How To


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.


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.
John Golab
Midwest Chapter – ARS



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


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.


Click on the titles below to view the detailed answers.

We recently moved into a new house. The previous owner had planted azalea bushes up to 35 years ago and she cherished them. While they are beautiful, they are very close to the house and have overgrown the front window. We also need grading done around the outside of the house,and the azalea bushes are in the way. Is there any way of drastically trimming them down without killing them.

Answer (from Tony Greco)
Azaleas can be drastically pruned but I wouldn’t cut them down more than 50% at one time to not put too much stress on the plants. If you need to remove more, consider trimming back in two stages.

Azalea roots grow laterally and close to the soil surface so if you’re regrading, try to avoid putting too much soil over the roots.

I would also wait for the plants to bloom before pruning. This way you can enjoy the flowers and then prune before the growth spurt that immediately follows bloom. Fertilize the plants after you prune them with an acidic fertilizer and mulch your plants with pine bark mulch. It may take a year or so for the plants to completely revive but they should be fine.

Try not to let the plants become overgrown. It’s better to judiciously lightly prune right after they bloom to keep them in bounds once they are the size you desire. Also, don’t wait until late summer to prune since the next year’s flower buds form in late summer. You don’t want to prune off next years flowers.

I live in Northern Indiana which I believe is Zone 5. I have a shaded area under mature pine trees. The branches on the trees have been trimmed up 10′ to 12′ above ground level. To the immediate East of the shaded area is the neighbors pool fence which blocks some of the morning sun. To the immediate West is a garden shed that blocks much of the afternoon sun. I would like grow Azaleas and/or Rhododendrons, if possible, as part of the Understory. I would welcome any suggestions you can provide as to what Azaleas and/or Rhododendrons, if any, will grow in the above described environment.

Answer (from Tony Greco)
It sounds like you have a sheltered area you’re working with which should be ideal. The pine tree should provide a nice acidic area which rhodies like but make sure the surface roots of the tree don’t compete with the rhodies for moisture. Ensure your planting mix will retain moisture under the pine tree so you’re plants do not dry out, especially the first year or two as they develop a root system. Refer to our web site for more info on soil makeup.

Although rhododendrons and azaleas are considered shade plants, they do require sun to set flower buds and stay compact. Any Zone 5 plants should do well for you but if you don’t get much sun I would probably stay away from deciduous azaleas as they like quite a bit of light. Evergreen azaleas may be better azalea choices.

For large leaf rhododendrons, I would consider the Yakushimanum hybrids such as Ken Janeck since they stay compact with beautiful flowers and foliage. These may not stretch as much reaching for sunlight since they are naturally compact. Small leaf rhodies tolerate more sun and may not be suitable for this site.

I live 65 north of St. Paul, MN. What would be the best rhododendron variety for me to grow? I planted two English Roseum plants last spring on the NW side of my house. Now I’m worried that they may not survive our severe winter. How can I protect them?

Answer (from Tony Greco)
English Roseum is known as an “iron clad” meaning it is very hardy and grows in harsh conditions. We have members in zone 4 in northern Wisconsin and Minnesota that successfully grow this variety.

I’m going to assume you prepared your planting site as we recommend on our web site (see Upper Midwest Rhododendron Culture). Make sure your plants stay well-watered this fall as newly planted rhododendrons should not be allowed to dry out. Also, add a couple of inches of mulch such as shredded leaves, pine needles or pine bark mulch to protect the roots.

Direct winter sun can be a problem, especially with new plants but I don’t think this should be a major concern on the NW side of your house.

We do not recommend covering rhododendrons in the winter; if planted properly and mulched well you should be OK. If you really feel you need to add some protection, put a burlap screen around the perimeter of the plants leaving the top open and fill the cage with leaves. I would avoid having the foliage rub against the burlap.

If these are your first rhododendrons, do not become alarmed when the foliage sags on cold days. This is the plant’s way of protecting itself from harsh conditions. The leaves resume their normal appearance when the temperature returns to the mid-thirties.

