How to trim rhododendrons?

Pruning Rhododendrons – How To Prune Rhododendrons

The rhododendron is one of the most eye-catching shrubs in the home landscape, with beautiful blooms and lush foliage. Being popular shrubs in many landscapes, the topic of how to trim a rhododendron bush, including wild varieties like mountain laurel, is a frequently asked question.

Pruning Rhododendron Guide

Although there is often little need for pruning rhododendrons, especially in naturalized settings, these shrubs respond well to the occasional trimming. In fact, excessive growth may require a heavy pruning. Trimming rhododendrons is typically done for maintenance, shaping, and rejuvenation- as is the case for overgrown plants.

The most common type of pruning is maintenance pruning, which simply involves the removal of spent flowers and old, dead wood. It is important to remove the flower stems from the shrub once blooming has ceased. Allowing these dead flower clusters to remain can actually reduce the following year’s flowering. Cut near the base of the old flower cluster. Also, remove dead or diseased parts of the shrub, following the branch back to healthy wood and making your cut at that point.

Best Time for Trimming Rhododendrons

According to most professional landscapers, the ideal time for pruning rhododendrons is late winter, while the plant is dormant. However, any time between the first frost in fall and the last frost in spring (while the sap is low) will work.

Immediately following its lush spring growth, as new foliage is still hardening off, is one of the worst times for trimming rhododendrons. This will likely inhibit blooming.

How to Prune Rhododendrons

If you are considering pruning, you should probably plan to fertilize your shrub in late fall the year before. Doing so afterward may result in leggy growth. Since buds form on next year’s flowers, by the time blooming has stopped, they are already well advanced. Therefore, as the flowers fade, trim no more than 15-20 inches off the strongest branches. Cut back the plant to expose the inner branches. Follow the branch down to the last whorl of leaves you want to keep and cut just above those leaves, about 1/4 inch above the topmost leaf in this cluster.

Large, overgrown rhododendrons can be cut 12-15 inches from the ground when necessary. Rhododendrons often have three or more main branches rising from the crown of the plant. Each of these primary branches should be cut at a different height to produce a more natural-looking shrub. Cut about 1/2 to 3/4 of an inch just above a latent bud. Pruning above a cluster of two or three buds is even better.

Sometimes more severe pruning may be necessary, requiring cutting to about 6 inches or so from the ground. Their adventitious buds at the base of the plant will send up new shoots, but keep in mind that flowering usually will not occur for up to two or three years after this heavy pruning.

Ciscoe’s To-do List: Rhody work: When to prune, when to move

The best time to prune rhododendrons is right after the flowers fade. Then, the new growth that occurs after pruning will have time to set flower buds that will bloom the following spring. Branches will emerge most anywhere you cut because rhodies have latent buds where leaves once were. Plants that are getting too big or have become leggy can literally be cut to 6 inches above the ground and will quickly bounce back with thick branching.

Move rhododendrons before the flowers fade

Sometimes it’s a better idea to move rhodies rather than cut them back. Rhodies that are too tall for their space usually grow back so quickly that, by the time they’re ready to flower again, they’re bigger than before you pruned them. Leggy rhodies generally are in too much shade and will do much better if they receive morning light. If you want to move a rhody to a better situation, it’s best to do it before it starts growing actively (even if it’s in full bloom). While in the chiropractor’s waiting room after dealing with the gargantuan rootball, you can contemplate whether you have any small rhodies that need moving.

Don’t wipe out nature’s helpers

Most of us know that lady beetles are good bugs, but look closely and you’ll be amazed at the number of beneficial insects that work for you in your garden. Beneficial insects generally move fast and have big jaws so they can catch and eat the troublemakers. Larvae that look like caterpillars are often beneficial as well. If you notice little green maggots gobbling up oodles of aphids, it is probably a syrphid fly maggot. Keep an eye out for clever lacewing larvae that outwit their enemies by disguising themselves with the bodies of aphids they sucked dry with jaws that double as hollow feeding tubes. Bald-face hornets put on a great show as well, swooping down like eagles to carry off crane flies. And don’t forget the spiders. Considered the most beneficial of all, spiders eat more bad bugs than any other critter in the garden. If you value the work that beneficials do for you in your garden, don’t apply chemical pesticides. Beneficials are highly susceptible to insecticides, and they are much slower to repopulate than are the troublemakers. If you wipe them out, it will take years before they re-establish in high enough numbers to effectively keep the bad guys in check.

Need help learning how to prune rhododendrons & azaleas? We’ve got you covered! Fortunately, you can fine-prune these beautiful plants just about any time of year as long as you take care not to cut off all the flower buds.

