In Portland, full sun is best for Hardy Gardenias. In climates with very hot summers, light shade is preferred.
The best time to plant Gardenias is in spring so they have plenty of time to produce roots before being challenged with winter cold. Gardenias prefer to grow in acidic soils. Use fertilizers and soil amendments that do not contain lime that will ‘sweeten’ soil, or raise the pH.
If growing a Gardenia in a container, add trace minerals including iron. Gardenias dislike root disturbance, so add mulch to root areas rather than cultivating soil around roots.
Night temperatures between 50-55 degrees are necessary to form flower buds.
Cold hardy forms are ‘Kleim’s Hardy’, ‘Frost Proof’, and ‘Chuck Hayes’. They are hardy to USDA zone 7(0-10f) and should survive cold Portland winters. These varieties may still struggle with the amount of moisture many of our heavy soils hold during winter, so adding pumice and organic matter to the soil when planting to provide exceptional soil drainage is a very good idea. They are excellent plants for growing in containers as well.
Traditional tropical Gardenias enjoy living outdoors during summer and coming inside during winter, or living as houseplants year-round. They are hardy in zone 9.
Here are some of our favorites.
Gardenia jasminoides ‘Kleim’s Hardy’ White flowers are single form with six petals and yellow stamens. Fragrance is classic Gardenia. Leaves are dark green, glossy and evergreen. Grows slowly in a rounded mounding habit to 2-3 feet tall and wide.
Gardenia jasminoides ‘Chuck Hayes’: White flowers are double form with many fat petals and sweet scent. Evergreen leaves are dark green & glossy. Round habit to 4 feet tall and wide. ‘Chuck Hayes’ is not widely grown and currently difficult to find
Gardenia jasminoides ‘Frost Proof’: Large white double flowers with narrow petals and sweet Gardenia scent. Evergreen leaves are dark green & glossy but more narrow than other varieties. Growth is upright and vase-shaped, to 3-4’ tall. ‘Frost Proof’ is often available to nurseries during spring.
FACTS: HARDY GARDENIA
Family: Rubiaceae – also the family of Coffee Plant (Coffea arabica), Mirror Plant (Coprosma), and Quinine (Cinchona pubescens)
Common: Gardenia or Cape Jasmine
Characteristics: White waxy flowers have a strong and distinctive fragrance. Leaves are dark green and glossy.
Size: Size varies per variety and ranges between 12” tall and 5’ tall.
Hardiness: Until very recently Gardenias were tropical shrubs, hardy in climates with winters warmer and drier than ours. Many varieties are still in that group, but several new forms have been released in the past few years that are more cold tolerant.
Diseases: Gardenias are susceptible to several pests and diseases. Whitefly, aphids and scale are common insects that can affect Gardenias. When any of them are present they may be accompanied by black sooty mildew along stems and leaves.
Root rot can also occur.
Another common problem is "bud drop." Flower buds form, look like they are going to open, but turn yellow and drop off instead. Common causes include overwatering, underwatering, insufficient light and high temperatures and low humidity.