Question
Hello: I have a rhododendron plant about three or four years old. It had a few blossoms two years ago; nothing last uyear and it looks like nothing this year. What is the best fertilizer for these plants and should they get a LOT of sunlight?
Many thanks, bob miller, riverview, New Brunswick, Canada

Answer
Such a simple question, Bob -- I hope this long answer doesn't make you give up before you finish reading.  It took me a few days to check the Canadian angle on this.  I am, after all, the Long Island Gardener.

Here goes:

Rhododendrons are more complex than people give them credit for.  They are the foundation plant that you ignore until something goes wrong.  Often the "wrong" is so unobtrusive the problem lasts for years - or forever.

So the first thing I think you should do is get a good dry soil sample from under your Rhododendron soil.  Analyze the pH and other elements -- or lack of them.  Maybe it's lurking in the soil, your problem.

And while you're doing that, Bob, I will tell you what I tell almost everyone who writes in here with a problem.  Run down to your local Starbucks -- if there is one in your neck of the Continent -- and get one of their free bags of used coffee grounds.  In fact, for your Rhododendron, get a half dozen if you can.

They're FREE and they're the best thing going.

Besides repelling slugs, coffee grounds attract earthworms.  And they are a GREAT soil conditioner.  They come in these slick little sealed bags from Starbucks and they are terrific.

Including your Rhododendron in your morning coffee routine will only bring you good things.

Back to the matter at hand.  You have one of two problems - or perhaps both.

Let's see what the problem is here.

Is it the weather?

Rhododendrons thrive in light shade.  They are more inclined to leaf damage from sun that is too bright -- sunscorch on the leaves and drying out from lack of moisture would cause more problems, especially in warmer climates than yours.

In your cold Canadian climates, a rhododendron would actually perform best situated with protection from wind and from strong winter sun.  

Deep shade would inhibit flowering, but that's not the same as protected, partial sun.  Different varieties of course have different needs.

If your Rhodie is in deep shade, you can try light pruning of surrounding growth, including careful removal of lower deciduous branches to increase light and circulation below.

Rhododendron iron clads are famous for their hardiness.  Severe winters are sometimes difficult for certain Rhodies, which experience severe wind- or freezer-burn as the ground freezes and wind or sun dry out the foliage.  Some people run around protecting their Rhododendrons the way others protect their high-maintenance Roses and other shrubs.  You can burlap-wrap it, you can spray the foliage as protection.  I think it is easier to replace them with varieties more suited to the Zone.

Next, regarding Fertilizer.

Have you fertilized your Rhododendron?  How late in the summer did you do that?  Properly fertilized Rhododendrons may not be strong enough to take the normal winter chill without some damage.  Late summer growth of any shrub, triggered by late application of fertilizer, risks damage to buds at the end of long, gangly stems.  Favored fertilizers for spring application: Hollytone, an organic 4-6-4 formula containing sulfur, Magnesium, Iron, Calcium and trace elements.  Magnesium, Calcium and Iron may have nothing to do with flower production, a healthy Rhododendron can take more severe weather without damage.  And no, the Coffee routine will not take the place of a good dose of fertilizer.

Which brings us to the work of Dr. Robert Ticknor and researchers at Oregon State University.

After discovering that Rhododendrons need more Nitrogen than previously thought, Ticknor decided to develop what he believed was the perfect Rhododendron fertilizer.  It had a 10-6-4 formula.  This is good as a general rule.  

I think you should put your Rhododendron on a maintenance formula of this stuff and see how it does.  

And sporadically, to encourage flowering, try an application of superphosphate around the drip zone to build up buds.  

Note that this is very different from the commonly available 30-10-10 "evergreens" formula sold by most garden suppliers.  That much Nitrogen is great for foliage.  But it will do nothing for flowers and if applied past late spring will inevitably stimulate leaves well past the frost safety zone in autumn.  It will also inhibit flower production.
 
Feeding a Rhododendron during the Summer (which begins around June 21) is always asking for trouble.  

In your region, it produces tender growth that is vulnerable to early frost.  If damaged growth has also developed buds for next spring flower flush, flowering would most certainly be compromised as well.  

On the other hand, a regular deep Summer watering will encourage deeper roots for these shallow-rooted shrubs, increasing the odds they will get through Winter unscathed.
If you find there are buds but they fail to open, there could be several explanations.  

Blasting is a symptom of fungus.  It is also a symptom of frost damage, the predictable result of late fertilization. Buds turn brown or black and fail to open.  No flowers.

Did you trim or prune your Rhododendron?  Not likely since it was new.  But the question has to be raised, at least for future awareness.  Flower buds form late in the season, through early fall; pruning will remove them.  Although new leaves are created every spring, Rhododendrons take another year to produce flowers.

Cold weather will sometimes destroy flower buds.  It sounds very suspicious to me that your Rhododendron blossomed for two years -- then after a year of growing at your house, flowering stopped.  And although all Rhododendrons will thrive in shade, some do need more sun than others to bloom.
Warm weather too early in the year kicks off vulnerable spring growth that will be killed by freezing temperatures.

Can you tell me the name of your Rhododendron, Bob?  If we can look it up and get specifics, we might be able to confirm it is strong enough to grow at your house.

And in the meantime, you can try these other remedies.  See if things get better.  Remember The Starbucks.

Thanks for writing.