Need pruning tools? Courtesy of our friends at Fiskars, we’ll be giving away a pair of bypass pruners and a folding handsaw to one lucky reader. Read on for details…(this giveaway is now closed.)

Before we began, this foundation evergreen rhodie was filled with dead material.

The reality is, sometimes you get to pruning when you have the time. Not every plant can be pruned successfully at the same time as every other plant, but rhodies are quite forgiving on timing.

Simple rules to follow: don’t prune them if they are flowering (unless you want to cut a few flowers for a vase) or about to flower (you’ll miss out on their best season) or if it is frozen outside (and you might break a branch along the way). And, if pruning in spring, summer or autumn, be sure to check for hornet nests. They love to build their summer homes in rhodies and other medium-sized shrubs — especially when they’re dense with dead wood! But remember: rules are made to be broken, and your garden and plants may require a bit of rule bending.

In all honestly, we knew how to prune rhododendrons, but we just hadn’t gotten to cleaning this one up in a couple of years. Trees and shrubs grow just fine without us cutting, shaping and coiffing them all the time. But, the messy interior of this rhodie was an eye-sore we could no longer ignore. So, on a day that eventually topped 90F, we spent a couple of hours beautifying this blooming beauty. We got done well before temperatures soared. No sunburns here and by far the plant is in better shape than before.

All we needed for the job: a pair of bypass pruners, a folding handsaw, and a tarp.

Before pruning, this rhodie had lots of interior dead wood. To make cleaning up all the detritus we would be cutting, we spread a sheet of plastic under the shrub before we began.
A tarp works great too.

Did you know that all azaleas are rhododendrons? Yep, Rhododendron is actually a genus of plants into which the azaleas are also classified! So, of course it makes sense that knowing how to prune rhododendrons would set you up for success in pruning your azaleas as well. Generally speaking, azaleas are twiggier, so you’ll just be in for making a lot more small cuts than you would on bigger rhodies.

After the rhodie was cleaned of dead wood, we removed rubbing branches.

Fine pruning is not shearing off the outside to “shape” a plant. Often the sign of a good pruner is you can hardly tell they’ve cut anything from your plants. Still, when we were done with interior cuts, sunlight & air could flow through the plant. This helps reduce pest and disease issues in many garden situations.

After we finished pruning & deadheading this rhodie,
it almost looks as though nothing was done from this side.

Because the plant hides a foundation & utility area, we chose not to lift all the lower limbs, which would expose the ugly part of the house from a nearby patio. Eventually, we may decide to plant something under this rhodie at which time, we will limb it up. But no way was planting going to happen during a heatwave in July!

Ready to dive in but still need the right tools for the job? In the comment section for this post, tell us about the most beautiful rhodie or azalea in your garden. Is it colorful, fragrant, filled with pollinators, or what makes it a plant you’d love to get cleaned up.

One person will be chosen via random.org on Monday, July 28, 2014 to win a pair of Large Bypass Pruners and a 7″ PowerTooth Folding Handsaw to be shipped to them from our friends at Fiskars. Comment period to enter closes at midnight PDT on Sunday, July 27, 2014. (This giveaway is now closed.)

(More details on our relationship as paid writer with Fiskars at the end of this post, but to be clear we have received no compensation for this post or tool giveaway. And, learn more about using random.org at the end of this article.)

Next up: a Visual step-by-step guide to deadheading your rhododendrons.

It’s a total myth that your rhodies won’t bloom if you don’t deadhead them!

Somebody’s mother a long time ago must have lied to their kids to get them to do the deadheading dirty work for her. In fact, rhodies will bloom abundantly even if you never remove a single deadhead. But, you’ll have to look at those ratty seed heads all over your evergreens throughout the growing season. And, you’ll be looking at a jumbled mass of dead material, piling up in the interior of your plant over the years. Removing deadheads when you prune rhodies is really an aesthetic choice not a critical move to keep them flowering.

If you want to deadhead your rhodies, here’s how:

After a rhodie has bloomed, if you wish to deadhead it, grip the old flower head between your fingers, taking care not to damage flower or leaf buds or stems surrounding the old parts.

Carefully, bend the flower remnant from between any new growth.
We like to do this by hand rather than with clippers.
Hint: you might want to wear gloves on rhodies that are stickier than this one.

Rhodie deadheads usually snap out easily & cleanly.
If part of the deadhead remains, pinch that out as well.

