Azaleas as Bonsai
by Jim Robinson (6/2/93)

Azaleas make a very impressive and satisfactory bonsai for the person who is willing to put up with some of the problems associated with azalea culture in the Rocky Mountain Region. The major problems one has to contend with are soils, protection from deep freezing in the winter, but with adequate cold for dormancy; protection from excessive heat in the summer; and, supplying adequate, but not excessive water at all times. These issues will each be treated separately.

Soils
Probably the most critical aspect of container culture of azaleas is the quality of the soil mix in which the plants are grown. In general, most authorities say the soil must be open, porous, have good drainage, good aeration and an acid (pH 5.0-6.0) chemical reaction.

I. Some soil mixes:
60% inorganic material (sharp sand, gravel, scoria, Turface, Terra Green, perlite, decomposed granite)
40% organic material (pine bark, fir bark, coarse peat moss, oak leaf mold)

OR
40% peat moss
40% redwood, fir, or pine bark
20% pumice
AND
70-80% of particles from 1/4 to 3/8
20-30% smaller but not fine.

OR
100% of particles the same size: 1/4, 1/8, or 3/8: none fine.

I am getting more convinced that a mixture of inorganics will be best for azaleas. I think Turface (or Terra Green, but is smaller in particle size), scoria (lava rock), sharp sand or gravel and granite make a good mix. The Turface adsorbs (that is, it attracts to its surface) nutrients, retains them and makes them available to the plant roots. Lava rock is porous and holds fertilizers and provides a spongy airy surface. Sharp sand and gravel deflect roots and increase root branching. Our granites contain iron and as they breakdown make this mineral available to the plant.

I have generally made my mixes of uniform particle size, that is, 1/8” mineral and bark, etc. The reason for this is to assure that air spaces between particles are provided. A mix of particle sizes could completely remove all air spaces in the soil. As I have moved to 1/4” particles this year, I am finding that the plants are drying out in less that 24 hours on hot days. I think we can include up to 20% finer particles without reducing air spaces too much. I have started adding a smaller particle size mix (in this case 1/8”) to the surface and letting it wash down over the summer.

II. Soil pH can be lowered by using soil sulphur, ferrous sulfate, magnesium sulfate (Epsom Salts) or ammonium sulfate. Use these chemicals in dilute amounts at frequent intervals. Avoid aluminum sulfate or other aluminum compounds as aluminum is too toxic to azaleas.

III. Mycorrhiza are essential soil plants for the survival of azaleas. As with some conifers, the roots of azaleas do not have microscopic root hairs and depend on mycorrhizal fungi for the adsorption of water and nutrients. Soggy soils, soils that do not drain well or lack oxygen and over watering can kill these fungi, and hence, the azaleas themselves.

IV. Watering will depend on the particular soil mix, the amount of humidity in the air, the temperature and the wind. Azaleas must be moist at all times, but not soggy. Aeration of the roots is of critical importance for mycorrhiza to flourish and hence, for healthy azaleas. 

Light and Temperature
Light  is required for all plants to make food. Plant "food": is not food to plants. It is only the minerals necessary to produce chemicals in the many different plant tissues. Plants make their own food from carbon dioxide and water, but the energy needed for this food-making process is light. I place my azaleas in strong light early in the spring and then in filtered light--like under a lattice--until the cool days of fall. Some azaleas can be grown in full sun, but if the leaves unfolded from the bud in the shade they will be thin and will usually burn if placed in full sunlight.

With the shortening of days and lengthening of night in the fall, shoot growth is retarded and bud formation begins. Once buds are set, azaleas should be kept as cool as possible--40-50 degrees, but for Kurume and Satsuki azaleas, temperatures need to be kept above freezing.

Lengthening of daylight in the spring when accompanied by increased temperature, stimulates most azaleas to the break dormancy. If plants are kept cool--around 45 degrees--flowering can be delayed. To get azaleas to bloom place them in a space at 60-65 degrees. About 4-6 weeks are required for flowering at this temperature range. Flowers last longer if the plant can be kept cool. When temperatures are above 70 degrees flowers will drop soon after opening.

Azaleas need to be kept cool in summer. This can be accomplished by keeping their surroundings moist so that the evaporation of the moisture will produce a cooling effect. I have small gravel on the ground under my azaleas. The pots are on 2"x 2" wooden slats about 2' off the ground. Overhead watering is provided each day which not only waters the plants, but the gravel. Evaporation of the water during the day cools the immediate environment for the plants. I use Toro low angle lawn sprinkler heads for 20 minutes early each morning. When the temperature gets above 90 degrees and is dry, I water again for 10-20 minutes in the early evening.

