Can I keep my rosemary plant happy all winter? This question comes up every fall. The easy answer is, “You can try.” There are also things you can do to improve your chances of success with keeping rosemary over the winter.
Start with a cold-hardy cultivar (variety) if you plant, to try to overwinter rosemary in the ground in other than a truly frost-free hardiness zone (that would be here on Long Island).
The U.S. National Arboretum website reports on trials of more than four-dozen cultivars of rosemary, and how they fare on all scores. Check their link to see if yours is one of the cold hardy varieties. It also makes sense for gardeners on LI to take the precaution of taking cuttings from the collection, in case winter causes havoc that cannot be anticipated with the herb-garden inhabitants. (You may pinch or prune your rosemary back a bit–not to start cuttings, but to freeze the delicious sprigs for use in winter like this, and keep the plant tighter.)
What is hardest on woody plants, including rosemary, is wet soil followed by temps that drop way down near zero then warm and drop again with considerable frost heaving. Proper location and care will help keep the well-chosen plant happy. Help it along with some lime or bone meal worked into the soil and provide good drainage, with six hours a day or more of direct or very slightly filtered sunlight. Try one of those extra-hardy cultivars planted in a sheltered spot, such as by a wall, and then mulch heavily after frost.
Most people who try to overwinter rosemary inside are familiar with the problem of powdery mildew. Unfortunately this fungus is favored by the indoor conditions that are typically provided in the average home.
It appears as a white or gray dust on the leaf surface. The fungus is a parasite, which survives solely on the nutrients it takes from the host plant. While breeders have worked hard to produce other plants that are resistant to powdery mildew, they have not been successful with rosemary.
There are many safe and effective treatments for powdery mildew, all of which have greater success the earlier they are applied. There are effective home remedies involving baking soda and dish soap, but the residue it leaves behind looks as bad as the fungus it is treating. Probably the best treatment is using Neem oil which seems to control the fungus completely with one to three treatments. Neem oil is organic and safe to use on edible plants.
Giordano’s carries a number of products in different forms (spray, powder) that will be successful against powdery mildew as well as other pesky problems like scale, spider mites, white flies, and aphids. Ask our staff which solution is best for your problem.