Heavenly scent and stunning flowers ensure these plants remain popular despite being difficult to grow, says Miranda Kimberley.

G. jasminoides AGM - image: Floramedia
G. jasminoides AGM - image: Floramedia

Gardenias can be tricky to grow, inside and out, but because of their heavenly scent and the attractive contrast of their creamy-white waxy blooms and glossy evergreen foliage they retain their popularity.

Most people are familiar with the houseplant gardenia, which can be difficult to get to thrive in centrally heated houses. Leaves and buds surprisingly drop off even when they are given considerable attention. But treated well they can be beautiful additions to the home.

There are also some gardenia that can be planted outside, though debate continues to rage about the limits of their hardiness. Some consider them to be hardy, perhaps more say borderline hardy. These are bred from the species Gardenia jasminoides Award of Garden Merit (AGM), which is the one grown as a houseplant, but the varieties 'Kleim's Hardy', 'Ice Diamonds', 'Perfumed Petticoats' and 'Crown Jewel' are definitely tougher.

The only other species grown in any number in this country is G. magnifica. This is potentially a larger shrub and has correspondingly larger flowers. It is said by Heather Godard Key of the Old Walled Garden to be a much easier plant to grow than G. jasminoides AGM, but it is not widely available and is definitely best kept indoors, unlike its relative varieties that can be risked in borders.

In their native habitats in China, Taiwan and Japan, gardenia are used to bright light and high humidity, so this does explain some of the difficulties growing them in a house that has a dry environment created by central heating, with pockets of dull light.

To counteract these factors, place the houseplant types in a brightly lit south-facing window during the winter, moving to one with bright but indirect light during the summer to prevent leaves getting scorched or wilting. Place them on a layer of wet pebbles or Hydroleca to create humid conditions. Give plants plenty of water in the spring and summer, but do not saturate them. Reduce the watering in the winter, but do not allow the compost to fully dry out between watering.

Gardenia are best grown in acid soil or compost, but they can tolerate soils towards neutral. Ericaceous fertiliser is recommended but be wary of over-fertilising because they can be sensitive to too many salts in the growing medium. For potted plants, apply a weekly high-nitrogen liquid feed in spring and summer and in winter feed them with a balanced fertiliser about once a month.

The "hardy" types can be planted in sheltered borders but are best grown in pots so that if we do get the far below freezing temperatures seen over the past few years they can be protected in a conservatory or greenhouse over winter.

Problems that may be encountered with gardenia include yellowing leaves. This may be caused by iron deficiency or over-watering. Unfortunately, at the other end of the spectrum under-watering may cause browning on the leaf margins. Another problem to watch out for is bud drop. This may be caused by fluctuating temperatures or irregular watering.

What the specialists say

- Heather Godard-Key, owner, The Old Walled Garden Nursery, Warwickshire

"We grow three varieties of gardenia. G. magnifica is an outright tender one for the heated conservatory that becomes a very large shrub if it's allowed to, with gorgeous big double flowers. We also grow two of the 'hardy' ones. Good old 'Kleim's Hardy' and a new-ish variety called 'Crown Jewel'.

"My view on these is that they really should be classified as borderline hardy. They can take a frost, as I've had G. 'Klein's Hardy' down to -8 degsC. Some might call that pretty hardy, but we've had three successive winters with -18 degsC on the cards, so it's all a matter of degree.

"I would recommend them as ideal for pots, out for the summer and kept in cool or unheated glasshouses or conservatories for the winter. If kept warm they should continue to flower - and of course the scent, which is what it's all about, is fabulously lovely and evocative of a far more exotic climate than our own.

"One of my top growing tips is to keep it acid. They really do not like lime. So grow them in a good ericaceous compost with reasonable drainage. If possible, in areas of hard water, collect rain water and use that. I don't know of any problems with gardenia. I'm not too kind to mine and they do perfectly well."

In practice

- David Anderson, general manager, Seven Hills Garden Centre, Garden Centre Group

"Most people regard gardenias as houseplants, but in fact there is an outdoor variety, G. jasminoides 'Kleim's Hardy'. This should survive outside, planted in sheltered borders or containers in well-drained, fertile, neutral-to-acid soil, but you may want to bring the plant inside during the winter on really freezing nights.

"The houseplant gardenia varieties are commonly G. jasminoides. Unfortunately, plants have a well-earned reputation for being difficult plants for even the best green-fingered person.

"Gardenia thrive on bright light, high humidity and an even supply of moisture and nutrients. Now this is where the problems can start. Our houses are hot, with dry air from central heating, and can be gloomy on cold wintery days, which is exactly what gardenia do not need. In these conditions, the buds that should offer the promise of a white, waxy, fragrant flower soon fall to the floor. The glossy leaves turn dull yellow and they too begin to drop.

"But gardenia can be easily grown in the home with a bit of TLC and perseverance. To match the plant's native environment, first make sure you give it plenty of bright light, preferably direct sunshine for at least half a day. Moving plants closer to southern-exposure windows will help. To help increase humidity, try grouping plants together on trays of wet pebbles. They will need a steady supply of water and nutrients, but don't overdo it. Monitor the soil frequently for moisture content and water thoroughly as the top inch of soil dries. Use a drip feed for fertilising but make sure that it is formulated for acid-loving plants.

"At my garden centre, we tend to concentrate on the houseplant varieties. Space and simplicity are needed to appreciate this remarkable plant. I have a dedicated area in my houseplant section just for gardenia. I use a few upsidedown white pot covers to place them on, which elevates them and shows off their foliage and flowers. Then I put a few other plant specimens and some church candles underneath them to build a display that I think creates a soothing image of peace and tranquillity."

Species and varieties

G. jasminoides AGM (H1c) - or the cape jasmine - is an evergreen shrub that is capable of reaching 180cm tall in a warm climate, but in ours forms a compact plant, often sold as a houseplant. It has nice, deep-green shiny leaves and heavily scented, double, creamy-white blooms between June and August. Height: 60cm.

G. jasminoides 'Kleim's Hardy' can tolerate down to -8 degsC so it is relatively hardy. To be on the safe side, it can be grown in a pot so that it can be placed outside in the summer and brought in over winter when the temperatures are too low. It bears lovely, fragrant, creamy-white flowers throughout summer and autumn with a peak in early summer. Height and spread: 90cm.

G. 'Crown Jewels' is another borderline hardy variety, alongside the well known G. 'Klein's Hardy'. It has pearly white, highly fragrant, waxy double blooms and flowers from July through to September. Slightly more spreading than the old favourite. Height: 60cm. Spread: 120cm.

G. 'Ice Diamonds' is a new borderline hardy type that bears loose, double, ice-white flowers from June to August. It is a compact, evergreen shrub, best planted in patio containers or in sheltered borders next to a path or doorway. Height and spread: 90cm.

G. magnifica is potentially the tallest of the gardenias. In a warm climate it can reach up to 2m, but in ours it will not pass 120cm. It has glossy, dark-green leaves and large, fragrant, double, creamy-white flowers in summer and autumn. It flowers less prolifically than the other species but it has larger, showier flowers. It is a tender species so should not be grown outside. It is suited to a heated conservatory and said to be an easier indoor plant than the other species. Height: 120cm. Spread 90cm.

G. 'Perfumed Petticoats' is a new double, hardy variety. It is a slow-growing type with fine, evergreen foliage and heavily fragrant flowers between May and October. Ideal for containers and low borders. Height and spread: 60-90cm.

Thank you to Floramedia, which supplied most of the images for this article from its photo library

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