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In the Magazine

Growing Gardenias

In the Garden > Flowering Plants & Shrubs

Gardenias are an outstanding flowering shrub with an exquisite perfume that creates a magical, romantic atmosphere in the garden. They flower at their best in the warmer months of the year usually from November to May. Their large creamy white flowers and glossy green leaves also make them very attractive garden plants.

Characteristics of the Shaw's garden

Don Burke awarded John and Carmen Shaw's garden the inaugural Burke's Backyard gardening excellence award because the gardenias in their garden were the best he had ever seen. What is their secret to growing such healthy gardenias in an area that is not ideal for this semi-tropical plant? The location is subject to frost with winter overnight temperatures falling to -4C. The owners have created a microclimate around the house so that the plants would not get frost-bitten in winter. The house is brick and there is extensive brick paving which absorbs the heat from the sun during the day and radiates it out at night creating a frost free zone. (See diagrams).

Agricultural piping was laid throughout the garden before major works began thereby ensuring excellent drainage. As well as that, the gardenias were planted in raised garden beds with an irrigation system installed so that the sprinkler heads were underneath the foliage of plants. Undoubtedly the most important key to the success of the gardenias is their nutrition. The area was previously a dairy farm which was heavily fertilised to ensure constant grass production for the cattle. Nutrients were also recycled in the form of manure.

The existing richly fertilised soil was then made even richer by the addition of a special soil mix from Australian Native Landscapes (ANL). To every four parts of ANL's standard organic mix they added one part of duck litter and one part of mushroom compost. The organic garden mix itself is made up as follows:

  • 50% black soil
  • 20% coarse sand
  • 0% of organic mix composed of composted sawdust, composted pine bark fines, spent mushroom compost and coffee grounds.

As you can see, this is an incredibly rich mix. The mix was added throughout the garden to a depth of at least 200mm (8"). This had the effect of raising the height of the beds.

In addition to the rich soil, once the gardenias were planted they were fertilised yet again with a slow release fertiliser, Multicote by Haifa Chemical Ltd ( N:P:K 18:2.6:9.9) 8-9 month release fertiliser. During the early establishment the plants were watered several times per week but since establishment are only given a good soaking once a fortnight via the well designed irrigation system. The plants are fertilised with Multicote each spring and again during summer. The plants are growing in full sun through to part shade.

It is generally believed that an acid pH is critical in order to grow good gardenias. The Shaw's soil tested at pH 6-7 which is only slightly acid. In summary the conditions in the Shaw's garden provided excellent drainage, warm microclimate, rich organic soil, regular deep watering and lots of fertiliser. These conditions are similar to those in nurseries where potted gardenias are grown.

Details

Common name: Gardenia, Florist's Gardenia.

Botanic name: Gardenia augusta. The most commonly grown variety is called 'Florida'.

Other varieties:
'Prof. Pucci', 'Fortuniana' and 'Magnifica', which have larger flowers. 'Golden Magic', which has flowers that turn yellow with age. 'Radicans' is a dwarf form with a prostrate habit (up to 0.5m high by 1.5m spread or 20x50"), making it an excellent ground cover. It has smaller flowers and leaves and tends to have more cold tolerance than 'Florida'. 'Aimee Yoshiba' is a newer variety which has large deep green leaves and is said to be the best of the larger flowering gardenias.

Other species: G. thunbergia (the star gardenia) has a tubular flower and more cold tolerance than the G. augusta. It is a native of South Africa. It is a better choice for Adelaide and environs.

Climate (see map): Gardenias are warm climate plants which are at their best in a mild, humid climate. They are seen to perfection in frost free areas north of Sydney and Perth but will grow in Adelaide and Melbourne in a warm spot. The gardenia is native to eastern Asia (China, Taiwan and Japan).

Best look:
  • Gardenias are excellent for mass plantings or as a hedge. Grown as a standard, gardenias look great in a pot.
  • Will perform well for years in a large pot growing in a quality potting mix.
Good points:
  • Fragrant and beautiful creamy white flowers which are seen from late spring to late autumn with the main flush in the months leading up to Christmas.
  • Can be picked to use as a button hole or posy (but handle flowers with care as they bruise easily).
  • Attractive evergreen small to medium shrub (1-2m or 3-6' tall and maybe as wide) with deep green foliage.
  • A versatile shrub which can be grown as a hedge or a standard or used as a background planting.
Likes:
  • Slightly acidic, cool, moist, well drained soil.
  • Some shade, particularly from the full summer heat in a hot or tropical climate. In cooler areas they are quite happy in full sun.
Care:
  • Water well, particularly from spring to summer when plant is flowering.
  • Protect from hot afternoon sun.
  • Prootect root system with a mulch of lucerne hay, compost or well-rotted wood chip.
  • Little pruning needed unless being grown as a hedge.
  • Feeding in spring and summer with Osmocote, Multicote or Dynamic Lifter.

Hates:

Frosts and cold climates although as seen in the Shaw's garden, warm microclimates can be created in frosty areas.

Downside: As with all popular plants there are some common problems with gardenias.

  • Buds falling or failing to open and going brown: Gardenias tend to keep producing flower buds right through autumn even though their growth is slowing. They will often hold these buds right through winter and drop them in spring. This is fairly normal. The buds can also drop as a result of being damaged by weevils or leaf hoppers.
  • Yellow leaves (particularly seen in spring): In the past leaf yellowing has generally been attributed to a magnesium deficiency and treated with applications of Epsom salts (sulphate of magnesium) but the plant is probably more in need of an all purpose fertiliser and a good watering. Apply fertiliser in spring when weather begins to warm and yellow leaves begin to show.
  • Nematodes: If the yellow leaves don't pick up after fertilising and particularly if there is wilting and if you have a sandy soil, check for nematodes on the roots. Nematodes cause cream, warty lumps on roots (about the size of a match head), yellow leaves and wilting. You could treat the soil with Nemacur, but this is a potent chemical which we do not recommend for general use. If the area is sunny, a planting of marigolds may also help deter the nematode.
  • Scale and mealy bug: Scale is commonly found along the stem and on the back of the leaves. Mealy bugs tend to hide among leaf nodes. Treat scale with PestOil. Follow up applications may be necessary.The presence of these insects also suggests the plant is under stress. Make sure it is well watered and correctly fertilised.

Availability

Readily available in pots at nurseries, particularly during spring and early summer when plants are in flower. Prices range from $8.95 for 150mm (6") pot. $14.95 for 200mm (8") pot and $30.00 for 250mm (10") pot. A 1.5m (5') standard gardenia will cost $80 - $120.

Multicote fertiliser is available from Elders throughout Australia. Cost is $24.75 for 4 kg and $90.20 for 25 kg.  

Copyright CTC Productions 2006

Disclaimer:  Burke's Backyard and Backyard Blitz do not accept payment to promote products. All recommendations are genuine. Details on the fact sheets are accurate at the time of publishing, however prices and contact information are not updated and may change.

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