Too Much Water

• Agapanthus does best in moist, well-drained soil. The classic symptom of over-watering is yellowing leaves near the bottom of the plant. Allow the top several inches of soil to dry between waterings, and if grown in a container, make sure the drainage holes are not blocked. Good drainage is especially important during the winter, according to North Carolina State University.
  
  

Too Little Water

• Chronically under-watered plants can exhibit yellowing leaves, typically starting near the top of the plant. The leaves will wilt and turn pale green, then yellow. Brown spots also may appear along leaf margins. Agapanthus needs extra watering during windy weather and hot spells.
  
  

Dormancy

• It's normal for the leaves on agapanthus to turn yellow and then brown when the plant is going dormant in the fall. Plants in the warmest parts of their range (U.S. Department of Agriculture Hardiness zones 7 through 11) may not go dormant during the winter, although they usually have a rest period in which flowering is reduced.