Gardenias do not like having their roots disturbed. Avoid breaking up the soil beneath the shrubs as much as possible, and use mulch to limit the need for pulling weeds. Gardenias are also heavy feeders, so be sure to add plenty of compost and organic material or fertilizers in the spring. It is optional to apply fertilizer again in the summer to encourage continued blooming (avoid high-nitrogen content fertilizers during this time). Do not fertilize in the fall, as this will stimulate new growth that will be frost-tender going into the late fall and winter.
Pruning should be done after flowering is finished. Remove straggly branches to improve the overall shape and thin branches as needed to improve air circulation. Good air flow throughout the shrubs will reduce the pest and disease issues that are common with gardenias, such as powdery mildew, leaf spots, dieback, anthracnose, sooty mold, whiteflies, mealybugs, scales, aphids and spider mites. Ensuring the shrubs are healthy and not under water or nutrient stress will also mitigate issues. Using drip irrigation instead of spray heads can help reduce leaf spot.
Sometimes, you may notice flower buds or leaf buds turn black at the tips. This is usually due to sudden temperature changes such as a cold snap, and will not affect the health of the shrub. Gardenias are very sensitive to cold temperatures. During a polar vortex or other extreme cold weather, you can protect gardenias by piling straw around the shrubs and wrapping in burlap if they are not too large.