Answer: First, the bad news - Diazinon is not registered for use on gardenias. It is phytotoxic, meaning it will burn the foliage and residuals can affect the stems and the roots of the plants. For aphid infestations, Insectidical Soap can be used on gardenias - just to make sure the foliage isn't damaged, wash the plant off with plain water 10-15 minutes after spraying with Insecticidal Soap. You'll need to repeat the sprays to get any new pests that hatch from eggs. (Follow label directions.)
Gardenias are fussy plants. They like to be given ample water, ample nutrients and ample warmth. The soil they are to be grown in must be acidic, otherwise the leaves turn brown and the buds drop. They need regular treatment with iron and doses of aluminum sulfate to keep the soil acid and keep iron chlorosis from turning the leaves a pale color. Feed them monthly during the growing season with an acid food, one prepared for camellias, azaleas and rhododendrons. Many successful gardeners use Miracle-Gro or Miracid as a foliar spray and as an aid in correcting iron deficiencies.
Gardenias prefer being planted in a place where they get full sun in the morning and shade in the afternoon. Plant them in the ground with the crown (the part where the stem arises from the ground) raised slightly above the existing soil. This will help prevent root rots. Gardenias do not like their roots disturbed by cultivation. Cover the area underneath gardenias with a thick layer of mulch. Growing yours in a whiskey barrel is just fine. Once established in a place they like and given proper care, gardenias will bloom their hearts out, providing an abundance of fragrant flowers for you to enjoy.
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