In recent years, herbs have gained a larger presence in my home gardens. Not only do these plants provide wonderful, fresh garnish for many of our favorites recipes, but they can also be a source of ornamental value as well as great pollinator plants when in bloom. Many herbs are touted as tough perennial plants, and many are so, but rosemary (Rosmarinus officinalis) can be a challenge to keep going in the garden.
Rosemary is aromatic herb that grows as a perennial evergreen shrub, typically hardy to zone 7 in the US. It is a Mediterranean native whose Latin name means “dew of the sea”, presumably due to its frequent occurrence close to the sea shore. In ancient Roman times, this plant was valued mostly for medicinal uses only. During the Middle Ages, it joined a host of other herbs that came into favor among European royalty for their culinary value, paving the way for it to become the herbal mainstay we know it as today.
In the garden, rosemary can be planted in spring and will grow into a sizeable plant over one growing season. However, since central Illinois is primarily zone 5, this plant cannot overwinter without some help. There are few varieties, such as ‘Arp’ and ‘Hills Hardy’ (also known as ‘Madalene Hill’), noted to be hardy to zone 6. So, it may be possible to overwinter these plants with some careful protection and the right planting site.
Rosemary requires a well-drained site to thrive, with full sun (greater than 6 hrs per day of direct sunlight). For overwintering outdoors, select a site that is protected from weather extremes, like a spot along a sunny, south facing wall of your home. Upon first frost, add a layer of heavy mulch to protect the plant for winter and cross your fingers.
It may be possible to nurse your plant through winter, but single digit temperatures are often fatal for rosemary. A small, portable cold frame structure may be a good alternative for trapping enough solar heat to inch your plants above that critical low temperature at night. Believe it or not, snow cover can actually insulate and protect plants from extreme lows, so don’t sweat it if there is a sufficient covering of snow. Problematic conditions arise when there is no snow cover and temperatures reach extreme lows. Consider adding a temporary, additional layer of insulating mulch to protect plants from these extremes.
Most central Illinois gardeners have the best luck overwintering rosemary in a pot. The entire plant can be move back outdoors in spring or multiple plants can be started from cuttings as rosemary readily propagates from stem-tips. The overwintering process can be tricky since our homes just don’t quite provide that pleasant Mediterranean climate throughout winter.
We typically keep our homes a little too warm and too dry for rosemary. Your goal for an overwintering spot should one that has cool temperatures (around 50-60 degrees) and very bright sunlight. Although rare, an ideal location would be a sunny basement window. I have a neighbor that has had good luck using a partially heated back porch that receives a lot of light for overwintering rosemary.
I have not had good luck with overwintering rosemary indoors. I just can’t seem to find a cool enough location that has sufficient sunlight. However, I do have a basement with plenty of space and the perfect combination of temperature and humidity, but simply too little light. One alternative is to put your plants under a grow light to sustain them through winter in a cool enough location. However, I just cannot justify the energy expense (for the lights) to keep a few plants going that I could otherwise start from cuttings next spring.
My wife has two rosemary plants, grown in pots all summer, which she brought in to overwinter last week. As we have debated the best location to store them, we’ve both agreed that no place in our current home is perfect. However, we do plan to give it a try again and keep our fingers crossed that the sunny, south facing kitchen window will stay cool enough to sustain the plants. Next year, we plan to attempt overwintering outdoors in our newly established herb garden, right outside the kitchen windows. Gardening wouldn’t be fun if you didn’t push the limits sometimes!