Answer: Gardenias grow in a variety of soil conditions in Florida but they do best in well-drained soil high in organic matter. Soil pH is important because it affects availability of mineral elements and should be maintained between 5.0 and 6.5 for most Florida soils. Where soil pH is above 7.0 because of naturally-occurring lime (like limestone, marl, or sea shells), a constant effort will be needed to avoid micronutrient deficiencies, notably iron. Since there is no practical way to permanently lower the pH of such soils, growing a more tolerant species than gardenia may be wise.
Proper fertilization is important for gardenia growth and flower production. Most established gardenias grow well with two or three applications per year. One application is normally applied around February and another in October. A third application may be made during the summer.
A complete fertilizer with a ratio of approximately 3:1:2 or 3:1:3 (e.g. 15-5-10 or 15-5-15) of nitrogen(N), phosphorus(P) and potassium(K) is generally recommended. For each application, apply a maximum of one pound of nitrogen per 1000 square feet. This rate is easy to calculate from the information given on the fertilizer bag. Simply divide the nitrogen percentage (the first number of the analysis) into 100. For example, if you purchased 15-5-10 then you would divide 15 into 100 which would equal 6.6 pounds.
Therefore, 6.6 pounds of 15-5-10 will supply one pound of nitrogen to be distributed over 1000 square feet of landscape area. This would be approximately ? pound per 100 square feet.
Frequently plants will become yellow (chlorotic) due to a deficiency of one or more micronutrients, usually iron. The deficiency can often be corrected by acidifying the soil or by foliar application of the deficient nutrient. Elemental sulfur added to soil will result in a lower soil pH but the decrease will only be temporary if the soil contains natural lime. One technique is to dig a small hole about a foot deep and 8 to 10 inches in diameter near the dripline of the plant. Mix 2 to 3 tablespoons of agricultural grade sulfur into the soil taken from the hole, and return the amended soil to the hole. Repeated every year, that volume of acidified soil usually prevents micronutrient deficiencies commonly associated with high soil pH. Foliar applications of iron are also effective. Follow the directions on the product label.
The leaves' loss of normal dark green color may be due to any of several causes, not just nutritional deficiencies. These potential causes include insufficient light, over watering or poor drainage, too low soil temperature, nematode damage or diseases. Tip burn, which occurs particularly at vein terminals, causes the leaves to lose their color and die. This may be caused by inconsistent watering. Some leaf yellowing on older leaves is normal. This may occur during the winter months, before new growth appears, and is typical of many broadleaf evergreens.
Hope this information helps!
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