Answering the Four Most Asked Questions About Rhododendrons.


Sitting at information booths at flower shows.

1- Why did my rhododendron die?

2- Should I fertilize my rhododendrons?

3- How do I prune my rhododendrons?

4- Why do my rhododendrons bloom every other year and some never bloom at all?

–Why did my rhododendron die?

I–Rhododendrons die because we kill the through lack of knowledge of their specific requirements.

1.  Death causes can always be found in the roots.

A.  No adventurous or tap roots

B. Extensive mass of fine roots invade a relatively small volume of soil

C. A fungus, mycorrhizae, on the roots

1. Symbiotic relationship

2. Triples effectiveness of roots

3. Change NO nitrogen to NH nitrogen which is needed by plant

4. Very important for mature rhododendrons

5. Mycorrhizae’s requirements

a. Air

b. Moisture

c. Correct ph 5 ½ to 5

1) To raise ph use lime, 5#/100 sq ft/ph unit

a) Use ½ during winter, rest next winter

2) To lower, use flowers of sulfur NOT ALUM

3) Chlorosis, indication of wrong ph

d. Slow temperature change of soil

e. Low salt concentration in soil

f. Soil temperature less than 75 F

D. Two common horticultural practices that satisfies all requirements

1. Natural mulch on top of the soil

a. Leaves, pine needles or wood chips, never peat moss,

never compost

b. 3 – 5” thick

c. As mulch decays, organic resins and tars enter soil and

help to flocculate it

d. Keeps soil cool in summer

e. Acts as a blanket and prevents rapid temperature




2. High amount of organic matter in soil

a. Holds water in soil

b. Buffers fertilizer (salts)

c. Adds mass to soil, thus slows temperature change

d. Can’t do for established plants. Rely on mulch


2. Planting new plant

A. Location

1. Some sun in early AM

2. Some shade from 1 to 4 PM in summer

3. Little wind

4. Good water drainage, no bog

B. Hole

1. Twice diameter of root ball

2. No deeper, ½” too shallow not bad

3. Reserve top 3” of top soil from hole and discard rest

4. Mix peat moss or compost (if no lime used) with reserved

top soil and use as backfill

C. Scrape off ½” of soil from root ball, preserving roots as much as possible

D. Place in hole and backfill with mix

E. Tamp backfill with hands, not feet

F. Mulch out beyond drip zone of plant

G. WATER directly on root ball

3.  How it dies if planted in May or June

A. No capillary action in soil, thus neither lateral nor vertical water

movement in soil

B. New growth doubles leaves which act as an umbrella

C. Cool soil temperature, thus no root growth

D. Summer arrives and air temperature rises, rapid loss of water, little


E. Wilt

F. Water plant, bounce back

G. Second wilt, partial bounce back, some die back in center of plant

H. Three wilts the first summer and the plant will die within three years

1. By then purchase guarantee expired

2. Plant looks miserable until it dies

3. Doesn’t bloom

I. Prevention

1. Water with hose to root ball EVERY WEEK EVEN IF IT                RAINS until Oct 15th

4. Answer

A. Plant from last week in August to third week in September

1. Same planting technique and location

2. Soil now warm, quick root growth

3. No new leaf growth

4. Cooler weather, less water loss

5. Only have to water 4 or 5 weeks

B. This is also the best time to transplant

5. Other reasons rhododendrons die

A.Phytophthora cinnamomi

1. Root rot

2. Warm, very wet soil

3. No cure

4. Usually seen in summer after heavy thunder storm

5. Cultivars vary in susceptibility

a. ‘Nova Zambia’ very susceptible

B. Weevil damage to roots

  1. Taxus weevil larva
    1. Leaf notching
    2. Control adult with insecticide or tanglefoot

2. Japanese beetle weevil stage

II– Should I fertilize my rhododendrons?

1. Yes

2. February & March

3. 22-4-4, 50% organic or 10-6-4, 50% organic

4. Holytone is fine

5. No need to buy fertilizer for acid loving plants

6. Cotton seed meal fine except smell. Not heavy in nutrients

7. About 1 cup for a 5’ wide plant, broadcast over root area, on top of mulch

III– How do I prune my rhododendrons?

1. Don’t purchase a plant that will overgrow its area in the garden

a. Rate of growth varies with cultivar

b. Note rate of growth before purchase

c. Flush of growth, once a year

1) Constant from year to year

2) Note number of years leaves are held on plant

a) Produces leggy or dense plant

3) Note leaf characteristics

a) Shape and size

b) Indumented rh. Smirnowii & rh. Yakushimanum

2. Incidental pruning

A. Right after bloom

B. Leaves below where you want to cut

C. Common for azaleas

3. Major pruning

A. No leaves below where you want to cut

B. Cut one branch of multi-branched plant

1) St Patrick’s Day

2) Hope for the best

C. If breaks form on cut stem, next year cut the other branches

D. If no breaks form, decide whether you really want the plant as is

IV– Why does my rhododendron bloom every other year or not ever?

1. Bloom cycle

A. Buds develop in late July to early October

  1. If no buds in October, no flowers next year
  2. Mature plants bud early, juvenile plants later

2. Difference between vegetative and flower buds

A. Vegetative bud thin and long

B. Flower bud very fat.  Can be multiple buds.

3. Buds can be killed over winter

A. Squirrels

B. Winter cold

1. Hardiness

a) Cultivar specific

b) Can’t tell by looking at plant

c) Sometime on plant tag

d) ARS hardiness ratings

1)      H-1 Hardy to –25 degrees F

2)     H-2 Hardy to –15 degrees F

3)     H-3 Hardy to –5 degrees F

4)     H-4 Hardy to +5 degrees F

4. Deadheading

A. Reason for alternate year bloom if not done

1. Removal of spent flowers on rhododendrons, deciduous azaleas, but not evergreen azaleas, to prevent seed formation

5. Flowers

A. Sunlight required for flowers

1. If grown in dense shade, very few flowers

B. Different cultivars bloom at different times

C. From March 15th to August 25th

D. All colors in the rainbow except green (leaves) plus white found in        flowers

E. Truss is made up of flowers, not florets

F. Consider rhododendrons as garden plant or exhibitor’s plant

1. Problem for hybridizers

G. Judging

1. No official scale, use general scale for flowers

a)      Be wary of consideration of size unless you are expert and know correct name of cultivar

b)      We don’t allow rhododendron entries with multiple buds

2. No ARS certified judges

3. Points to look for

a) Freshness

b) Tiers of flowers in truss, looking for 3 or 4

c) Size of truss compared to size of collar of leaves

1) 60 – 70% or more is good

4. Picture frame for azaleas, usually 12” x 15” or 15” x 20”