Allan Murray is a past president and Bronze Medal recipient of the Cowichan Valley Rhododendron Society. His 2-acre garden, Misty View Farm at Cobble Hill, reflects the use of this fertilizer that he developed about 18 years ago. The mix is used to feed most of the alpines, trees and rhododendrons in the 20-year-old garden.
The quality and quantity of the flowers on your rhododendrons next year depend greatly on what and when you feed them this year.
Nitrogen, phosphorus and potassium are three elements that you need to keep plant growth in proper balance, in terms of growth,
flower set and leaf colour. A properly fertilized plant is hardier and can withstand more cold than one underfed.
Applied in the right amount:
Nitrogen: promotes normal growth and healthy green leaves.
Phosphorus: promotes root development and increases flower production; without ample phosphorus the quality
and quantity of blooms is reduced.
Potassium: has overall value to plants providing them with vigour and disease resistance. Rhododendrons without
sufficient potassium will look unhealthy and their appearance will disappoint you.
Deficiencies of these elements in Rhododendrons show the following symptoms:
Nitrogen: lighter leaves than normal for the variety, stunted growth; stalks too slender; few new side shoots.
Phosphorus: foliage darker than normal; shoot growth retarded; sometimes a yellowing between the veins on the
lower leaves; generally the leaf colouring is purplish, especially on the stalks.
Potassium: localized in older lower leaves; dead leaf margins and tips occur and the leaves appear mottled, yellowing
being at leaf margins and moving towards the centre, finally the margins curling under and turning brown; older leaves
usually drop.
Too much nitrogen will burn the foliage, destroy the young feeder roots and force excessive vegetative growth. Burn is
generally at the ends and edges of leaves and will be on any area of the whole plant, not just the sunny side. Give as much
water as possible to wash the fertilizer out of the roots.
Before you fertilize:
• Look for a soil pH of 4.5 to 5.5
• Examine the previous year's growth, bud set, leaf colour and size
• Note the position of the plant: sun/shade, water, especially overhead water
This information will help you determine how much fertilizer your specific plants need. For example,
water use or rainfall will determine the amount of fertilizer needed: overhead and sprinkler systems may require
more fertilizer applications per year as the water will wash away some of the fertilizer; drip or restricted water
systems may reduce the number of fertilizer applications per season.
CHEMICAL FERTILIZER
Chemical fertilizer comprises non-organic material; it is quick acting and usually cheaper and more convenient
than organic fertilizer. Green Valley's 10-8-6 rhododendron fertilizer, which was developed by Tom Brown, Head of the
Geology' Department at UBC and a member of the Vancouver Rhododendron Chapter, comprises:
- 10% nitrogen
- 8% phosphorus
- 6% potassium
- 5% magnesium
- 1.8% iron
- 6% sulphur and trace minerals
To apply chemical fertilizer: mid March first application; mid April second application; first week of May third
application.
Later fertilization is not recommended: soft succulent growth is the first to be damaged by a freeze; the plants need to
be in a mature woody condition for when the cold weather arrives. Sprinkle 3-4 oz. per square yard at the drip line of the
rhododendron taking care to avoid spills on the leaves; it will bum the leaves if left on.
ORGANIC FERTILIZER
Allan's organic fertilizer is made of once living materials or byproducts of things once living, e.g., blood meal, bone meal,
manure, compost. It is slower than chemical fertilizer but feeds longer. Organic fertilizers are ideal for rhododendrons
from the standpoint of both nutrients and humus. It is made as follows (measure by volume):
- 4 parts alfalfa meal
- 4 parts canola meal
- 4 parts blood meal
- 2 parts dolomite lime
- 1 part rock phosphate
- 1 part bone meal
- 1 part kelp meal
- 1 part greensand
To apply organic fertilizer: The fertilizer, with its trace elements, will release slowly for 2-3 years but should be
applied every year for maximum benefit. When to apply depends on the weather. Generally speaking: beginning of March
spread magnesium sulphate (Epsom salts) at the rate of 1/2 lb. per 100 square feet; mid-March sprinkle organic fertilizer
around the drip line of the plant, 2-3 oz. (by volume) per square yard.
Siegfried Kemmler is a member of the Cowichan Valley Chapter on Vancouver Island, British Columbia.