How To

Assessing & preventing winter leaf “burn” on a “rhody”:

Question:

5-1-19 William B., Ypsilanti, Michigan

I’ve got sick rhodos (see photo).  In addition to root rot and a nasty winter, I didn’t get the sprinklers on during our heat wave last summer so I’m sure that has added to the trouble.  Who is the go-to specialist for advice?

Thanks.

Bill

Answer:

Hi Bill-

It has been a rough winter for Midwesterners like yourself.  Although it wasn’t so bad here near Lake Michigan, many other members of our Midwest chapter who were exposed to extremes in cold & wind weren’t so lucky.  I certainly sympathize with you.  I’m basing my suggestions on what you’ve said and the photo you provided.

As you pointed out, your “problems” just didn’t start with this past winter – summer drought conditions probably weakened your “rhodies”.  I’m afraid that these twin “assaults” of a dry summer & a frigid winter have set your plants back “big time”; it will take time for your plants to recover(if ever in some cases).

Precautions can be taken to enhance their chances of survival (see below) but they go only so far in face of weather extremes spurned on by climate change:

1. Start with hardy varieties (flower bud hardiness of minus 15 to minus 20 degrees).   I know it’s “sexy” to go with those flowers of a less hardy variety but ultimately, you “pay the price” – better to go with some of the hardy varieties first – those which are hardy to minus 25 degrees and beyond and are known as “ironclads”.  After you have had success with those, you can “dabble” in some of the less hardy varieties. That said, even the most experienced “rhodoholics” (like myself) have lost plants.  “Seasoned veterans” will admit that they have killed “that plant” several times!  🙂

2. Site plants with wind & sun protection – structures or groupings of shrubs make ideal wind barriers and pine trees (in particular) offer year round protection from the sun.  Believe it or not, even the late winter sun can cause significant “leaf burn” in rhodies.

3. Make sure the plants are well hydrated, not just in the summer but particularly going into the fall & winter. (Having said that, there’s a risk in over watering in the summer – that’s why it’s important to use a porous soil mix with pine bark mulch as its major component.)  This minimizes the chance of Phytophthora root rot from occurring.  Mulching your plants is important also.  I use oak leaves, pine         mulch & pine needles
4. One of the reasons for a plant’s lack of hydration can be a “pot bound” condition similar to what you encountered with the container grown plant.  It’s important to not only “score” the root ball with 1/2″ deep cuts on the 4 sides and bottom but also to soak the root ball in water for 15-30 minutes PRIOR to planting.  “Teasing” the roots out somewhat will complete te process.  The idea is to encourage the roots to become “involved” with the surrounding soil.

Words only go so far – I would suggest checking out our videos on our Chapter website (I believe one of the lessons presented by Ted Nyquist covers the cultural tips I’ve mentioned).

Roger D.

Follow up question:

Does the browning of the leaves look like heat and/or winter damage to you?

Follow up answer:

It looks like winter burn to me.  Parts of my “rhodies” exposed to the winter wind exhibit the same look; the effect just is not as wide spread on mine because the plant was protected to a partial extent.

That said, there is another possibility and that is:  over fertilization.   The tell-tale sign is “burnt” leaf margins but that usually manifests itself shortly after the fertilizer is applied.  Only you can make that determination.  Chances are the plant involved here is suffering from a combination of both frigid winter wind and over fertilization.

Roger D.

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Transplanting a large (large-leafed) rhododendron:

Question:

2-27-19 Meg. B., Norton Shores (near Muskegon), MI

I stumbled upon your website and hoping you can provide some insight or expertise. We have a very large rhododendron, I can send a picture via e-mail, and I am looking for information about transplanting it. I absolutely love it but we are doing an addition to our home and it’s the way. Any insight on who to contact for moving it/if it’s possible/if it’s worth it/will it survive, etc. ? We are located in Norton Shores, MI.

I have attached the photo from last year. It’s probably 15’ wide if I had to guess. I think small leaves but do not know the exact name.

 We would like to move it as soon as it is safe to do so. And would plan to keep it somewhere in our yard so it won’t be going far.

We are about 1 mile in front Lake Michigan!