And one final lesson: know your buds. When learning how to prune rhododendrons, it’s important to know a flower bud from leaf and stem growth. This way, you can quickly avoid cutting off all the flowers when you fine prune your plants. (And, if you’re hiring a maintenance team, you can quiz them about what’s what on your plant. If, for instance, they want to shear your plants into a hedge in February, odds are they’ll be cut off all your potential flowers.)

Within a month or so of blooming, rhodies will put on new buds like this that will flower in the following spring. (Okay, sometimes they open in fall or winter, too.)

Some rhodie branches will form vegetative buds that open leaves & new stems like these.

Pruned and deadheaded properly (or even not at all), rhodies and azaleas can live for many generations. They provide evergreen interest during the dull days of winter, a blast of spring color & pollinator forage in every color of the rainbow, and they provide the backdrop against which summer & fall color will glow.

Next May or June we’ll enjoy another week or two of blood red blooms on this beautiful rhodie. Fact is: it would have bloomed whether we pruned it or not!

(Fine Print: Garden Mentors is a paid writer for Fiskars. However, we have received no compensation for this post. The winner of the Fiskars tools will be selected by random.org. First to comment is number one and subsequent comments will be counted sequentially. Contact information will be provided to Fiskars, which will ship the tools to the winner. This giveaway is now closed. )

Wondering when to prune? Right after azaleas and rhododendrons are finished blooming is the best time to do any pruning they may need.

These shrubs make blooms that flower on the prior year’s wood. So you have to be careful when pruning them since buds for next spring will be made this summer, and you don’t want to prune those off. Don’t wait to prune your azaleas and rhododendrons until late summer or even later, or you probably won’t get the flowers you’re looking for next year.

For rhododendrons with large leaves, you want to prune right above their “growth joints” the place where the plant is starting to grow. Just beneath that point is where dormant buds are, so take care not to cut them off. Small-leafed rhododendrons and azaleas can be pruned anywhere along their stems.

Cutting back a rhododendron heavily can stop the plant from flowering for a year or two. You can prune pretty heavily if needed to get better shape for your bushes, just know that they may not flower for a year or more if you do so.

What should you prune away? Check the inside limbs for any that are dead or look weak. Any limbs on the ground or crossing over other limbs should be pruned away, too. You’ll be giving your bush better air circulation and a less hospitable environment for insects and disease.

Deciduous azaleas (that lose their leaves in the winter) differ from rhododendrons in that they can actually be sheared into a hedge. Anywhere you cut them on their stem, they will branch out. If you’re pruning evergreen azaleas, you can shear them after they flower.

Here’s a great how-to video on how to prune your rhododendrons:

How to Prune Rhododendrons and Evergreen Azaleas

Scientifically, they all belong to the genus Rhododendron. From there, they break down thousands of species, varieties and cultivars. Fortunately, the same basic pruning principles apply to all of them. So, even if you aren’t sure which kind of rhodie was planted in your garden, perhaps generations ago, these tips should help you keep it healthy and blooming beautifully for years to come.

Rhododendrons can range in size from tiny miniatures like R. impeditum, which gets no bigger than about a foot tall and wide, to towering, bushy trees that create colorful spring canopies above our heads. Their leaves may be shiny or dull, green, grey or variegated, fuzzy beneath or bare, curled at the edges or flat. Plus, you’ll find a rhodie blossom in nearly every color of the rainbow. And, although we generally think of these plants as spring bloomers, some varieties like the ‘Christmas Cheer’ cultivar will bloom in the dead of winter. And those that bloom the earliest in the year will sometimes surprise us with a second bloom later in the fall. Plus, the adaptable evergreen rhodendron provides great year-round interest in just about any garden – just be sure to match your plant choice to your planting zone when buying your plants. Rhodies are fairly simple to prune. Although many can bounce back from a heavy cutting to the ground, it may take years to regrow. Instead, pruning a little bit each year is the ideal way to keep your rhodies and azaleas looking good all the time.

Take note: Rhododendrons will bloom even if you don’t deadhead them. Deadheading, or removing flower clusters using pruning shears after they have begun to fade is really more of an aesthetic choice. Think about it: plants have flowers that bloom, are pollinated and become seedpods. In nature, nobody is out there cutting spent flowers off wild shrubs. And those plants bloom year after year in an attempt to replicate themselves by forming new seed, which it hopes will form new plants. The same is true in your own garden. If you do choose to deadhead, try to remove spent flower heads right after blooming and before the tender branching buds begin to grow, or you may find yourself breaking lots of new growth accidentally.

What’s wrong with my Rhododendron?

Glendoick Gardens Home page Mail Order Rhododendrons Glendoick Garden Centre

Glendoick Gardens, Perth PH2 7NS Rhododendrons and Azaleas.

