Gardening in Minnesota is not for the faint of heart -- especially if your own heart tends toward heat-loving Mediterranean herbs. Thyme (Thymus spp.) is a classic culinary and ornamental herb known for its tolerance of sun-baked, sandy, rocky soils. How well thyme survives the classic harsh Minnesota winter depends in large part on the type of thyme you grow, as well as how you prepare it for the colder months.
Minnesota Growing Zones
According to the U.S. Department of Agriculture, northern and north-central parts of Minnesota are largely cooler than other parts of the state, and are classed as USDA plant hardiness zones 3a and 3b. A middle band, which extends into northwestern sections of the state, is rated as USDA zone 4a. The lower section of the state is USDA zone 4b, as is the area bordering Lake Superior. A recent change to the USDA's zone map finds a small south-central section of the state to be in USDA zone 5a.
How thyme fares in your Minnesota garden depends to some extent on the type of thyme you grow. To add to the confusion, gardening resources don't always agree on the woody herb's hardiness. The University of Minnesota Extension program rates common thyme (Thymus vulgaris) as hardy to zone 4. But the Missouri Botanical Garden considers common thyme hardy only to zone 5. If you've had good luck raising other perennial Mediterranean herbs, chances are common thyme will survive in your garden. But if you're in doubt, choose a hardier cultivar or take measures to protect the herb.
Several cultivars of thyme are rated as reliably hardy to at least USDA zone 4. To add citrusy zest to your walkways, grow lemon thyme (Thymus citriodorus), with its pink flowers and leaves that smell and taste of lemon. Wild thyme (Thymus serpyllum) is more low-growing than common thyme. The groundcover herb grows only about 1/4-inch high. It bears light-purple flowers from late spring through midsummer. As its name implies, the leaves of caraway thyme (Thymus Herba-barona) carry the scent and taste of the caraway seed. Its height and spread is similar to wild thyme, and it bears light pink flowers in midsummer. Silver thyme (Thymus argenteus) is an upright thyme, with variegated silvery-white and green leaves, as well as pink flowers. Juniper thyme (Thymus leuchotrichus) is a creeping type, with needle-like leaves and a pine-like smell.
No matter what part of Minnesota you live in, preparing your thyme for the state's harsh winters will increase its chances of survival. Cut the woody herb back in late summer so it has time to recover before the first frost. In early fall, give it one long, deep watering session to protect plant tissue from cold snaps. Piling on additional mulch extending about 12 inches from the base of the herb -- while not touching the plant itself -- will help protect the plant's root system from cold and frost. Add this extra mulch only after the ground freezes, however. Heavy mulch in early fall will prevent thyme's roots from receiving the last bit of warmth from the waning sun. Similarly, rake away heavy mulch later in early spring so the soil around your thyme plant will warm up more quickly.
If your Mediterranean herbs tend to fall victim to cold weather no matter what measures you take, grow them as patio plants. This measure is probably the safest course for growers in USDA zone 3. Both creeping and upright thymes look well by themselves or paired with plants in containers. Grow them in a sunny spot during warmer months, and bring them inside when the weather turns colder. If you'd rather not grow thyme in pots, set a ring of rocks or bricks around each plant to provide some extra protection from retained heat.
- USDA: Plant Hardiness Map
- University of Minnesota Extension: Growing Herbs in Minnesota
- Missouri Botanical Garden: Thymus Vulgaris
- Lesley Bremness: The Complete Book of Herbs
- Photo Credit John Foxx/Stockbyte/Getty Images