Soil pH requirement. If the soil pH is not in an acceptable range, some plant species have a difficult time taking up certain nutrient elements. The result can be a chronically yellow and poorly growing plant. Azaleas and other acid loving plants need a low soil pH. Check the pH with a soil test. In some naturally high pH soils, acid loving plants should not even be considered for the landscape.
Some plants are very susceptible to insect and disease problems.
You can spend a lot of time and money on pesticides to keep those plants looking good. Excessive use of some pesticides can also be environmentally unsound. The problem can be reduced from the start if you are careful about the plants you select for your landscape. You can count on scale insects if you use Japanese euonymus. Rate of growth and mature size.