Rhododendron Expert Glendoick’s Ken Cox discusses Rhododendron Problems, Pests & Diseases

Rhododendrons are quite easy to grow as long as you have well drained but moist acidic soil. They wont tolerate deep and dry shade and poor drainage/heavy clay soil. And they wont grow in areas with chalky/limey soil unless in raised beds.

Rhododendrons which are not happy tend to show this by their leaves turning yellow. There are several reasons that this may happen. So why has my rhododendron got yellow leaves? Reasons include: because they are too wet, too dry, planted too deep, starved (needing feeding), soil too alkaline, or soil is compacted:

Why does my rhododendron with yellow or chlorotic leaves:

This is a general sign of unhappiness and can have many causes:

1. Rhododendron drainage is poor: solution: lift plant and improve soil structure or move to better drained spot. You need to do this before the plant starts to suffer as root problems caused by poor drainage can be fatal.

2. Rhododendron is planted too deep? The rootball should be at or just below the soil surface. If you cant feel the rootball then it is probably planted too deep. Lift the plant. If it has been in for a long time you may see new roots forming near the soil surface. Replant at the correct level. Do not let piles of mulch bury the plant. 10cm of mulch is more than enough. Do not use membrane and bark as rhododendrons dont like this.

3. Rhododendron is starved. Apply fertiliser April-May to Late June.

4. Rhododendron is suffering from lack of water: often due to competition from tree roots. Feed and water the plant and it will probably pick up. Think about thinning the shade or moving the plant if it is suffering.

5. Soil is too alkaline (unlikely in Scotland) pH 6 and higher. Soil acidity is measured by the pH scale. 7 in neutral. Chalky soils tend to be pH 7-9. Rhododendrons need a pH of 4-6. If you have alkaline soil, you will need to build raised beds and plant in peaty soil. Avoid watering with tap water if it is chalky. Use rainwater collected from rooves instead. You can have your soil tested but it is better to ask around as gardeners generally know if the local soil is alkaline or acidic. In Scotland virtually all soil is naturally acidic. Farmers do lime soil for crops but this washes out over 3 years or so. You can lower/acidfy the soil pH before planting by applying sulphur as alumineum or iron sulphate.

6. Mineral Deficiency Rhododendrons need specific minerals. Most of these are found naturally in soils. But in alkaline soil, rhododendrons take up too much calcium and not enough iron. Adding iron chelates or sequestrine can help but most of this time this is an expensive waste of time. The chorosis is far more likely caused by one of the reasons listed above: poor drainage, stravation, drought or poor planting.

7. Soil Too Acid Some rhododendrons dislike very acidic soil. Particularly those in Section Pognanthum. We use dolomitic limestone on these to bring the pH to around 6 which they prefer. They grow on limestone in China.

Why has my rhododendron got crinkly leaves?

1. Caused by late Spring or early Autumn frosts

When the new growth buds begin to elongate before unfurling, the tips of the new leaves can be puckered by frosts causing them to distort when they unfurl. This is mainly cosmetic but if it keeps happening you may need to move a plant to a more sheltered site or cover it up in frosty weather.

2. Caused by Sap-suking insect (aphids). For insects use a contact or systemic insecticide. They tend to strike in dry weather and you’ll find them on our under the leaves.

photo (right) insect damage

Spots on rhododendron leaves. What causes it?

Fungal Disease

Powdery Mildew

symptoms are pale spots on upper leaf surface, with corresponding brown/grey patches underneath:

Control with fungicide from late May onwards.

Any rose fungicide will do: myclobutanil (systhane), fungus fighter, roseclear etc.

If the infections are mild, just live with it. Some hybrids such as ‘Elizabeth’ are very susceptible and are better destroyed.

Rust

Black spots on upper surface, lower surface with orange patches: Control with rose fungicides. Roseclear, etc.

a few rhododendron hybrids such as Anna Baldsiefen, Arctic Tern and Azaleodendrons are very susceptible.

Powdery mildew on deciduous azaleas.

This is a different species.

It tends to attack those with R. luteum and R. occidentale in their breeding: so paler colours.

It coats the leaf upper surface with white in late summer.

Unsighly but not usually harmful though if the plants defoliate early for a few years they can be weakened.

Black spots with no patches on leaf undersurface

Some varieties eg ‘Mrs GW Leak’ get this. Nothing to worry about, not a disease.

What has been eating my leaves?

