A couple of suggestions - you will get much better info if you can provide the correct botanical name. Common names are meaningless as they vary widely from area to area and often apply to more than one plant. And if you are referring to Tibouchina, your question is more appropriate to the Shrubs forum than to Groundcovers.

Tibouchinas are likely going to be marginally hardy in zone 8 - they are quite cold sensitive. They also need full sun - 4 hours a day is not sufficient. And if grown in containers, watering needs are critical. Containers dry out rapidly and this can happen in an unequal fashion depending on the soil mix and the sun/heat conditions. Yellowing and dropping of foliage is a pretty sure indictor that watering needs are not be correctly met.