Friday, January 30, 2009

Rosemary in Winter

Rosemary in Cuxa Cloister Rosemary in flower

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?

???Kevin Wiecks


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), (January 30, 2009).

“Pest and Disease Control in Your Greenhouse,” (Greenhouse Growing), (January 30, 2009).

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Comments (10)

  1. Wendy Erickson Says:

    This past summer I got a nice rosemary standard marked down to $5 at the end of the season. After losing one the winter before (that one was NOT $5!) I have been determined to keep this one alive. It lives in an indoor porch which averages a temp of 65 degrees, gets full sun in a south facing window with some neighboring plants, and I have a humidifier running right in front of it! I water it every day or three (it’s in a clay pot) and so far so good. I fertilized it in December and it has some new growth too. I must be nuts because I paid $50 for the humidifier…Go figure. But if it survives, it will be worth it!

  2. Kate Foy Says:

    I visited the Cloisters for the first time on Thursday. How delightful to see the gardens wearing their winter white and to imagine the summer and other seasonal looks.

    Re Rosemary, I have no problems with mine, either seasonal or pestilential. It delights in the baking heat of Queensland, Australia and grows sturdily and rapidly. I delight in its perennial presence especially for cooking and as an aromatic vase specimen. Good luck with the potted versions and thanks again for the Cloisters experience.

  3. thea mcginnis Says:

    Kevin, thanks for all the info on Rosemary. I have had relatively no success keeping rosemary over the winter, even when i bring it into the house. i lived in upstate ny in a lovely farmhouse, but i don’t think the home temp was consistent enough to offset the move trauma. However, I now live in Arlington VA which is probably zone 6ish, garden-wise. At the end of the summer, my sister brought me three plants from Tudor House, a historic home in Georgetown that has beautiful gardens. They were end of the season bargains, I believe. Anyway, two were lav enders (munstead i think) and one died, the other one is outside, still green and alive. But the third is a Rosemary. It grew taller thru the fall. This past week of snow and ice storms and freezing temps hasn’t turned it black. it’s still going strong, even covered with snow. I have no idea what kind it is, but i’ve never heard of a rosemary surviving normal winter weather.i run it through my fingers every morn, enjoying it’s scent. what do you think?

  4. Rebecca Bourgault Says:

    I discovered this blog a few months ago and I am delighted about its existence. Thank you!

    I would like to suggest an entry on the subject of natural plant dyes as I understand that woad, weld and madder provided medieval artisans with sources of primary colors, not to mention buckthorn and black walnut as alternatives. Are these plants found in your gardens?
    Also, I was wondering if we know anything about what plants were used in the dyeing of fibers used for the weaving of the Unicorn Tapestries? Have chemical analysis been done? Thank you again for a beautiful blog site.

  5. Barbara Bell Says:

    For several winters, I have over-wintered rosemary plants in my home with great success. Cool temperatures and humidity are the key. The plants are given a southern exposure in a room that is about 60 degrees. When there is new growth, which is usually leggy under these conditions, it gets cut back. I like to consider the plants to be in “suspended animation”, and I do not wish to stress them with regular strength fertilization or the need to support new growth. It is wise to check the soil every three to four days to see if the plants need water. Put your finger in the soil, it is the only way to know if they need watering. Each plant might have different watering needs depending on the type of pot (unglazed clay permits the soil to dry out quickly) or if the plant is rootbound, so check each plant and water accordingly. When my plants were smaller and easier to manuever I would put them in a steamy shower stall once a week. It was also a convenient way to hose off the plants (with cool water!) keeping any infestation at bay. Now I run a humidifier near them. And don’t forget to harden-off any plants before relocating them outdoors for the growing season, and the same rule goes for bringing them in for the winter. For the winter to spring transition place the plants outdoors in a semi-shaded spot with protection from drying winds during the day, then bring indoors for the evening. Do this one week prior to leaving them outside for the growing season. In the fall, bring the plants indoors at night for one week before bringing them in for the dormant season.

  6. Kevin Wiecks Says:

    Thank you for all the wonderful comments. It is very interesting, as well as helpful, to hear the different approaches to overwintering this plant.

    Wendy, rosemary always drops in price prior to winter. Retailers also realize the difficulties. It sounds like you are off to a better start with the new one. The humidifier is a great idea, as Barbara also suggested. Just be careful that humidity levels don’t get too high. Fungal diseases can be a tremendous problem with rosemary.

    Kate, I am glad you enjoyed the visit. Make sure you come back in the spring.

    Thea, It sounds like you have a hardy rosemary cultivar. The straight species Rosmarinus officinalis would not overwinter in those conditions. Chances are your plant is either ‘Arp’ or ‘Hill Hardy’ (syn. ‘Madeline Hill’). Both cultivars are grown in the research gardens at Cornell University in Ithaca, NY. They are nice specimens that overwinter successfully within the blistering winters of the Finger Lakes. However, as mentioned in the post, they are much slower growers.

    Rebecca, we will certainly do a post on dye plants in the future. They are a very important part of our collection. We have an entire bed devoted to plants that were adapted as artists’ materials. This bed does contain weld, woad, madder and buckthorn. Unfotunately, there is no black walnut. All of the colors in the Unicorn Tapestries were taken from dyes derived from woad, weld. and madder. We will revisit this subject. Thank you for the suggstion.
    Barbara, thank you for the comment and welcome to the blog.
    Barbara is one our devoted gardeners at The Cloisters.
    Expect to read more on the subject of Rosemary. I will be continuing this post on Friday, February 13th.

  7. thea mcginnis Says:

    Kevin, thanks for the info on hardy Rosemary. I didn’t know there was such a variety. It’s funny, but i lived not too far from Cornell when I lived in NY and couldn’t keep a rosemarygoing over winter. My sister is a docent at the Tudor House (a George Washington extended family home) and I will have her check what variety the Rosemary she picked up there is. Now it seems I’ve got a NY Rosemary that does quite well down here. (mid atlantic region).

  8. Elizabeth Chadwick Says:

    Just came across your fascinating blog while browsing this evening. I have a rosemary in my garden - Nottingham UK, England. It dwells in a sheltered bed near a bay window and lives there throughout the year. I was amazed to learn you have to over-winter yours indoors. Ours is a prolific creature that flowers constantly and thrives - even in winter. Of late our winters have been mild and where we are it doesn’t go much below -5 at the coldest. but our Rosemary is rampant and shows no sign of being winter-challenged!

  9. Miriam Says:

    I bought a rosemary plant at Whole Foods Market in NJ three years ago and planted it directly in my garden which has Southern exposure and it has been growing beautifully ever since. I have cut lots of branches (14 to 15 inches each) to give to my neighbor because if I don’t control it it kind of takes over every other smaller plant around it. It stays green all year and has very cute flowers. I’m sure it will take the big Northeastern that we are expecting tomorrow night. On the other hand I have tried to keep various Rosemary topiaries that I have gotten oner Christmas alive inside the house so I can plant it out in the spring with no success. I threw out the last one today, black as charcoal.

  10. Mike Bertoglio Says:

    Visited Cloisters last week for first time. Awesome garden.
    Last winter in Washington State we lost our Rosemary in pots to a freezing rain. However, the two plants I have sited near the wall or our house-Northwest survived.
    What about your Bay Laurel- did that one survive?

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