Winter care of satsuki azaleas

Satsuki azalea's are originally from Japan and usually grown in pots. While many parts of Japan have short and cold winters, Japan experiences a relatively temperate climate, being an island. And in Japan, satsuki have been grown in pots and given winter shelter for at leat two centuries. This can create some challenges when growing satsuki in other parts of the world. In this article, I will go through some of the challenges of making sure your satsuki make it through winter.

For those growing satsuki outside of Japan, there are generally 3 types of challenges:

1. Cold winters after cool short summers in temperate climates
2. Long, very cold winters in continental climates
3. Winters with a strong rainy season

Western Europe like The United Kingdom & Republic of Ireland, the Netherlands, Belgium, large parts of France and Germany, and some coastal parts of Scandinavia, experience winters comparable in cold with Japan. However, the winters are longer and often lack snow cover. These parts of Europe are generally USDA zone 7 or 8. Additionally, the winters will be wetter. But the most important factor is that the summers are much cooler and shorter. The inability of satsuki to get in the proper amount of growth has been one of the reasons why satsuki have suffered in unusually cold winters of western Europe. And why satsuki are not widely grown as landscape plants in western Europe. The climate change of the last couple of decades has made this part of the world more suitable to growing satsuki. Making the summers wetter and the winters milder and shorter. Locations were people may decide to grow satsuki that have very cold winters include areas with continental climates in North America, as well as many parts of central and eastern Europe. These areas are often USDA zone 6 and 7. Climates with zone 5 are arguably too cold for growing potted satsuki azaleas, as even a heated greenhouse will offer quite some challenges. These regions in North America usually do have the longer and hotter summers. The same can be true for central Europe. But parts of eastern Europe that are zone 6 usuually experience both short summers and long cold winters. Finally, there are regions that experience a strong rainy season during winter. Japan famously has a summer rainy season, and thus less rain during winter. Regions like the Pacific Northwest of North America but also coastal parts of the United Kingdom and Scandinavia and experience high levels of precipitation during plant dormancy.

There are two main ways in which winter cold can damage plants. The first is that the water inside the plant tissues freezes. The formation of ice makes the water expand and turns it from a liquid into potentially sharp crystals. Ice crystals forming inside or near living cells can cause cell membranes to be ruptured, which can result in cell death. The second way winter can damage plants is through dessication. When water freezes, it can no longer flow as a liquid. Plant roots are only able to take up liquid water. When the soil around the roots freezes for a prolonged time, the plant does lose water thorugh evaporation, risking dessication. The plant is much more at risk if frozen soil occurs during a period of strong winds with a very low humidity. A third way in which azaleas can be damages is bark split. If there is still too much moisture in the trunk, the bark can split off from the trunk, causing daamage. If this happens at the base of the plant, the entire plant could die. Unusually early or late frosts, frost period with extremely low temperatures, frost periods with strong winds, or consequitive freeze and thaw cycles can damage azaleas. Besides the cold, snow, falling ice, or the winds of winter storms may damage satsuki bonsai through physical force. When cells are damaged or destroyed by the winter cold, the tissues will turn brown or black and become necrotic. This will become evident within days. When a plant dessicates, it will wilt and then turn brown.¹ Azaleas will recover from most winter injuries, but if they are bonsai it may negatively affect the quality of the tree or interfere with bonsai development or training. Additionally, satsuki bonsai may already be tapping into reserves because it was severely (root)pruned earlier in the year. Therefore, a plan for winte protection should be in place. Especially for the more valuable specimen.

Plants protect themselves from winter by going dormant during the winter months. Satsuki and all other evergreen azaleas, go dormant during winter. Dormancy is triggered by a combination of shorter daylength and lower temperatuers. Azaleas will set flower buds and seize all growth. Many varieties will also shed their larger spring leaves, retaining only the smaller leaves directly around the flower bud. These spring leaves will change colour as their chlorophyll is broken down, revealing the colour of the underlying anthocyanidins pigments, just as decidious trees and their autumn colour. There are also varieties that will display autumn colour on all of their foliage. As the azalea enters into winter dormancy, numerious biochemical changes occur and gene expression patterns change. Azaleas will then be able to reduce the levels of water in their tissues, which increases the concentrations of solutes. Watery solutions that are high in sugars and salts will freeze at temperatures below zero degrees centigrade. Additonally, azaleas will produce anti-freeze proteins, directly preventing ice crystal formation inside the cells.² For an azalea to go dormant, it is essential that they experience the shorter photoperiod as the daylength reduces during summer and early autumn. Next, they should experience chilling temperatures around 5C. At this point, the satsuki should show several of the signs of dormancy. But to become resistant to the coldest temperature, the azalea first needs to experience moderate freezing. Only a moerate frost will prime the azalea to be as resiliant as possible during the coldest part of winter.

