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Journal ARS Article

Vol. 47: No. 4: Year 1993

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Fertilizing Rhododendrons the Organic Way

Terry Richmond
Port Alberni, British Columbia

When fertilizing rhododendrons we should look to nature to show us the way.  In nature mulching and fertilizing is a continuous process with the current year's mulch being gradually transformed in subsequent years to usable fertilizer.  Nature's rhodo food begins with a leaf, needle, twig, petal and fruit fall - in short, any and all matter that falls to earth or flows into their area in ground water.

Rhododendrons, because of their environment and the shallow layer of organic matter in which they grow, have evolved a massive root system consisting of literally thousands of tiny, shallow running feeder roots.  These roots are extremely efficient in extracting life sustaining plant nutrients from their immediate area.  Root systems will be much smaller in a benign climate because a smaller amount of nutrients is required to maintain plant health.  Conversely, rhododendrons in exposed and/or harsh conditions will have a vastly increased root system to extract every ounce of nourishment from their surroundings.

So how do we fertilize rhododendrons in our garden?  First, any literature on fertilizing rhododendrons assumes that your plants are growing in the correct medium.  Again, as in nature, this medium should be extremely high in organic matter, well drained, well aerated and moderately to slightly acidic.  Fir and pine bark, composted oak leaves and evergreen needles, decayed wood, well rotted sawdust, coarse peat moss, reed sedge and topsoil high in organic matter are some of the materials that can be combined in endless combinations to provide excellent growing mediums.  Growing medium acidity or pH value is not nearly as critical when growing plants in an organic medium using primarily organic fertilizers.  One good quality compost for rhododendrons contains oak leaves, evergreen needles, alfalfa and washed seaweed.  Between the various layers an organic nitrogen such as canola meal, fish meal or blood meal can be added.

A word of caution!  Rhododendrons, because of their previously mentioned tiny feeder roots, can be easily damaged through over-fertilization, especially when using high analysis chemical fertilizers.  Elements to be cautious using include nitrogen, iron, sulfur, boron, sodium and calcium.  Contrary to popular belief, rhododendrons do not hate calcium.  In actual fact the reverse is true.  They will gorge themselves on available calcium until they make themselves sick.  With respect to iron, a few years back a respected rhododendron grower suggested I supply more iron to help combat the effect of full sunlight in my exposed garden.  He was undoubtedly right, but I supplied so much iron sulfate that severe leaf scorching occurred.  A little fertilizer goes a long way, especially with small plants.

I fertilize in early spring around the end of March using all the organic fertilizer and soil amendments that I can obtain.  When I combine ingredients I try to duplicate natural fertilizer analysis.  For instance, in canola meal (6-2-1) and in fish meal (3-2-1) the nitrogen is two to three times that of phosphorus and three to six times that of potassium.  Three advantages of organic fertilizers over their chemical counterparts is in their trace element and humic content and in their extended time release of nutrients.  Some fertilizers in the following list contain up to 34 trace elements, while seaweed is reported to contain every element presently known.


Blood meal: nitrogen and trace.
Bone meal: phosphorus and calcium and trace.
Fish meal: complete N-P-K and calcium and trace.
Canola meal: complete N-P-K and trace.
Cottonseed meal: complete N-P-K and trace.
Powdered alfalfa: complete N-P-K and trace.
Worm castings: complete N-P-K and trace.
Powdered rock phosphate: phosphorus and 32 trace.
Green sand: potassium and 34 trace.
Kelp meal: potassium and all trace.
Dolomite: calcium and magnesium. 

My base organic fertilizer and filler recipes in volume parts are as follows:

 Fertilizer Recipe:
   2 parts fish meal
   2 parts canola meal
   2 parts alfalfa
   1 part worm castings
   1 part dolomite lime
   part rock phosphate
   part bone meal
   part kelp meal
   part green sand

 Filler Recipe:
   5 parts sand
   5 parts double screened fir bark or 5 parts composted fish waste.

The filler, equal in volume to the fertilizer total, is used to prevent clumping of the meal type fertilizers and to minimize the dust problem associated with mixing finely ground or powdered materials.


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