A cold hardy gardenia is a variety bred to withstand extremes of cold in comparison with most varieties of gardenia. This is an important distinction. By the standards of much of the continental United States, there is no such thing as a gardenia that is seriously hardy in cold weather.
Gardenias are best adapted to tropical and subtropical conditions. Gardenias are native to Africa, Asia and the South Pacific. They haven't yet been bred to endure severe winter cold. It is better to grow gardenias as hothouse plants, house plants or seasonal indoor/outdoor plants if your USDA climate is colder than USDA Zone 7. Beyond that zone it is possible to grow gardenias with limited success in micro-climates or in warm years, but hard frost will cause flower-drop and a hard freeze will kill a gardenia. The most consistently recommended climates are USDA Zones 9 and 10. Unless you are planting a specifically cold hardy gardenia you shouldn't expect gardenias to grow well outside that range.
Basic Gardenia Facts
- Gardenias are tropical and subtropical plants.
- Gardenias grow best in a moist and humid climate but with limited direct water hitting the plant.
- Even a cold hardy variety is unlikely to prosper as an outdoor plant beyond USDA Zone 7.
- The colder your climate, the more carefully you should plan your planting and care of a gardenia if you intend to grow it outdoors.
Cold Hardy Gardenia Varieties
There are a small number of varieties that can be trusted to endure non-tropical conditions. Those few are commonly recognized and promoted. The Oregon Association of Nurseries recommends Kleim's Hardy Gardenia and the Chuck Hayes or Oregon Gardenia. Other nurseries add the recently developed variety Frost Proof, but no varieties are recommended for climates colder than USDA zone seven.
Chuck Hayes is a reliable variety with a classic, fully double blossom with a rich, traditional gardenia scent. Suited to container planting for those in cold climates it can grow to as much as four feet in height.
Kleim's Hardy Gardenia
Kleim's Hardy carries a single blossom similar to apple blossom in shape and form. The scent is again traditional gardenia, though not as intense as some. The shrub is low-growing to three feet, and well shaped for landscape use.
Semi-doubled blossoms resemble narcissus when partially opened, with an outer star-like disc of petals and an inner cup. As the flower fully opens it becomes an informal, loose lightly doubled rosette. The bush is large, well suited to landscape and hedge use. The plant takes more sun than many gardenias, and is considered the most cold tolerant of the species to date.
Limits on Gardenias
Only these few varieties can withstand more cold than a tender variety. Those recommended for Zone 7 can survive limited exposure to temperatures as low as 0 degrees F. Beyond that limit, other things must be considered. Gardenias are subject to mold and mildew when water comes in contact with the leaves or petals. High humidity is an advantage, but rain and watering systems can damage plants. It's not even wise to plant gardenias anywhere condensation can cause dew to drop down from neighboring plants. Soil should be moist, acidic, and well-drained, with plenty of organic matter added. Provide regular care and pruning for gardenias to ensure good air circulation within and around the plant.
Even chill resistant gardenias will do best when you take the time to prepare the planting site and develop good conditions for growth. The planting site should be in soil with a pH of 4.5 to 5.5. If your soil is not naturally this acidic you can prepare using organic material: peat moss, compost and pine mulch are all recommended ways of increasing soil pH. Soils with a high organic content also tend to drain better than clay soils, while retaining enough water to provide for the plant. Set your plants well apart to make sure there is good ventilation. You should also avoid planting directly against a fence or wall to ensure ventilation. Water from below, using a drip or underground system if possible, to keep leaves and petals dry. If you live in a zone with variable weather that pushes the limits of a cold hardy gardenia then you should pay particular attention to micro-climates of your garden. A sheltered, south-facing site with good sun and a dark wall behind to absorb heat can improve the chances of your gardenia surviving a cold snap.
Make good use of cold tolerant varieties, micro-climate, and the possibility of container planting. With these methods you can enjoy gardenias with their lush, elegant perfume as a special summer treat regardless of where you live.