Caterpillars

Holes in the leaves (internal) and irregular chunks of eaf removed (as opposed to notches around the edge) are the symptoms of catterpillars. Often they have gone by the time you notice. Some varieties are more commonly affected than others. Moving the plant away from overhanging branches or cutting them back can help. Insecticides can be used.

Frost Damage to flowers:

Flowers caught by frosty nights in Springtime: you can protect with fleece or move plants to a more sheltered site. by a wall, or in woodland for example. But try to avoid deep shade.

Why has my rhododendron died?

The causes of failure are listed in order: commonest cause of failure first.

1. Drainage:

More rhododendrons are killed by waterlogging and poor drainage than any other cause. If the soil is boggy, heavy clay or compacted, rhododendrons will not grow for long and will likely die. Dark brown dead roots= Phytophthora caused by poor drainage. (NOT the same as Sudden Oak Death.)

2. Incorrect Depth of Planting:

Rhododendrons are surface rooting and MUST NOT be planted deep. The top of the rootball should remain at or just below the soil surface. If you bury the roots under soil the plant will die. Beware of putting deep mulch on top of rootballs. Roots need to breathe.

3. Variety not hardy enough?

Check hardiness rating and for bark split. 2009-10 Winter was very severe all over Europe. Worst affected were plants in containers as the roots froze solid. Most rhododendrons and azaleas came through well and most of those damaged have grown from below, at Glendoick. Northern Europe had very severe conditions in Spring 2013 but what I observed many plants, especially dwarf lepidotes are producing young growth and will recover. Bark split is typically caused by late frosts in Spring or early Autumn frosts when sap is running in the stem of the plant. It can heal up but a bad case of barksplit can kill young plants. It can often take 6-9 months before you notice a sudden collapse. Check the stem at the base of the plant.

4. Vine weevils?

Look at stem just above ground. If bark has been eaten, usually girdling stem: vine weevil are probably the cause. The bark is eaten by white grubs in the soil usually in late summer or over winter. Adults notch the leaves in spring. Vine weevil are more of a problem in pots and indoors than in the garden. Most commercial plants are protected by compost insecticide. Beware amateur gifts of Primulas, strawberries and other weevil favourites in case there are grubs in the compost.

5. Honey fungus

(Armillaria mellea) Roots are full of black bootlaces with white core). Fungus comes from old treestumps. Very little can be done about this. You can try to use heavy duty polythene to prevent bootlaces getting through the soil. Some tricky species are particularly susceptible to honey fungus. (more details below)

What’s been eating or sucking sap from my rhododendron leaves?

1. Vine weevils and rhododendron

These nocturnal black flightless insects are more damaging at the larval stage as white grubs in the ground (see above). The adults cause cosmetic damage by notching the leaves of rhododendrons. This is usually where there are overhanging conifer or other branches. Cutting these back often solves the problem. Or move the plant into a more open site.

2. Caterpillars

These munch random shapes into leaves and sometimes holes in the leaves too. By the time you notice the damage they have normally hatched and gone. Cutting back overhanging branches can help prevent ending up on your leaves.

3. Scale Insect on rhododendrons and camellias

This is a serious problem in the UK now and getting worse. Few larger woodland gardens are free of it these days. Symptoms: Look for sticky black secretions on the upper surface. The small scale insects 1mm wide are like translucent circles stuck on the lower leaf. When they hatch they leave a white fluffy cotton-like patch on the lower leaf surface

The only way to get rid of it is to prevent the adults hatching and laying on the young leaves in summer. Deltametherin (sold commercially as Decies) and for amateurs as ‘Provanto’ is effective. Mix with a wetting agent for best results to help the pesticie adhere to the leaf. Other synthetic pyrethroids include lambda-cyhalothrin (Westland Resolva Pest Killer) and cypermethrin (e.g. Py Bug Killer). Petroleum oil has also been recommended.

As the insects are on the underside of the leaf, it needs a particular approach,: You need a very high pressure sprayer which makes tiny droplets of spray and add a wetting agent. You might need to spray twice in the season. It is crucial for the lower leaf surface to be coated/covered. The time to spray is June-July/August to stop the young newly hatched crawler insects moving fronm the old growth to the new young growth. At this point they are most vulnerable to insecticide. Old scales may remain on the plant for some time after the death of the insect. To check if treatment has worked, slide your thumb across a group of scales. If scales are dead, they will be hollow and the coverings will flake off easily. Living scales will leave a colored, wet residue on your thumbnail.

Hard pruning a badly infected camellia or rhododendron can be the best way to get rid of it. For a large garden with a bad outbreak I recommend a mixture of cutting back and burning and spraying. Get a contractor in to spray if you dont have access to a high pressure, small droplet commercial sprayer.