You should receive a lot of snow where you are which is good. The snow acts as an insulator protecting your plants.

As for other varieties to try, any of the ironclads such as Nova Zembla (red) and Catawbiense Album (white) should work. Also the lepidotes (small leaf) such as PJM and Olga should do very well. And of course the Northern Lights Azaleas developed at The University of Minnesota are very hardy.

A group of plants from Finland have been introduced recently that are extremely hardy. The following is a list of these Finnish hybrids that are all hardy to minus -35:

  • Haaga – Light Scarlet
  • Helsinki University – Pink
  • Mikkeli – White
  • Peter Tigerstedt – White w/reddish flecks
  • Pohjola’s Daughter – white with pink

I would recommend shopping at good, reputable nurseries rather than the big box stores which sometimes carry varieties that may be too tender for a given climate.

My niece rents an apt and bought 2 gorgeous rhododendrons in pots for her patio. She wants to take them into her garage or basement over the winter. I don’t think this is possible, is it? Can she take them indoors with normal light and keep them in her apt?

Answer (from John Golab)
Yes indeed, the potted rhody can be kept indoors near a window so some light is present. The coolest area inside the apartment is best. No mention of what city is indicated, so I will assume a Midwestern city with normal winter weather.

The potted plants can also be placed in a garage, potting shed, or basement provided there is sufficient light to keep them alive. They cannot survive in the dark. Artificial lighting can also be successful, much the same as germinating seeds. In all cases, water as necessary, not excessively.

One side note, the name or hardiness was not given for this rhody. Generally, storing the plant in a potting shed with sufficient light, unheated, will gain 15-20 degrees, as compared to the outside temperature.

Transplanted from a nursery a mature plant this late spring in early June, along with a vibrant azalea, should I trim either plant before fall? Any tips to ensure a good blooming season next year?

Answer (from Tony Greco)
I would strongly suggest you do not prune your azaleas or rhododendrons this late in the summer (August) if you want to maximize your bloom production next year. The time to prune is shortly after they bloom in late spring or early summer.

The flower buds most likely have either formed already or will shortly appear on your plants. If you prune them now, you will be losing this flowering potential next spring. It is also very unlikely there will be adequate time this year for new growth and bud development to mature properly. In the Chicagoland area, I have found when pruning late in the year, the new growth may not fully develop and harden off properly before winter and can be damaged.

If you have an unsightly branch or two that needs to come off, go ahead and trim it, but I would wait until next year for general pruning or shaping if you can.

I can only recommend you keep your plants well-watered and mulched this year for maximum bloom next year. It is important they do not dry out, especially for the first year or two after planting, or their growth may be retarded which can affect bud development.

My two newly planted rhodies died last fall and I am uncertain as to why. Can you recommend specific ones that like the west Michigan loamy soil?

Answer (from Tony Greco)
Janet, sorry to hear about your rhododendrons not making it last year. 2005 was a tough year for plants mainly due to the drought we had. Not sure the reason for your problem but the lack of water could be part.

I trust the plants were planted and sited properly, but they do require quite a lot of water the first year until their roots get established.

You have wonderful growing conditions in western Michigan, as a matter of fact we get our plants for our chapter spring plant sale from commercial growers in the Saugatuck area. You should have a lot of choices. One variety that I see growing very well in Michigan is Scintillation. I grow this variety myself in the Chicagoland area and it does extremely well for me. Our website has a photo if you’re interested in a pink. Our website is relatively new and we plan to post a lot more photos in the coming weeks.

Calsap is another beautiful plant, white flower with a dark purple eye.

Some other varieties I can recommend are Pearce’s American Beauty, Janet Blair, Ken Janeck and the ironclads including English Roseum (lavender pink), Catawbiense Album (white) and Nova Zembla (Red). Actually, all of the varieties on our website will grow well for you. It’s really a matter of choice as to flower color, plant size etc.

I’m not sure where you purchase your plants but you may want to search out plants from local Michigan growers. I can always tell how the soil from Michigan growers is so sandy compared to the container grown pot-bound plants from the big box stores.