Rhododendron roots access most of the nutrients they need from the soil. Planting in a good location gives the roots and plant most of the minerals needed for healthy growth. Rhododendrons may not require annual feeding, especially if they are healthy with good growth and flowering. Feed rhododendrons only if they are planted in soil with poor pH or acidity or if the plants are growing slowly. Yellowing leaves are another sign that the rhododendron may require fertilisation.
Test the soil pH before planting a new rhododendron, following the instructions in the test kit. Rhododendrons need a soil pH between 4.5 and 6 or the roots cannot absorb the necessary iron from the soil.
Apply agricultural sulphur to the bed at the rate indicated by the soil test to lower the soil pH, if necessary. Work the sulphur into the planting site at least one month prior to planting the rhododendron.
Spread a 2- to 3-inch layer of compost over the site before planting. Dig the compost into the top 12 inches of the bed. Compost adds nutrients to the soil that the rhododendron roots can easily access.
Mulch over the roots with a 2-inch layer of sphagnum peat moss or compost after planting. The mulch leaches nutrients into the soil surrounding the roots. Replenish the mulch layer each spring.
Fertilise the rhododendron with a 12-4-8 or similar fertiliser blend after flowering. Apply the fertiliser at the rate recommended by a soil test or on the fertiliser package for your plant size.
Newly planted rhododendrons don't require fertilisation after planting. Begin fertilising the spring or summer following planting.