Site selection is important when considering where to plant azaleas and rhododendrons. They prefer well-drained, light, acid soil, plenty of moisture during the growing season, protection from winter winds and no early morning winter sun. Early morning winter sun tends to heat up the leaves and buds, letting water from them transpire while the roots are in the frozen soil and unable to supply moisture to the plant. This leads to desiccation and the browning of the leaves and death of the flower buds. Plant them where they will receive dappled sun during the summer.
It is recommended that a soil test be done before preparing the soil for planting. Soils with a pH below 7.0 are considered acidic, 7.0 is neutral and values greater than 7.0 are alkaline. Generally azaleas and rhododendrons like a soil pH of 4.5 to 5.5. You may like to devote and develop a garden area solely for growing plants that require or do best in this type of soil. To create acidic conditions, mix Canadian peat, soil and sand together very well in your desired garden area. If further acidity is required, incorporate powdered sulfur at a rate of 0.8 to 2.4 pounds per 100 sq. ft. to lower it by one unit. Once the correct pH has been established, it would be good to use an acid-forming type fertilizer to keep the pH in the right range. Acidic fertilizers include ammonium sulfate and ammonium nitrate. Follow label directions when applying so as not to burn the plant by applying too much. Check soil acidity every three years as alkaline water supplies can cause the soil to be less acidic. Providing the correct soil acidity level promotes efficient nutrient uptake by the plant. Nutrient deficiencies caused by alkaline soils will appear as yellowing of the leaves.
Azaleas and rhododendrons also like cool roots. A 2-3 inch layer of mulch such as pine needles, peat moss or oak leaves will keep the roots cool and add to the acidity of the soil as they decompose. Water well during drought conditions and just before dormancy.
As always, Happy Gardening!