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Ask Dr. Hort: How to grow healthy gardenias

Love gardenias? So do thrips

Thrips are tiny, black 1/16-inch insects that love gardenias and can cause the flowers to turn brown and fall prematurely. A healthy plant discourages thrips, but if you've got problems, spray with the systemic insecticide acephate. Systemics become part of a plant's sap, and as those suckers feed they get a dose and die. Try something like Bonide Insect Control Concentrate or Ortho Systemic Insect Control Concentrate. Don't use more than is needed, or it will kill beneficial insects. And as always, read label directions for use. In addition, the use of neem oil can help control thrips, aphids and whitefly.

The healthy gardenia

Q: My gardenia plant is not blooming. When it did bloom, they lasted just a short time and then quickly turned brown. What should I do? Loretta

A: Is your gardenia getting enough sunshine? How healthy is your plant?

Light and health are two of the primary factors for gardenias to set buds. Your plant should get at least six to eight hours of full sun and the leaves should be a solid grass green, not mottled yellow with green veins, a sign of common iron deficiency.

A quality gardenia fertilizer, such as a 10-0-10 plus secondary nutrients, such as magnesium, iron and manganese, applied three times per year, in February, July and October, will correct the problem.

(Pinellas County residents should note that in July, during the fertilizer restriction period, use Southern Ag Essential Minor Elements Plus Potassium.)

That's not a chocolate center you're seeing

Q: Can you tell me why my gardenia blossoms have dark centers? Bud Staebell

A: Of all of the questions I get on gardenias, from bud drop to iron deficiency and thrips control, yours stands apart. The anthers and pistils are brownish in color as you dig down into the flower. This is normal.

But if tiny black insects spring out, it's thrips. A few thrips are okay, but too many cause flower drop. Read about thrips treatment in the box below.

When black soot and scale attack

Q: I have a scale insect problem with my gardenia. It also has black soot all over the leaves. I have sprayed a treatment for scale but it does not seem to help. The plant is pretty large, so it's also difficult to cover it adequately with a spray bottle. Do you have any suggestions? Thanks. Karen Rhodes, Palm Harbor

A: It is not so much the product as it is how thorough the application is to control scale. Scale secretes copious amounts of honeydew from sucking plant juices — which the sooty mold fungus lives on. Sooty mold is harmless to the plant other than reducing photosynthesis. Insects love to prey on plants that are unhealthy, so No. 1 is fertilizer application. (For details, see the item at left, "The healthy gardenia." No. 2 is water sufficiently. Make a soil saucer 6 inches high by 3 inches in diameter around the plant (remove grass or mulch) and fill the saucer with water every other day for six to eight weeks, then twice per week if there's no rain. No. 3 is to spray the entire plant (don't forget the undersides of leaves) with horticultural or neem oil every two weeks, for three applications.

It is very important to start your plan of attack when a new flush of growth has emerged following blooming.

The infamous bud drop

Q: Every year my gardenia fills with buds, and then 90 percent fall to the ground before flowering. Can you tell me why? Lynne McCaskill

Q: My gardenia bush is dropping its buds even though it has produced several flowers, albeit smaller than usual. Please advise. B. Phillips, Beverly Hills

A: Many factors are involved with bud drop on gardenia, Gardenia angusta, some within your control and some not. High night temperatures (above 55 to 65 degrees), a drop in humidity (they like high humidity), moderate wind and poor soil pH (usually planting too close to the house) are all factors that are hard to fix.

Here's what you can do: Take a soil sample to your local garden center for a pH evaluation (gardenias like 5.0 to 6.5). Make sure, during bud set and through flowering, that the root zone is kept evenly moist. A portable drip system works well and is available at home improvement stores. Last, establish a quality fertilizer program throughout the year and never fertilize during bud set. (That should help with the smaller bloom size, too.) Make sure that both nitrogen and potassium (potash) are 30 to 50 percent slow-release. Be aware that some areas have fertilizer restrictions at this time of year.

It's all about location, location, location

Q: I've had a terrible time with my gardenia (next to my house). I used neem oil to end black deposits made by something. I spent hours cleaning each leaf with a very, very mild solution of Ivory soap, canola oil and water. I added 1/4 teaspoon of a mixture made of equal amounts of liquid soap and oil to a sprayer containing 16 ounces of water. I also fed the gardenia with citrus fertilizer 6-4-6.

The result? The blooms do not open fully and die quickly. Please help. Thanks. Gerri Yerkes

A: Growing next to the house is not the best location because of a potentially high pH (pH governs nutrient availability, gardenias do best from 5.0 to 6.5). Fertilize with a 10-0-10 with at least 30 to 50 percent of both nitrogen and potassium (potash) in a slow-release form in May, September and February, plus S/A Essential Minor Elements in July and January.

Make sure the plant gets even moisture from bud set through flowering to help hold buds. (Read the accompanying box below on thrips and their treatment.)

Leaves give their lives for better blooms

Q: I finally have a potted gardenia plant that is thriving. But recently some leaves near the bottom started turning yellow so I gave it maybe 2 to 3 tablespoons of Epsom salts. Did I do something wrong or not give it enough? It is in a 12-inch terra cotta pot. A few weeks later, and I still have more yellowing leaves. CeAnn Novak, Palm Harbor

A: As the gardenia sets its buds, the oldest leaves are sacrificed by sending their nutrients out to support the new flower buds. Those leaves eventually fall and after blooming new leaf growth occurs. Evenly moist soil is particularly important since yours is in a pot, or you will increase bud drop. No fertilizing until blooming is finished, then use a fertilizer labeled 10-0-10 with 50 percent slow-release, which will be effective for the following three to four months.


For more information, read the University of Florida online publication "Growing Gardenias in Florida." Go to

Need help? Dr. Hort (Greg Charles) answers questions about garden problems. E-mail him at or mail questions to HomeLink, Features Department, St. Petersburg Times, P.O. Box 1121, St. Petersburg, FL 33731. Describe problem in full, and include your name, city of residence and contact information. If possible, include a good-quality photo. Fuzzy ones won't do. Photos cannot be returned.

Ask Dr. Hort: How to grow healthy gardenias 07/16/11 [Last modified: Friday, July 15, 2011 1:24pm]
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