Japanese Azaleas

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 Vuyk’s Scarlet and Vuyk’s 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 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.

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.