Fall for Rhododendrons

by Norman Todd  early 1990’s


In the gardens of coastal British Columbia the best time to plant broad leaf shrubs and trees is the fall.  In October and November, the soil is still warm, the autumn rains have begun, and conditions are the best for the development of new roots.  By planting in the fall, plants will become established before the burgeoning demands of spring signal the roots to send more nutrients for swelling blossoms and expanding leaves.


As long as the ground is not frozen the roots of the broad leaf evergreens are purposefully working.  To the human eye there does not seem to be much going on above soil level but the plant is still photosynthesizing.  Sugars and starches are being manufactured from the nutrients the searching roots are pumping up to the leaves.


When a plant is dug up and replanted, or taken from a container and put in the ground, the roots are always disturbed.  To assist them to grow into soil in their new environment, the roots of a potted plant must be teased apart quite vigorously so that they can make intimate contact with the welcoming environment of the new, freshly prepared home.  For rhododendrons this means an open, friable soil with a high content of organic matter, located in a well-drained site.  Rhododendrons hate standing in water:  they drown.  Their roots need loose, airy, humusy soil around them.


One of the best and easiest ways to give them this open soil and the right acidity is to incorporate bark mulch.  Up to 50% of the mix in the planting soil can be bark mulch – not too coarse and not dust either.  Many people like to use peat moss, but much of the moss available today is so fine that if it ever dries out, it is very difficult to moisten it again.  Furthermore, peat moss breaks down far more quickly than bark mulch.  The number one cardinal rule for growing rhododendrons is that the roots must never go dry.

If your soil is a black humus-rich loam, you are fortunate indeed and no amendment may be necessary.  You can plant the rhododendron as deeply as it was in the pot.  But if the natural soil is a heavy clay, then 8 to 12 cm of coarse sand should be dug in to a depth of 30 cm before the addition of the bark mulch.  Composted leaves, preferably oak, can be used instead of bark mulch.  If the soil is heavy clay, you can plant very shallowly, bringing the amended soil higher than the soil level at which the plant was previously growing.  Mixing some fertilizer in the planting hole is beneficial.  Avoid using animal manures that are less than a year old.  An excellent fertilizer you can make yourself is a mix of ground meals:  4 parts of canola, 4 of alfalfa, 4 of blood, 1 of bone and 1 of kelp plus 2 of dolomite lime and 1 of rock phosphate.


Most of the rhododendrons we grow are hybrids.  Hybrids result when two or more of the plants found growing in the ‘wild’ – the species – have been combined by cross-pollination.  Most of the species that have been used to make these popular hybrids come from climates that have relatively dry winters and high summer rainfall.  In the Victoria are, we have the reverse.  One of the consequences of our persistent winter rainfall is that the nutrients that are at the surface of the soil get leached downwards quite quickly.  Rhododendrons, which are very shallow-rooted can in fact be starved in our winter because all the goodness goes past them before they can catch their fair share.  If the roots can’t supply the nourishment, then the plant will draw from its stored reserves – its rainy day account, leaving less for production of flowers and foliage.  The regime that I follow is to feed sparingly with a chemical fertilizer (10-8-6 with all the minor elements), five times a year.  I start in November and feed every two months with the last feed on Canada Day.  Rhododendrons are not gross feeders, but they do like three meals a day – light ones in the winter, heavier ones in the spring.


Choosing the right variety for the right spot is important.  There are now more than 23,000 registered hybrids.  This is a daunting number but, from its very size, you can be sure that the right plant for your particular location can be found.  Some need almost no direct sun, some need full sun, some will grow to be trees and some will never be more than 2 cm high, some will bloom in December and some will bloom as late as August, some are deciduous, most are evergreen.  Colours range through the entire spectrum except for the pure gentian blue.  Even that is now a possibility with our increased understanding of genetics.


One of the convenient characteristics of rhododendrons is that because they’re shallow-rooted they are very portable.  For gardeners who would really like their plants to come with wheels so that they can move things around until their concept of horticultural artistic perfection is reached, rhododendrons come quite close to being ideal.  They can be moved at any time of the year.  The huge majority are planted in the spring and they do very well.  The very best time, however, for gardeners in our area is the fall.

(This article was discovered in the VRS archives, undated, but probably first published in the early 1990’s. It is both timeless and timely.)