Transplanting and Pruning
Azaleas can be transplanted anytime from late summer after the buds are fully formed until late spring. Some references suggest that after blooming is the best time, but transplanting can be done even when the plant is in full bloom. I prefer to transplant my azaleas in midwinter or early spring. I prefer this as many
Satsuki's bloom in May or June and the hot weather at this time of year makes transplanting dangerous, in my view.

Some bonsai books recommend that the roots be washed completely when transplanting, leaving no old soil on the plant. I have done this, with both successes and failures. I am convinced that soil must be removed up to the underside of the trunk, even if it takes a Dremel tool to cut away the old soil. Packed soil has no aeration and can support no feeder roots or mycorrhiza.

Some azaleas will begin growth prior to flowering. If new shoots develop at the base of flower buds, they should be cut off. If new growth starts on non-bud bearing shoots they can be left or cut depending on the desired styling design. Shoots left on prior to flowering will take energy from flower development and can result in some smaller flowers or bud drop.

After the flowers fade they should be cut off to the base of the flower stems. New shoots will develop from the basal area where flowers were. Reduce the number of shoots to a maximum of two, thinking about design when you cut!  Most bonsai references recommend that plants be pruned back to two or three leaves on every branch at this time, even into old wood below the flowering area. I have done this successfully. I have also severely cut back plants leaving only stubs of branches. This is drastic pruning and should only be done with healthy plants.

Fertilization
Ammoniacal forms of nitrogen are favored over nitrate forms as the ammoniacal forms produce an acid reaction and lower the pH, whereas nitrate forms raise the pH. Ammoniacal forms are chemicals such as ammonium sulfate and urea. Avoid potassium and sodium nitrates.

Fertilize regularly with a weak acid-type fertilizer. An example is Miracid, which has ammonium phosphate and urea, and also has chelated iron and other minerals such as copper, manganese molybdenum and zinc.

When I repot azaleas (and usually my other bonsai), I sprinkle a mix of 2 parts cottonseed meal, 1 part blood meal, and 1 part bone meal on top of the lower layer of new mix, add a little mix before placing the plant, and then add more after I have brought the mix level up to within about 1/2" to 1/4" of the top, stirring it in lightly. These materials are all organic and breakdown slowly. Nutrients from them will not be available to the plant for a week or so after watering, so burning the roots is not a factor.

If you have a highly porous soil, minerals will leach out easily. Fertilizing weekly with 1/4 of the usual amount per quart or gallon is a more effective way to fertilize than applying larger quantities less frequently. Small amounts of fertilizer can be applied each time you water. Do not fertilize azaleas after buds have set in the fall and do not begin fertilization in the spring until the flowers have faded. This is the general information you will find in most publications. I have experimented with placing one rape seed cake on my azaleas when I  bring them in for the winter. I have never had bud drop from this and the leaves have stayed green and healthy.

Disease and Insects
Root rot is the primary cause of losses of azaleas. Prevention by appropriate soil and careful watering is about the only way to prevent this disease. It is a result of compacted or fine particle soil that has lost its capacity to hold air. Oxygen is essential for root life in all plants, but especially for azaleas.

Dark blotches with well defined borders can be an indication of fungus infection. Spraying with a good fungicide such as put out by Ortho, Fertilome, Hi-Yield and other companies will control these fungal diseases. I have found both Maneb, put out by Hi-Yield (I found it at Paulino's Nursery) to be a useful fungicide.

Red spider and leaf chewing insects are the major pests I have encountered. Using Kelthane or other miticides will kill red spider. Apply as directed on the label. Hosing off the plants and providing a moist environment usually prevents red spider. Watch for notches on the leaves for chewing insects and spray with Blackleaf forty, Diazinon, Isotox, Insecticidal soap or other stomach or contact poisons to control leaf-chewing and sucking insects.

Species of Azaleas
The most commonly available species are from the Kurume group ( Rhododendron obtusum ) and Satsuki group ( Rhododendron indicum ). Other very fine azaleas, available from specialty nurseries--bonsai, or Azalea, or Rhododendron nurseries--provide additional species such as R. nakahara, R. kiusianum, R. thymofolia, and R. japonicum--that can be made into excellent bonsai. Some bonsai books and nursery catalogs list other species that can be sought for bonsai.

Final Comment
This short discussion of azaleas as bonsai is intended as an introduction to the subject. It should, in no way, be considered exhaustive! There are volumes written of the genus Rhododendron, of which azaleas are one group. Experience and reading in the many books and journals will enable you to increase your success with these unusual and beautiful plants.