Plants

Japanese Azaleas

Rhododendron japonicum

Folkert Siemens Folkert SiemensUlrike Hanninger Ulrike Hanninger

The Japanese Azalea is part of the Rhododendron genus, and it’s primarily valued for its abundance of flowers. With its compact growth, it’s ideal for small gardens or front yards.

Growth type
  • Deciduous wood
  • Dwarf shrub
Growth height (from)
from 50.00cm to 130.00cm
Growth width (from)
from 60.00cm to 150.00cm
Growth characteristics
  • spherical
  • sweeping
  • tight
  • bushy
Flower color
  • purple
  • red
  • pink
  • white
Flowering time (month)
  • April to May
Flower shape
  • 5-fold
  • funnel-shaped flower corollas
Flower characteristics
  • unfilled
  • slightly filled
  • hermaphroditic
Leaf color
  • green
page format
  • oblong
  • oval
Sheet properties
  • wintergreen
  • Autumn coloring
  • Winter coloring
Fruit color
  • brown
Fruit shape
  • Capsule
Light
  • scattered light to semi-shade
Soil type
  • sandy
Soil Moisture
  • fresh to humid
ph value
  • weakly acidic to acidic
Lime compatibility
  • sensitive to lime
Nutrient requirements
  • moderately nutritious
Humus
  • rich in humus
Decorative or utility value
  • Flower Decoration
  • Leaf ornaments
  • picturesque growth
Toxicity
  • non-toxic
Winter Hardness
  • conditionally hardy
Climate zones according to USDA
  • 6
  • 7
Use
  • Single position
  • Form cut
  • Group planting
  • Planters
  • Underplanting
  • Rebates
Garden style
  • Japanese Garden
  • Rhododendron garden
  • Stone Garden
  • Pot garden

Origin

From a botanical perspective, Azaleas are part of the Rhododendron genus. The term Japanese Azalea (Rhododendron japonicum) covers a group of plants that all have very similar characteristics and tendencies. They are various winter-hardy and semi-evergreen low-growing shrubs that look largely similar to Rhododendron simsii. As the name suggests, most original Japanese Azalea species stem from Japan. The most important parents of today’s varieties include wild Rhododendron obtusum, its variety amoenum and Rhododendron kaempferi. The first hybrids began emerging in Japan over 400 years ago. The history of the Japanese Azalea’s further development and cultivation, resulting in new hybrids and garden varieties as well as other forms, is confusing to say the least. Even experts struggle with differentiation and categorization.

Growth

Japanese Azaleas feature particularly shallow, densely fibered roots that primarily spread out among the top layer of humus. They generally form round to sprawling, bushy crowns with dense branching. Young shoots are very thin and have a red-brown outer layer. The bark on the oldest branches is generally light gray to reddish brown, exfoliating into thin, fibrous sheets. Growth height varies greatly by variety at between 20 inches to 51 inches. Older plants tend to be wider than they are tall.

Leaves

The leaves of the Japanese Azalea are small, elongated ovals. Many varieties form brown-red coloring in the fall and winter. In cold winters, they tend to lose most of their leaves, while some varieties retain their foliage until spring, which is then replaced by new leaf growth, depending on the temperature. Most varieties lose their colored leaves in fall, then carry smaller green winter leaves that are replaced in spring. This makes the Japanese azalea a great decorative garden plant even in winter. Botanists call these plants semi-evergreen or wintergreen.

Flowers

One significant characteristic of the Japanese Azalea is its abundance of flowers. The whole shrub may be covered in a sea of flowers that cover any leaves. The color palette ranges from white to salmon pink, baby pink to hot pink, and deep red to purple. The flowering period is April and May.

Location

In the garden, Japanese Azaleas best suit off-sun to partial shade. A certain level of sun is required so that the shrubs can form their typical very dense flush of flowers. Make sure that the plants are protected and not exposed to any harsh cold winds in winter. Ideally, they should be protected from the wind all year round.

Soil

As bog plants, Japanese Azaleas prefer evenly moist soils. The substrate should be permeable and loose as well as very rich in humus. The ideal pH value is acidic to slightly acidic, at around 4.5 to 5.5. Japanese Azaleas are very sensitive to lime, as are all rhododendrons. They display classic signs of lime-induced chlorosis: yellowing leaves caused by the high lime content of the soil preventing iron uptake.

Japanese Azalea in plant pots

Low-growing varieties of Japanese Azalea can also be grown in pots

Planting

The ideal time to plant Japanese Azaleas is early spring. Plants in containers can be planted out into summer as long as these are well watered. Fall is not ideal for planting as poorly rooted plants can be heavily damaged by frost. Depending on the final size of your variety, ensure 20 to 31 inches of distance between plants. Poor soil must be well loosened, drained and enriched with plenty of humus before planting. A base of rotted cattle manure and leaf compost is ideal. Loamy and clay soils will have to be replaced – otherwise the plants’ growth will be stunted as they fail to root in dense soil.

