Growing Rhododendrons and Azaleas

Bed Preparation for Ericaceae

Fraser M. Hancock | Mississauga, Ontario | 1982

Editor’s Note: This article appeared in the Journal of the Rhododendron Society of Canada, Bulletin 1982, Volume 11, No 2, pp 19-21. In 1982 Fraser Hancock was Production Manager at Woodland Nurseries, Mississauga. He is a graduate of the University of Guelph, B.Sc. (Ag.) in horticulture, 1982.

The growing of the plants of the family Ericaceae is a challenge for both hobbyist, gardeners and nurserymen alike. This is due to the exacting requirements of this family, especially in the aspects of soil characteristics and conditions necessary for optimum growth. The method of soil preparation outlined is the one which we use at our nursery for bed growing azaleas, rhododendrons, Calluna, Erica and other assorted plants which require an acidic, well-drained, aerated soil.

I must first mention that the soil base which we use is a fine sand, quite infertile in nature, with a fairly neutral pH. Since the sand does not have an appreciable calcareous component, the pH can be adjusted fairly readily through the use of ferrous sulphate and fine sulphur to the proper range of 4.5-5.5 pH along with our native acid peat. Soils with a clay component will be much more difficult to alter practically, and growers would be well advised to remove clayish soils from proposed planting areas to a depth of 10-15 cm (4-6”) and apply a more suitable well-drained, sandy loam raised to a level of at least 15 cm (8”) above normal grade. Sub-surface water from the surrounding calcareous clay soil should not pass through the raised bed or the pH will rise quickly, nullifying the advantage of incorporating the acidic media.

Dig up the bed to break up any “hard pan” which may restrict proper drainage, and ensure the bed does not have the ground water impeded in any way. If using a fork, remove any pernicious weeds such as perennial thistle or quack grass which will be difficult to control after planting. Soil from the pathways around the bed is thrown into the bed to raise it about 10 cm (3-4”) and crowned slightly to ensure quick drainage to the outside of the bed and to prevent water from puddling around the stems. Amendments are added as follows:

Per 100 square feet (3 x 3 m or 10’ x 10’):
1 × 6 cu. ft. bale coarse grade sphagnum peat
5-7 bushels pine bark compost
3 bushels sedge peat (black humus)
5 × 1 lb. Magamp (magnesium ammonium phosphate 7-40-6) slow release fertilizer
15 lb. fine sulphur (to lower and maintain acidity, from 7.0 pH to 5.0 pH)
1 lb. ferrous sulphate FeSO4 to lower pH quickly and provide iron.

These materials are added uniformly to the bed in layers, resulting in a bed of soil and media components about 15 cm (6”) in height. The bed is then rototilled or forked well, bringing up about 5 cm (2”) of the sandy loam and working the organic matter well down so that a homogeneous mixture results, leaving a bed of 15-20 cm (6-8”), which is about the depth of the root ball of a young rhodo or azalea. Ideally, for a bed where the plant will become a permanent part of the landscape, the bed should be even deeper, to allow better volume for the roots to explore over time. For our purposes the plants are in the bed for a maximum of 3-4 years so 20 cm is adequate.

A vital fact which bears constant repetition is the need to work the coarse organic materials well down into the bed, to allow large pore spaces where the root ball will be well aerated. I feel that some people confuse “aeration” with drainage. While drainage is crucial and does affect aeration, many soils can be well drained and very poorly aerated. Sandy soils are good for drainage, but in our location and in many other parts of southern Ontario these sands are overlying clay deposits which can reach saturation and not allow easy drainage. This causes the water table to rise, restricting aeration to the roots as water displaces the air. The type of sand predominant in our area is of a very fine texture which, when compacted by years of settling and water penetration have become very deficient in aeration porosity.

The root systems of Ericaceae seem to be very sensitive to high carbon dioxide concentrations and low oxygen levels that occur in poorly-aerated soils. The lack of large pores within the soil due to a tight soil matrix, or through pore displacement due to poor drainage will restrict the soil volume that the fine roots can explore.

We have found that the root systems will go down as deeply as there is good aeration. Where the organic component of coarse material stops in depth, so does the root system. A deep bed of suitable texture is important to give the plants the cool, moist root run that they desire, and also improves the plants’ ability to survive our severe winter conditions. A deep root system can delay and shorten the midwinter period when the entire root ball and the essential water source are completely frozen. This is especially true for the elepidote rhodos which, having a large evergreen leaf surface, are prone to dessication and tissue damage if the roots are frozen and the water is tied up in ice crystals and unavailable to the plant.

Mulching applied after planting, and maintained through the season helps to keep the roots cool and moist and slows the freezing process as well.

The organic materials we incorporate are very coarse in nature. The peat has fist-sized chunks which do not break down rapidly and offer a good media for the soil matrix. Plants dug up after 2 years are found to have a myriad of roots through these chunks. The typical peat, normally available, is milled by rototilling to a fine powder and vacuumed from the bog for ease of packing. The fine material is acidic and does offer exchange sites for nutrients but loses some moisture retaining characteristics and almost all aeration potential. We pay a premium for the coarse peat which is hand cut from the peat bogs, but find it irreplaceable in providing proper soil conditions.

pH The breakdown of organic matter through microbial decomposition called “subsidence” is a problem when a lot of organic matter is used in the bed. This is especially rapid when the pH of the media is above 6.5, since the microbial systems responsible favour alkaline conditions. With more rapid decomposition there is danger of severe depletion of nitrogen in the soil matrix. These micro-organisms require large amounts of nitrogen for their life processes and are much more competitive than the plants for this vital nutrient.

The amount of nitrogen provided in our bed preparation through the incorporation of the Magamp (7-40-6) is sufficient, and we therefore use soluble fertilizers intermittently through the year to keep the nutrients at their proper levels to encourage growth, and also to develop flower buds on our larger sale-sized plants.

Growth phases of the rhodos should be observed to time feedings when the plants need them most. Deficiency symptoms are expressed often through leaf colour and lack of proper development. Interveinal chlorosis is indicative of improper pH, which causes important elements such as iron (Fe) and manganese (Mn) to become unavailable to these acid loving plants. Pale, uniformly yellow leaves on rhododendrons and azaleas usually denote lack of available nitrogen. Azalea leaves showing reddish tinges also indicate nitrogen deficiency.

In general, feeding programmes mentioned in other articles should be used in conjunction with the bed preparation I have outlined. However, bed preparation is the single most important aspect of growing Ericaceae well, along with site selection. If enough forethought and work is done prior to planting to provide your acid-loving plants with the soil characteristics they need, a lot of headaches will be alleviated and your garden can be enjoyed as it ought to be.