html (January 30, 2009).