Answer 1:

Meg, I can tell you that moving a rhody of this size will require proper timing, moderate amount of care and “appropriate” means for transfer.  “Rhody” root systems are pancake shaped (no tap root) and shallow (probably 5-8″ deep in this case) – potentially that’s quite a bit of weight.  If you intend to move it a short distance, a heavy duty drop cloth or plywood will do coupled with “adequate” manpower.  Care will have to be exercised so as to avoid damaging, the roots, trunk & branches.

As I said in my previous email, you cannot begin to move the “rhody” until the ground thaws out.  Once you have moved it to a temporary (or permanent) location, be sure to apply pine bark mulch to prevent the roots from drying out.

Roger Dunlap

Answer 1a:

The advice Roger gave you is all good. What a beautiful Rhododendron. I suspect it to be

1. Roseum Elegans, an “Ironclad”(flower bud hardy to -25º) for the Midwest.

I would add the following to the transplant instructions:

Be aware the diameter of the pancake soil ball will  be slightly larger than the diameter of the plant.  It appears the ball is jammed between the two trees and you may suffer some loss on the root system getting it out of there. Don’t worry, once the ball is up and out , go around the base of the plant and prune out damaged roots. Once they are in their new location, the roots will grow back within a couple of seasons.

Additionally, when digging it out, go around the circumference of the plant with a fork and gently lift each section slightly to provide air under the plant. Once you have done this a couple of times ,you can go around again, this time with a shovel and actually dig the plant out. I suspect the rhody roots will be entangled with the tree roots so you may have to do some chopping. Remember, the roots are only about 8 inches down. Use this as your guide. Again, mulching in with Pine bark mulch is critical for both storing and planting once the plant is out.

 John Golab

Phytophthora (root rot) resistant rhododendron varieties

Question:

1-6-2018 – Pamela S. – Livonia, MI

I am a rhododendron fan living in Livonia, Mi. I would like to have a recommendation for a rhododendron resistant to Phytophthora. I have read that ‘Caroline” is such a plant.  I need to consider replacements for a number of ‘Roseum Elegans’ rhododendrons which formed a foundation planting on the front east side of my home. I would appreciate your advice and recommendations as well as the purveyor of your rhododendron of choice.
Thank you!

Answer:

In response to your questions regarding resistant varieties to Phytophthora, there are a number of them. Be aware however, that new and old hybrids alike are being studied for their resistance as we speak. New hybrids released have not been tested for their resistance to this disease, as this takes a long time and the proper conditions (high heat and high humidity) may not manifest themselves, depending on geographic location.
The other issue is availability. Many of the new hybrids have not hit the local garden centers or nurseries. While some are available
through mail order, they are not landscape size plants. If one is willing to wait until small plants reach landscape size (5ft in 10 years), this is another outlet for you.    Let me give you a few classes of rhodys that are normally resistant:

Let me give you a few classes of rhodys that are normally resistant

1. Lepidotes (small leaf) almost never suffer from the disease. e.g. Pink Diamond, Olga, Wally, Laurie, Pioneer Silvery Pink, Aglo, Staccato, Landmark and more.
2.  Finnish Hybrids (large leafs from Finland) e.g. Peter Tigerstedt, Haaga, Hellikki, Helsinki University, Mikelli.
3.  Old Ironclads (large leaf) Except for Nova Zembla (very susceptible) the rest are quite resistant. English Roseum,  Roseum Elegans, Catawbiense Album, Catawbiense  Grandiflorum, Maximum (late), Roseum Superbum, Everestianum.
4.  A few newer hybrids that have started to show up:  Hoopla, New Century

There are at least a hundred more but only available as small  Plants from Mail Order.

The second question you had concerns a source in your area.  A very good nursery near you is Rosebay Nursery, Saugatuck,  Mi. (269) 857-4852. They are primarily a wholesale nursery, so they don’t deal with warranties etc. Sometimes they will sell to the public with proper understanding. It would certainly be worth a phone call and at the very least, they can tell you who they sell to in your area. (Garden center, nursery etc.)  Mail order—- RareFind Nursery in Jackson, NJ, or Whitney Gardens, Brinnon, Washington.

Hope this helps. To learn more about growing rhodys, join our Midwest Chapter.

Regards,  John Golab,
Midwest chapter – ARS

Upper Midwest Rhododendron Culture

by John Golab
Most native soil in the greater Chicago area is very heavy clay compacted further by home developers. This soil condition requires amendments and modifications to grow healthy rhododendrons.