Care

Japanese Azaleas should only be fertilized sparingly, if at all. Use standard rhododendron fertilizer. It’s better to cover the ground each spring with a blend of horn shavings and leaf compost. Note: do not use garden compost – it contains far too much lime. Only mulch the plants once the soil has completely defrosted after winter. High temperatures and strong sunshine could otherwise cause serious frost damage, as the roots cannot tolerate increased evaporation. Thicker layers of mulch as winter protection should be removed in good time before new shoots. During the exceptionally abundant flowering period, the plants require a lot of water. Otherwise, the blooms quickly wilt.

Pruning

Azaleas are usually trimmed into a round shape, especially in Japanese gardens. Regular pruning is, however, not required to encourage flowering. Shrubs with loose or lopsided crowns can be cut back drastically to the old wood to reform. A note of caution: as all rhododendrons, Japanese Azaleas do not tolerate being cut back immediately after planting out. In most cases, root pressure will not be sufficient for new growth from the old wood.

Winter Protection

Although Japanese azaleas are aesthetically a real win for any garden, they are relatively rare in gardens. This may have something to do with the extensive protection required for overwintering in zones with cold winters. Winter hardiness also varies greatly – so inform yourself before making a purchase. However, all of these plants have one thing in common: they become more tolerant to cold temperatures as they age. Young plants must always be protected during winter. They don’t just need to be protected at soil level (e.g. with a coating of leaves): Fleece or burlap over the crown is also advisable. This also keeps out the winter sun that Japanese Azaleas react very sensitively to.

Use

Japanese Azalea in rockeries

Japanese Azaleas are also great plants for rockeries that aren’t too sunny

Japanese Azaleas provide year-round decoration for any garden. Thanks to their compact growth, they’re also ideal for smaller gardens and front yards. They are great for combining with other rhododendrons and shade-loving plants with similar site preferences. During flowering season, they are especially effective in front of evergreen hedges.

Aronense varieties and Diamond Azaleas boast particularly beautiful flowers. Both stay very small, so can even be added to smaller beds. Although Japanese Azaleas are generally considered garden plants, there are some varieties that have been grown in pots, tubs and stone troughs for centuries. So it’s worth a try with varieties that remain small.

Varieties

Japanese Azaleas can be allocated to various hybrid groups depending on their origin. Here is an overview of the most significant ones.

Kurume hybrids, named after the Japanese city of Kurume, have very compact growth not exceeding 31 inches in height. During flowering, the bushes are covered with countless tiny blooms with colors ranging from light pink to dark red or even purple. Very suitable for small gardens.


Kaempferi or Malvatica hybrids, however, feature relatively large flowers in orange or red. Newer varieties of these Japanese Azaleas also include purple.

The aim behind creating Vuykiana hybrids was clear: create new colors and shapes of flowers. The first results were mediocre and not too impressive, until further crossings came up with varieties such as Vuyks Scarlet and Vuyks Rosyred, which have very lovely flowers – yet leave something to be desired when it comes to winter hardiness. Frost protection is essential for these plants, no matter how mature. Typical characteristics of this group are flowers with a diameter measuring 1.97 to 2.75 inches as well as stocky growth.

Japanese Azaleas in the snow

Some Japanese Azalea varieties are winter hardier than others, and manage even cold winters well. A blanket of snow is always welcome as this provides natural winter protection

Arendsii hybrids were created by crossing the very robust Noordtiana. This is the only variety of Mucronatum hybrids that has been tried and tested in gardens, and it is very winter hardy. The flowers of Arendsii hybrids are generally light purple but can also range from pink to fiery red, and deal with cold well thanks to Noordtiana.

Japanese Azaleas are sometimes named after the cultivator or the place they were grown. Varieties therefore have names such as Rhododendron Kaempferi ‘Blue Danube’. Arendsii, Aronsense, Kiusianum and Diamond Azaleas all come from German growers. These are copyrighted brand names for particularly compact and heavily flowering varieties from East-Frisian Azalea grower Carl Fleischmann.

Adonis, a Kurume hybrid in white with fringed edges, pink-red Betty and Favorite, a Kaempferi hybrid with ruffled edges and early flowers, are also recommended Azalea varieties. Modern varieties are generally made up of so many different plants that a precise hybrid can no longer be specified.

Propagation

Japanese Azaleas can be propagated using cuttings. The chances of these taking root are best from mid July to early September. A tip: Cut the cuttings in dry weather. The plant should not be wet or damp at the time of cutting. In terms of shoot length, 1.18 to 3.15 inches has proven advisable. The success rate varies drastically depending on variety and origin. A special cultivation station with underfloor heating is essential. You should also use acidic, lime-free cutting substrate, ideally a blend of two parts pure white peat and one part quartz sand.

Diseases and Pests

Japanese Azaleas are very robust when it comes to diseases and pests. If the pH is too high or the soil is too solid, lime-induced chlorosis may arise, as the plants are not able to absorb enough iron from the ground. The most common diseases include leaf spot infections as well as Exobasidium vaccinii var. japonicum. Fungi from the Phytophthora genus are particular nuisances to the Azalea grower. They are responsible for root and stem rot. The most common pests include Otiorhynchus weevils, and spider mites may also be a problem in areas with very low humidity.

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