Gardenias, although not the easiest plant to grow, do surprisingly well in Dallas gardens, given the proper care. Good selections for zone 7 Dallas gardens, according to Bill Scheick, garden writer for The Dallas Morning News, include Klem's Hardy (plant in the sun), Frost Proof and Chuck Hayes. Even winter temperatures in the Dallas area seem to agree with these gardenias.
Test the pH of your soil. Gardenias in Dallas need to be grown in acidic soil with a pH of 6.0. You can purchase home testing kits at the large gardening centers or take a soil sample to the county cooperative extension office for analysis. Ask the agent for recommendations of appropriate soil amendments to bring the pH of your soil to the proper level. Examples of these amendments include sphagnum peat moss, aluminum sulfate and sulfur.
Choose a location in the garden in which to grow your gardenia. In Dallas, the shrub requires morning sun and afternoon shade.
Prepare the soil in the planting area by digging into it to a depth of 10 to 12 inches. Loosen the soil well with the gardening fork and remove any weeds and rocks.
Add a 3-inch layer of compost and any amendments required by the soil pH test to the existing soil and mix it to a depth of 8 inches.
Dig a hole 1 inch shallower and three times the diameter of the pot in which the gardenia is being grown. Remove the plant from the pot and place the roots in the hole. When planted, the gardenia should sit 1 inch higher than the surface of the soil. This will allow for the drainage required of the plant when grown in the heat in Dallas.
Back fill the hole with soil and mound it up around the top portion of the root ball so it isn't exposed.
Water the shrub until the water puddles. When it drains, lay down a 3-inch layer of mulch, completely surrounding the base of the shrub, but 2 inches away from the bark.
Give the gardenia 1 inch of water per week. You may need to water more, depending upon how hot and dry the weather is in Dallas. Keep an eye on the soil and if it appears to be drying out, water it. Heat and dryness are the biggest dangers to Dallas gardenias.
Fertilize the gardenia one month after it has been planted and continue monthly feedings until June. Once the summer heats up in Dallas, discontinue fertilizing. Use azalea food according to the rate suggested on the package. Always water the gardenia before fertilizing it.
Inspect the gardenia for signs of mealybug (Planococcus). Dallas-grown gardenias are especially attractive to this pest in summer, fall and early winter. A mealybug infestation looks like cottony patches on the leaves and twigs. Insecticides can be effective in ridding the gardenia of the pest. Contact the Dallas Cooperative Extension office for the appropriate insecticide to use in your garden.