Growing Rhododendrons and Azaleas

Rhododendron Soils and Their Preparation

Bob Dickhout and Lyall Crober | 1982

Source: Dr. R. J. Hilton. Bulletin of the Rhododendron Society of Canada. 1982, Volume 11, No.2

The soil for Rhododendrons and Azaleas should be well drained, coarse textured, and high in organic matter. Such soils are rare in Niagara gardens and it is nearly always necessary to improve the soil before planting.

Rhododendrons and Azaleas are by nature long-lived plants, but thorough preparation of the soil is necessary if they are to thrive.

Sandy soils can be made suitable by spreading 10 to 15 cm. of coarse peat moss over the bed and mixing it into the top 15 cm. of soil. If half the quantity of peat is first mixed in and then the remainder added, mixing is easier and more thorough.

Do not excavate deep beds in heavy clay soils, because the drainage may not be adequate. The best procedure is to build raised beds, containing a mixture of 50% sandy topsoil and 50% coarse peat moss. Excavate the bed to a depth of 20 cm., break up the subsoil and add 1 cm. of drainage material (eg. gravel, rock, etc.) Raise the beds 30 cm. above grade level using logs or stones to contain the medium. In a raised bed the plants will not be affected by soil conditions below them since the beds are well drained. Raised beds should be large enough to accommodate several Rhododendrons because the plants benefit from a large volume of prepared soil. Raised beds dry out more quickly so more water will be needed during dry weather.

Beware of most peat moss available today. Much of it is of poor quality: often it is very fine and holds too much water for good soil aeration. It is not suitable for mixing with heavy clay soils. Coarse or chunky peat moss should be used.

Rhododendrons and Azaleas in general require an acid soil with a ph of about 5.5 Soils with a ph higher than 5,5 should be acidified using either pelleted sulphur or iron sulphate. Add these acidifiers to lower a square metre of soil by one ph unit.

AcidifierClaySilty LoamSand
Pelleted Sulphur280 grams140 grams70 grams
Iron Sulphate1.0 - 1.5 kg550 grams275 grams

The sulphate mineral salts give a quicker response but the sulphur pellets are cheaper and safer to use.

Remember, a square metre is equivalent to 39.4 x 39.4 inches, and a pound is 454 grams.

For the do-it-yourselfers, soil ph kits are available from many garden centres and nurseries; or soil samples may be sent to:

University of Guelph
Laboratory Services Division
95 Stone Road West
Guelph, Ontario
N1H 8J7