Azalea japonica, the Japanese azalea

Azalea japonica

Azalea japonica is a small heath shrub that provides evergreen, bushy growth.

Key Azalea japonica facts

Name: Azalea japonica
Scientific: Rhododenron azaleastrum Tsutsusi
Family: Ericaceae
Type: shrub

Height: 3 ½ to 6 ½ feet (100 to 200 cm)
Exposure: part sun/shade plant

Soil: acidic, heath soil  –   Foliage: evergreen  –  Flowering: mid → end-spring

Its main appeal, however, is the massive blooming: beautiful flowers ranging from pink to red, purple and white.  Since it grows up to 5 to 6 ½ feet (1.5 to 2 meters) tall, depending on the species, Azalea japonica is the ideal shrub to fill in a heath plant flower bed. But it’s also excellent as an indoor shrub!

Planting Azalea japonica

Azalea japonica plantingIt is important to not over-expose your Azalea japonica to sunlight, so favor a spot that is partly shaded.

  • Well-drained soil is essential.
  • Adding heath soil is necessary.

To generate spectacular blooming, add heath plant fertilizer upon planting and then regularly during the growth phase.

Pruning and caring for Azalea japonica

Pruning Azalea japonica

It isn’t necessary to prune the plant. Nonetheless, if you wish to balance the shape or reduce the size of your azalea, wait until blooming is over.

  • Favor light and delicate pruning rather than drastic cutting back.
  • Remove wilted flowers regularly (deadheading) to spur appearance of new buds. This will also direct the plant’s energy to growing new branches.

Watering Azalea japonica

The surface-most roots of Azalea japonica are the most thirsty roots. They especially require frequent watering in spring and summer.

  • To reduce watering frequency, mulch the base of your Azalea japonica with maritime pine bark. This will retain moisture and cool the soil.
  • After a couple years, your azalea japonica will do fine without any watering, except in case of prolonged dry spells or heat waves.
  • If the water is hard in your area, do your best to recover rainwater. Indeed, hard water tends to reduce soil acid levels, which is the opposite of what Japanese azalea loves.

Growing Japanese azalea in pots

The shallow root system and small size of Azalea japonica makes it an ideal candidate for container growing on a balcony or terrace. Prepare heath plant soil mix and ensure the pot or container is wide enough.

  • To keep your Azalea japonica small, prune branches back after the blooming.
  • Cut about 2 to 6 inches (5 to 15 cm) off the tip of each branch. While you’re snipping away, try to bring the shape as close as possible to a sphere or mound.
  • During the growing season, turn your pot around weekly to spread growth around.

Depth of the container or pot isn’t an issue, to the point that you can save soil by filling the lower half with rocks or hollow space fillers.

Repotting Japanese azalea

It’s best to wait for the blooming to end before repotting. Flowers are very demanding in terms of nutrients, which is why you shouldn’t risk damaging the fragile root system at this time.

This is especially true if the pot is rootbound. Indeed, rootbound plants have roots that circle round and round in their pot. Usually this happens when repotting is delayed.

After the blooming, prune the branches lightly and shape the plant as described above, and proceed to repot immediately.

  • Use heath soil or peat-based potting soil. Azalea japonica likes acidic soil.
  • Include 1/4th compost or ripe manure because peat in itself doesn’t have many nutrients.
  • Choose a pot that is only one size larger than the previous. It’s enough for the next two years and jumping to a larger pot isn’t necessary.
  • In case the roots are girdled and rootbound, unwind them and cut them short, so that they fit in the pot without circling.

Growing Azalea japonica as a bonsai

azalea japonica bonsaiSince Japanese azalea has long been grown as a bonsai under its more specific ‘Satsuki’ and ‘Kurume’ varieties, why not try it out yourself?

