The Worst Enemies of Rosemary

One of the most popular kitchen herbs in the world, rosemary is one of the first perennials herbs added to most vegetable gardens. Rosemary can grow into head-high bushes where winters are mild, but temperatures below 15 to 20°F (-6 to -9°C) can kill the plants. But cold is not the only problem a rosemary-loving gardener is likely to have. Five species of powdery mildew are known enemies of rosemary, particularly plants that are kept indoors. In addition, deceivingly beautiful rosemary beetles are surprising gardeners in the UK with their herb-hungry appetites.

In From the Cold

Frozen soil usually spells the end for rosemary plants, though several varieties have shown good winter hardiness at the National Herb Garden in Washington, DC. Simply adding mulch may be enough to help the most cold-tolerant varieties though moderate winters, but I have had much more luck by bringing the plants indoors. This is easy to do if you grow a plant in a sunken pot through summer. In fall, the pot can be lifted, cleaned up, and brought indoors in a matter of minutes.

Dodging Powdery Mildew

Whether you have a plant you have lifted from your garden or a fragrant rosemary tree youíve adopted for the holidays, be prepared to see patches of dusty white powdery mildew pop up as winter advances. This fungal disease is encouraged by the warm temperatures and limited air circulation of indoor life, so try to find a very cool, drafty spot for indoor rosemary. Plants may be happier in a chilly unheated garage than in your warm kitchen.

Should you see powdery white patches on some branches, promptly prune them out and then spray plants with a mixture of one part milk to five parts water. When applied in bright light, the milk acts as a short-lived fungicide. The milk solution wonít work on all plant diseases, but it works great for outbreaks of powdery mildew on rosemary kept indoors in winter.

Meet The Beetles

Native to the southern Mediterranean, the green-and-purple striped rosemary leaf beetle made its first garden appearance in the UK in 1994, and has since spread throughout most of the country. Groups of this metallic beetle can strip leaves from rosemary, lavender and sage, and they will occasionally eat thyme plants, too. These beetles can complete their life cycle right in your garden (they pupate in soil around plants), so keep a sharp eye out for these unwanted visitors. The adults are active from spring through fall, when gardeners are advised to shake them onto a sheet or open umbrella for collection. Or, try the method we Americans use to collect Japanese beetles, which is to shake them into a shallow pan of soapy water on cool mornings. Wet beetles canít fly away, and the soap cripples their ability to swim, which makes for more efficient collecting.

By Barbara Pleasant

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