As the insects are on the underside of the leaf, it needs a particular approach,: You need a very high pressure sprayer which makes tiny droplets of spray and add a wetting agent. These two things allow both the upper and lower leaf surface to be coated/covered. The time to spray is June-July/August to stop the insects moving fronm the old growth to the new young growth.

The chemical we use is deltamethrin.

RHS WEBSITE has the following options

  • The synthetic pyrethroid insecticides deltamethrin, lambda-cyhalothrin cypermethrin: contact sprays.
  • Systemic neonicotinoid insecticide acetamiprid
  • Organic Sprays: fatty acids plant oils and extracts, natural pyrethrum. These pesticides have very little persistence and so may need several applications during the scale’s egg-hatching period.

4. Aphids on rhododendrons

Whitefly and greenfly can suddenly appear in large number on the underside of young leaves and on the stems, particularly in dry weather, sucking out the sap and causing puckered or wrinkled leaves.

Vireya rhododendrons seem particularly prone to attack.

Spray with a contact insecticide or systemic insecticide.

Pyrethroids: Deltamethrin (e.g. Baby Bio Houseplant Bug Killer, Provanto Ultimate Bug Killer, Sprayday Greenfly Killer, Provanto Ultimate Fruit & Vegetable Bug Killer),

lambda-cyhalothrin (e.g. Westland Resolva Bug Killer)

cypermethrin (e.g. Py Bug Killer).

Organics: Natural pyrethrum, Fatty acids, Plant oils.

If you dislike using chemicals, soap concentrates can be used. It may take several doses to clear the infestation.

With both chemicals and soap sprays, it is important to target the insects on the leaf undersides. This requires a fine spray to allow the water droplets to adhere. There are also a number of biological controls which are most effective under glass. These include Aphidoletes (midge) and Aphideius (predatory wasp).

5. Lacebug (Stephanitis) on rhododendron

A serious problem in some areas. There are two different species of these sap-sucking insects, one which attacks rhododendrons and the other azaleas. Lace bug damage on leaves causes a greyish, or whitish leaf upper surface with a discolouring on the lower surface. The effect is most severe on plants grown in full sun and in shade it may not be a problem, where there are more lacebug predators to keep them under control. This problem is becoming serious in parts of Germany and probably in adjacent countries.

Warm-Blooded Pests

Rhododendrons are toxic to many animals which usually know not to eat them, but there are cases of sheep and cattle poisoning from time to time. Horses and cattle are more likely to cause breakage than to graze your plants; this and the soil compaction that they can cause means that is advisable to keep farm animals fenced from your rhododendrons. Birds can cause considerable damage, both in dust bathing and in looking for food, especially around small, newly planted stock. Indoors and in frames, mice and voles can be troublesome, especially in cold and snowy winters.

Deer and rabbits. Both these animals can cause great destruction in the rhododendron garden. We find Scottish rabbits can eat most evergreen azaleas and species deciduous azaleas such as R. albrechtii. When grazing is lush and there is plenty to eat, rhododendrons are often left well alone. The problems usually occur when there is snow on the ground, especially when the ground is frozen solid. Often rhododendrons are one of the few evergreen plants which the deer and rabbits can easily reach and so considerable damage such as the loss of leaf tips, branch tips or more seriously, the stripping of the bark, may occur. Ideally fencing is the perfect option but this is expensive, especially where deer are concerned. To fence effectively against rabbits, use netting at least 1 metre wide and bury 10-15cms in the ground to prevent them from digging underneath. In really severe infestations, it may be necessary to poison rabbit warrens using sodium cyanide or similar gasses. Such chemicals are potentially very dangerous and countries have strict regulations as to who is allowed to use them. An effective deer fence needs to be 2m high for Roe deer (the most common deer species in the U.K.) but may need to be higher where larger species of deer occur. Several commercial animal repellents have been developed over the years. The most common is aluminium ammonium sulphate (sold under the name ‘scoot’, ‘stay off’ etc). Grazers, a calcium compound is effective as a foliar spray to keep rabbits from grazing plants.

In a last resort, someone will need get a gun out, as the populations of deer and rabbits in some areas are far larger than the environment can sustain.

Rhododendrons: Other Challenges and problems

Sunburn

Is quite common after hot summers in many regions. Mild sunburn turns leaves bright yellow while a more severe attack will burn the leaves to a crisp. Move sunburned varieties into a more shady place if the problem persists. In Scotland sunburn is rare!