Preparing The Soil

Because rhododendrons require an exceptional amount of air in the soil, the soil should be loose, porous and organically rich with a pH range of about 4.5 to 6.0. A standard mix consists of equal parts of good top soil, peat moss, sand and shredded bark.

To prepare soil conveniently:

• Put three to four inches each of peat moss and sand on top of the soil, then rototill.
• Add three to four inches of finely shredded pine or cedar bark, then rototill again.
  • (The bark provides a long-lasting porous soil structure that encourages good root development.
• Add other materials if you wish: perlite, decomposing oak leaves or pine needles.
• Use ferrous sulfate (Copperas) or sulfur to reduce the pH. One pound of Copperas or two pounds of sulfur per 100 square feet will reduce the soil pH by one unit.

Planting

Large-leafed rhododendrons prefer filtered sunlight. A northern exposure close to buildings is a preferred location. Small-leafed rhododendrons, such as PJM or deciduous azaleas, flourish in direct sunlight. Protection from wind is important.

• Most plants are sold balled and burlapped or in containers. Remove them from containers (or remove cord and burlap), then soak plants in a tub of water for five minutes or until bubbles stop forming.
• Add a fungicide, such as Subdue or Ridomil, to the water. (This step is strongly recommended.)
• If a plant is root bound, cut and distribute the roots horizontally before planting.
• Plant rhododendrons in prepared soil. The top of the root ball should be level with or preferably slightly above the surrounding soil.
• Sprinkle a small handful of rhododendron-azalea fertilizer around the drip line.
• To maintain good green plant color, add an iron chelate with trace elements if the soil pH is above six.
• Remember that first year plants need more water than more established ones.
• Keep plants mulched year round, especially during hot weather. Shredded bark, pine needles and partially decomposed leaves all make excellent mulches. Mulch should be two to three inches thick.

Rhododendron culture & hardy varieties advice for Iowa

10-25-17 Wes H. – Des Moines, IA
My garden is quite small and is typical lot for an Arts and Crafts bungalow built in 1919. There is no lake influence to lessen the Midwest climate here in Des Moines. I live in the city limits in an older neighborhood with established large shade trees. Iowa’s climate is harsh, with cold winter temperatures that can reach 20 below, as well as high heat and humidity in the summer. The rhododendrons I grow need to be able to take the extremes in temperatures and growing conditions . . . I have some tried and true varieties and have grown them successfully for years: R. catawbiense ‘Album’ and ‘Boule de Neige’ and R. maximum. In addition, I have two deciduous azaleas developed in Minnesota (‘Orchid Lights’ and ‘Pink Lights’) within their Northern Lights series. With those plants well-established, I have decided to branch out with other rhododendrons. Last year I planted: Calsap, Dark Lord, Florence Parks, Ginny Gee, Great Smokey, Helsinki University, Ingrid Mehlquist, Mary Fleming, Peter Alan, Purple Passion, Silberreif and Tapestry.
I have found that planting on my soil works best to plant the rhododendron on top of the ground with amended soil (tree and shrub mix, peat moss and cotton gin trash). I make collars of chicken wire to hold the mounded soil in place with pegs and keep the squirrels and raccoons from digging up the plants. Each mound is mulched with a layer of pine needles. The mounding technique appears to give my plants the drainage they need to thrive. Also, I only use Holly Tone for fertilizer and only use it twice a year, per the recommended dosage. And I do not fertilize newly planted rhododendrons.
I decided not to let the failures in 2016 deter me and ordered additional rhododendrons for 2017. The list for 2017 was: Casanova, Chionoides, Crete, Cunningham’s White, Edith Bosley, English Roseum, Kalinka, Ken Janeck, Lee’s Dark Purple, Roseum Elegans, Solidarity and R. catawbiense ‘Grandiflorum.’ They all appeared to be the better for having been planted in April and having the chicken wire collars installed.

10-25-17 Question:
I also have a question about a new variety of rhododendron that I have recently seen in gardening catalogs. Do you have any experience with ‘Abbey’s Re-View’ that appears to be capable of blooming twice in a single growing season–once in early spring and again in early fall? It is also supposed to be hardy in zone 5 (my zone), but I have doubts. My experience with rhododendrons so far has taught me that the heat and humidity of summer can be as difficult for rhododendrons as our harsh winter weather. Do you have any words of wisdom for my rhododendrons in Iowa?

