Hello, When hydrangeas and rhododendrons lose their leaves early and fail to thrive, they are responding to stresses in their environment. Stress may be the result of several things such as lack of water or there may be something wrong with the site where they are planted. When a plant is stressed it also may attract insects and diseases which may cause it to defoliate. I would need to see a picture of the leaves to learn what caused them to yellow. Panicle hydrangeas thrive in a sunny location with even soil moisture. Rhododendrons, on the other hand, are more of an understory shrub and do well in partial shade but too need an evenly moist soil. As to fertilization requirements, unless you have had the soil analyzed, it is hard to know what the plants need. To obtain a MSU soil test mailer go to MSU Soil Test.com. This will help you decide how the soil should be treated and whether these plants are suited for that site With respect to watering, this has been a fairly dry summer in our area and many plants have shown the need for extra water even though they never needed it before. I also noticed that there is groundcover myrtle/vinca planted under the shrubs. Hydrangeas and rhododendron have shallow roots which would compete with the myrtle for water which means that you would have to water much more than you had previously. For a more thorough answer you may wish to consult with a certified arborist. You can find arborists in your area listed at http://www.treesaregood.com.