Lichens

Often form on old, straggly specimens, particularly when they have few leaves and are lacking in vigour. Evergreen and deciduous azaleas seem particularly prone. You can scrub the lichen off, but often, it is a sign of poor soil conditions, lack of fertiliser or old age, and it can best be dealt with by rejuvenating the plants by pruning and fertiliser or by throwing them out and starting again. Lichen tends to grow on weak plants rather than causing the weakness.

Phythopthora and other serious fungal diseases

Root rot/wilt

Phytophthora cinnamoni is a root disease which is usually fatal and often kills a plant extremely quickly. The symptoms are (usually) a sudden collapse during the growing season, usually of the whole plant or several plants in one area of the garden. Check the roots and cut into the stem of the plant. The disease is characterised by the roots turning a deep brown colour (rather than white as healthy growing roots should be). If you scrape away the bark at ground level, you will find the cambium layer below the bark has been stained a dark reddish-brown colour. The disease is caused by inadequate drainage and warm or hot soil temperatures. The combination of these is very often fatal to rhododendrons. It is most common in areas with hot summers but can occur anywhere if poor drainage is allowed to occur. Phythopthora cinnamomii is most active at a soil temperature of 29-22C (68-72F). Plants in containers are particularly susceptible, especially if watered overhead with sprinklers.

Root rot susceptible rhododendron varieties:

  • Most members of subsections Taliensia (e.g. R. phaeochrysum),
  • subsection Neriiflora (e.g. R. dichroanthum),
  • Species such as R. souliei and R. lanatum.
  • Most larger yellow hybrids (which are almost all derived from R. wardii), such as ‘Hotei’ and ‘Goldkrone’.
  • Some smaller leaved or alpine species and hybrids especially Section Pogonantha.
  • Drought and flooding makes plants more susceptible.
  • If you grow any of these species, make sure that drainage is perfect. Mound plant or used raised beds if you have high rainfall.

Root rot avoidance:

1. Ensure that the planting area is well prepared with coarse organic and inorganic matter to ensure free drainage and maintain aeration in the soil. Freshly composted bark has been shown to have some root-rot resistant properties

2. Ensure that the soil where the rhododendrons are growing is not allowed to become compacted by people or animals walking over it.

3. In warmer climates, growing in shade and mulching will help to keep the soil temperature down. In Germany, most varieties of rhododendron are grafted, which increases tolerance of poorly-drained soils. In areas with heavy clay soils, the best practice is often to plant above soil level, either in raised beds or by mounding up the soil around the root of the plant.

Stem Diebacks

(Botryosphaeria (canker), Phytophthora cactorum, P. ramorum, P. kernoviae) These can be serious diseases of rhododendrons particularly in regions with considerable summer heat and or humidity. They are characterised by the sudden wilting and death of a branch or part of a plant and tend to attack where plants have been physically damaged in some way. Check the inside of the wilted stem for diagnosis: it will normally have turned brown, usually with a dark reddish-brown core. The dead stem must be cut out well into green wood in the healthy part of the plant and the dead branch should be disposed of or burnt but not composted. Ensure secateurs are sterilised using bleach or alcohol to avoid spreading the disease. Good hygiene, adequate air circulation and moderate use of fertiliser all help prevent the diseases. Drought stress increases susceptibility to the disease. Botryosphaeria can begin as leaf spot and go on to cause stem dieback. There is no effective chemical control for this disease. For Phytophthora, fungicides such as mancozeb, metalaxyl and fosetyl-aluminium can be used but not by amateurs.

Phythopthora ramorum (Sudden Oak Death)

A recently described disease which has caused much worry. I suspect it has been around for many years and that scientists have only just recognised it as a separate species. The disease has been found on a wide variety of plants including Rhododendron, Camellia, Viburnum, Pieris, Kalmia, Taxus, and the symptoms are similar to those of Botryosphaeria die back. It can spread from Rhododendrons onto other plants such as larch trees which are usually killed by it. Rhododendron ponticum is by far the worst host for it. Indeed most outbreaks in gardens seem to be traced back to infection on R. ponticum. if you can get rid of this species, I’d recommend doing so. Thought to be spread by water-splashing, its effects on rhododendrons are often not very severe, causing limited stem die-back.

Symptoms

The characteristic symptoms are shown in the photos below. Look out for the dieback starting as staining up the petiole from the base of the leaf. If you see symptoms like this you should have your plants tested. Outbreaks of this disease are required to be notified to agricultural authorities and infected plants must be destroyed.

I suspect that over time this will just be perceived as just another fungal disease, particularly prevalent in mild and warm areas such as California and Cornwall, England and which we will just have to learn to live with.

Our nursery is regularly tested for the disease.