11-13-17 Answer:
In reading your description of the rhodys you’ve tried, the planting mix and the mulch employed,
there is not much I would change but I’ll offer a few suggestions.
1. Add IRONITE and granular sulfur to your planting mix (a handful of each per plant). The IRONITE works quickly to increase acidity, while the sulfur takes almost 6 months to work, resulting in increased soil acidity for years. The quick acting Ironite fades relatively quickly and is gone after 6 months.
2. Spray foliage with MIRA-ACID, in April, May and June.
3. Since the winters in Iowa are pretty cold, you might think about using burlap wrapped wind enclosures for the non-Ironclads or those that are new and you suspect may be “marginal”.
4. Add pine bark fines to your next plantings. I use a 50% quantity in my planting mix. Pine fines are available here in bulk or bagged quantities from ACE HARDWARE stores.

As I read your selections for last year I said to myself “that won’t make it, that won’t make it “ etc.. Your next paragraph verified all those I guessed might fail. The reason being all but Calsap are rated for Zone 6. I would suggest Silberreif be tried again. I have had mine for almost 20 years but is protected by a white pine. It is an extremely good bloomer.
Regarding your 2017 “batch”, you will have to protect Kalinka and Cunningham’s White during the winter. Also Cunningham’s White tends to succumb to high temperature and humidity in the summer. It’s not you – it’s the plant – just not great for the Midwest. Maybe you’ll get lucky.
Added later: I have no experience with ‘Abbey’s Re-View’. I have heard of a new series being developed down South that bloom twice a year or more. I question its hardiness though.
Hope this helps.
Regards,
John Golab
Midwest Chapter – ARS

Pruning

Question:

I have some magnificent rhododendron bushes – about 7 feet tall. They are getting leggy and thinning out. What can I do? Can they be trimmed to promote new growth? Their large size is quite unusual for northwest Evanston.

Virginia M.
Evanston, IL

 Answer: 

Hello Virginia,

You are very fortunate to have a rhody at 7ft. They are such slow growers here in the Midwest. Your plant is undoubtedly 20 years old or more.  For an older plant such as yours, pruning to reduce the size and to encourage bushier growth has to be done carefully over a period of at least 2 years and maybe more.

To begin with, cut no more than 1/3 of the top growth during the first year. During the course of the season, latent pips will appear on the stem below the initial cut.  These will develop into your next branch. Once you observe where this pip(protrusion) appears (it can be near the cut or well below the cut), the branch stem above it can be removed to the pip, as no growth of any kind will appear above that point. It may be that you get only one pip but more are certainly possible. By the second year, you will be able to determine whether the height and new growth meets your objective.  If not, then once again repeat the process of cutting down no more then 1/3 of the height and locate where the new pips come out.

Pruning an old plant for bushiness is generally a more difficult process.  It is much easier to shape and increase the density during the first ten years of the plant’s life.  If no pips appear, do not be discouraged. Eventually your pruning may force new growth from the very bottom, then you will be on your way!!!!

John Golab
Midwest Chapter – ARS
June 6, 2017

Controlling Pests

Healthy, well-grown rhododendrons are relatively free from diseases and pests.

• Control root weevil leaf and root damage, as well as injury from other sucking or chewing insects, with Orthene; use as recommended on the label.
• In warm, humid climates, consider a preventive program of spraying with Subdue or Ridmil for fungus dieback.

Winter Care for Rhododendrons

by Tony Greco
Quite a few of our chapter members live in Wisconsin and Minnesota and grow rhodies very successfully. It is important to first of all select hardy varieties and plant them in a somewhat sheltered location. Refer to “Upper Midwest Rhododendron Culture” for culturing information.

Soil preparation is extremely important. We do not recommend wrapping the plants with burlap in the winter. We found adding additional mulch around the perimeter of the plant in the winter (such as shredded leaves, pine needles or pine bark) is beneficial and helps keep the ground from freezing and thawing. You can also put some pine boughs in the ground around the plant if you feel you want to add some additional winter sun protection. With regard to snow cover, snow actually works as an insulator which is good and also protects leaves from winter sun burn.

The small leaf rhododendron varieties such as PJM and Olga are more tolerant of extreme weather conditions and perform well in exposed areas. I have found large leaf varieties become more tolerant of winter weather conditions after they have become established after a few years.