  • use soil that is rather acidic, as for heath plants, and avoid lime
  • water enough that the soil remains moist but never wet
  • use organic azalea fertilizer, but only during the growth phase
  • prune your bonsai Azalea japonica just after the blooming
  • if you want to shape it with wire, proceed with caution because branches break easily

Repot your Azalea japonica bonsai very carefully every two years. Root systems tear apart easily so again, proceed with caution.

Azalea japonica diseases and pests

Your Japanese azalea might have to fend off red spider mites, leaf gall, and root rot.

Leaf gall on Azalea japonica

If leaves start swelling, curling over and growing fleshy bulges, you’re facing a case of leaf gall.

This is a fungal disease induced by the Exobasidium vaccinii germ.

  • Remove infected leaves immediately and burn them.
  • If it persists, spray a natural fungicide like horsetail tea.
  • Try to pick and remove all galls before the fungus matures and releases spores. The gall will turn whitish when this happens.

Root rot on Azalea japonica

Here are a few symptoms of this damaging disease:

  • trunk and roots start taking on a dark red or brownish color
  • leaves shrivel up and fall off

By the time you notice this on your shrub, sadly, it’s usually too late to heal the plant. You’re facing a case of root rot caused by Phytophthora bacteria.

  • Pull it out to protect nearby plants.
  • To avoid this fungus, ensure the soil drains very well. You can also spray the ground with a natural fungicide.

Different types of Azalea japonica varieties

We’ve talked about the ‘Satsuki’ and the ‘Kurume’, two hybrids only propagated through cuttings. But there are many more types of Azalea japonica to be explored!

  • Azalea japonica varietiesSatsuki – often called ‘late-blooming’ because it only blooms at the end of spring (May to June).
  • Kurume – semi-dwarf hybrid that blooms profusely.
  • Encore – developed by a Lousiana grower, Robert C. ‘Buddy’ Lee. Blooms from spring to fall.
  • Robin Hill and Gartrell hybrids – a group of hybrids brought to us by a different Robert, Robert Derby Gartrell. in the late XXth century. Larger flowers, very hardy.

Thanks to the many varieties, it’s only a question of time until you find an Azalea japonica that hits your sweet spot!

Is Azalea japonica the same as Rhododendron?

Three key reasons explain why Azalea japonica are often grouped within the Azaleastrum section of the Rhododendron family:

  • the fact that they’re evergreen,
  • hardy
  • and have leaves and flowers that are smaller than actual rhododendron shrubs.

So, in a way, they’re simply a group within the larger Rhododendron family.

How is Azalea japonica different from other azaleas?

Japanese azalea is said to be an evergreen azalea. This means it keeps its leaves in winter. The plant only sheds older leaves when newer ones have taken over, except for the rare bout of really cold weather. On the other hand, regular azaleas are often deciduous and will lose all their leaves every year in the cold season, even during mild winters.

Additionally, azaleas of the deciduous type have larger, longer leaves. This contrasts with Japanese azaleas that have smaller leaves, at most 2 inches (5 to 6 cm) long.

All there is to know about growing Azalea japonica

This shrub is actually a small-flowered rhododendron. It bursts forth in spring with a beautiful bright and colorful bloom.

Moreover, Azalea japonica grows into a nice round shrub. This dense shape is perfect because it highlights the generous blooming and the deep green leaves.

Note that Azalea japonica might lose its leaves in case of strong frost spells, but this won’t have any incidence on the plant itself.

  • Azalea japonica can resist freezing temperatures down to around -4° to 5°F (-15 to 20°C).

When grown indoors, Azalea japonica helps purify indoor air from ammonia. It’s that unpleasant smell from house-cleaning products and pet urine.

Smart tip about Azalea japonica, the Japanese azalea

To enhance shrub growth, mulch the ground around it to retain moisture.

Favor any acidic mulch such as pine bark mulch.

Images: 123RF: Piotr Milewski, CC BY 2.0: Paul van de Velde, CC BY-NC 2.0: Berverly Vealach, CC BY-SA 2.0: brx0; Flower Council Holland