Armillaria Honey Fungus

This fungus can attack and kill rhododendrons in the woodland garden or areas where tree stumps are prevalent. Sometimes sending up conspicuous fruiting bodies, the main part of the fungus lives underground, sending out vigorous rhizomorphs (commonly known as bootlaces), which colonise both dead and living tree and plant roots. Gardens with lots of old tree-stumps in the ground are at most risk Of course, one can glibly recommend that all tree roots be removed but this is impossible with the enormous root systems mature broad leaf trees such as elm, beech or sycamore. Digging out a dead plant killed by honey fungus will usually reveal the tell-tale black bootlaces which have run through the root ball and into the stem, virtually strangling it in severe cases. Symptoms vary from yellowing of leaves, poor leaf retention, poor growth length, partial dieback of branches on the plant, or sudden total collapse. Often a last gasp attempt to flower heavily will be made. There is little doubt that stressed plants (by water logging, drought etc) are more susceptible to the fungus. Certain species such as R. lacteum, most of subsection Taliensia and many in subsection Neriiflora are particularly at risk, while many hybrids seem to be able to withstand having the fungus in their roots, though it can reduce their vigour.

There is very little else you can do about Armillaria, apart from using artificial barriers to keep it from spreading. The fungus usually remains close to the surface, seldom deeper than 30-45cm in lighter soils, though it is said to go deeper in clay soils, so heavy duty plastic and other materials can be used to make impermeable barriers rather like underground walls. Alternatively, plants can have an underground wall made in a circle around the root ball. Raised beds with a solid lining are another solution, but ensure there is adequate drainage: try to construct on a slope if possible. We have tried using permeable membranes but the fungus appears to be able to penetrate these.

Petal Blight (Ovulinia)

This fungal disease is one of the most upsetting diseases as it ruins the flowers you have waited so long to enjoy. Infected flowers first exhibit small spots which appear water-soaked. These rapidly enlarge turning the petals into a slimy grey mass with sinks limply onto the leaves below. It can even strike before the flowers open. It takes 2-3 days for the flowers to be completely ruined and a whole bush or group of plants can quickly be affected. The destroyed petals dry stick to the foliage and white patches which turn to black fruiting bodies are produced which will infect the following year’s flowers. The disease usually occurs during moist weather at flowering time, especially if accompanied by warmth and poor air circulation. Watering overhead so that it wets the flowers is to be avoided if possible. If you have an infected plant brought in from another source, it is well worth removing all blooms immediately so that the spores cannot be spread. If you have a large amount of infection, it may be necessary to spray with a fungicide containing myclobutanil, from when buds start to show colour at weekly intervals until the flowers go over. Keeping the foliage and especially the flowers as dry as possible is the best way to avoid the disease.

Azalea Galls

This unsightly fungal disease is characterised by green, pink or red swellings on the leaves and shoots. It is found on wild populations of some species and also found in gardens on some species and hybrids, and especially on evergreen/Japanese azaleas and R. ferrugineum and its hybrids. The diseased part should be picked off and destroyed: the problem usually occurs only once a season with the main flush of new growth. Many of the fungicides used to control mildew and rust also seem to have an effect in reducing this disease. Many authorities claim that galls are spread by an insect but I have never seen any firm evidence for this.

Updates From The Farm

The Pacific Northwest has one of the finest rhododendron growing climates in the country. They are an integral part of many landscapes, and their variety is astounding. As the bloom season winds down in early June, it signals a perfect time to groom and prune your rhododendrons to keep them from outgrowing their space.

There are two types of rhododendrons. Elepidotes are large leaved rhododendrons. These are the types of plants that most individuals would associate as being a rhododendron. Elepidotes hold their leaves in whorls. Plants can become quite large over time. Lepidote rhododendrons have smaller leaves and are usually lower growing. Their leaves and branching rise from buds all along their branches. This group includes evergreen azaleas.

This time of year, as your rhododendrons finish blooming, you can keep them looking their best by doing some grooming. On elepidote rhododendrons, the flowers are often born in clusters, known as trusses. Breaking off the spent bloom trusses sends energy from seed production into growth. This process is known as deadheading. Lepidote rhododendrons usually drop their spent flowers naturally, or during dry weather, they can be brushed off the plant.

This is also a good time to prune rhododendrons. Light pruning now will not adversely affect next year’s bloom set. Elepidote rhododendrons can be pinched to promote compactness. To do this, break off single growth shoots. The the bud will push again, usually with multiple, shorter shoots rather than the single long shoot. Lepidote rhododendrons can be pruned anywhere along their branches.

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