Gardenias can be grown outdoors year round in several regions of Texas. They will grow well in roughly the bottom third of the state covered by USDA zones 8a to 9b, where winter temperatures stay at 15-degrees F or above. They can successfully be grown in containers and overwintered indoors (in a sunny location) in cooler areas of the state, according to Clemson University.
Provide a growing location that has daily lightly filtered to moderate shade conditions. Ensure that there is no competition for root space, as gardenias do not like their roots disturbed in any way and do not tolerate transplanting well. If growing in a container, choose one at least twice the size of the root ball to allow the shrub to grow for several years without being transplanted.
Provide a nutrient-rich planting soil with lots of organic matter that remains easily drained. Amend weak soils with generous amounts of aged manure, compost and peat moss, adding a bit of coarse clean sand in clay soils. Add these to planting soil or simply top-dress the plants with an inch of the compost and manure as mulch, once a year.
Water your gardenias consistently, year round, to keep the soil evenly moist but not soaking wet. Never allow the soil to become dry beyond the top 1/2 inch of soil crust. Irrigate down at the soil line only and never over the top, leaves or flowers of the shrub, as this can invite disease and can badly spot the glossy foliage. Winter watering will be less than summer, so monitor the plants carefully through the seasons and adjust watering accordingly.
Feed your gardenias in the spring and early summer and with a slow release or organic fertilizer designed for acid-loving plants. In Texas, March and June are ideal. Apply over the surface of the soil according to product label directions. Spread evenly around the root zone and drip line of the shrub tree and water in well until the soil is drenched to a depth of at least 6 inches.