It is important to note that plants gain more resistance to the cold of winter as they experience gradually colder weather. This means that the plants are at their maximum hardiness near the end of winter. As a consequence, in late autumn or early winter without any low temperatures, the satsuki will not have achieved it's full hardiness yet. This process is called 'cold-acclimation' and it should play a key role in your winter management plan.³ The goal is to avoid exposing your satsuki to sudden cold temperatures or temperature extremes. And therefore, we should actually make sure the satsuki gets to experience these gradual colder temperatueres. What should be avoided is to 'protect' the satsuki from early mild frosts that follow a gradual decrease in temperatures, as this robs the satsuki from it's opportunity to acclimitize to the cold of winter. Such misguided 'protection' may actually cause the satsuki to get damaged later in winter. If a satsuki is prevented from achieving cold-acclimation, then it should be protected from frost during all winter, regardless of any winter hardiness guidelings found for this specific cultivar. Once the satsuki is cold-acclimatized, unusually warm spells in the middle of winter may be just as harmful as unusually cold spells, as these warm spells may undo some of the cold-acclimatized and reduce winter hardiness.

Satsuki azaleas are a diverse group of hybrids, in part because some satsuki contain genetics from species other than R.indicum. But also because they were developed in different parts of Japan. Among satsuki varieties, there are two dinstinct groups: Northern Satsuki and Southern Satsuki. Many of the Southern varieties are hybrids of R.eriocarpum. This can give these satsuki rounder leaves, their purple or lavender colour, or a stamen count above 5. Additionally, the R.indicum species used to create this hybrid may also come from a population in Southern Japan. For example, many satsuki varieties are descedent from natural hybrids of R.indicum and R.eriocarpum found on the Island of Yakushima, south of Kyushu. This island is subtropical and ranges from zone 10 at the coastlines to zone 7 at the central mountains at 1000m altitude. Some of these natural hybrids were brought to the Tokyo area in the early 20th century. Other satsuki varieties were bred and hybridized in the south for a longer period of time. Some examples are 'Wakaebisu', 'Kunpu', 'Asahizuru', 'Joga', 'Takasago' 'Izayoi', 'Gumpo' and 'Hakatajiro'. From them, we have more modern cultivars like 'Kaho', 'Eikan', 'Gyoten', Aikoku, and 'Shinnyo-no-Tsuki'. While these varieties also are better adapted to hotter summers, they lack the winter hardiness of some other satsuki. And these traits may be inherited by the newer varieties that originate from them.

The Northern varieties originate from the mountains of central Japan, namely Honshu. This area is located roughly 1000km Northeast of Yakushima. Many of the R.indicum populations and temple gardens where satsuki were cultivared, for example Nikko temple, at around 500m in altitude and right at the base of the 2000m tall Japanese alps. These Northern varieties therefore have narrow pointy leaves, lacking R.eriocarpum genetics and directly descendent from the more cold hardy populations of R.indicum.. Examples of these varieties are: 'Osakazuki', 'Chinzan', 'Kinsai', 'Kozan'. 'Nikko'. 'Nyohozan' and 'Korin'.

Besides these clear two groups, there are many other varieties that don't fit the above neat two groups. Two varieties that seem heavily aligned with R.indicum., having similar narrow leaves and compact growth are 'Kikoshi' and 'Yata-no-Kagami'. However, 'Yata-no-Kagami' is associated with Kyoto, a warmer and coastal area of central Japan. Two more satsuki that are known to be fairly winter hardy and associated with Nothern Japan are 'Kogetsu' and 'Matsunami', but their foliage is dissimilar from R.indicum., suggesting some hybrid origin.

There are many more older varieties, that today are rarely grown, but that have given rise to todays cultivar. These are 'Seidai', 'Kyokko Nishiki', 'Gobi Nishiki', 'Chitose Nishiki' and 'Fuji Nishiki'. All have unknown parentage, but it seems likely these are already hybrids with at least some genetics from R.eriocarpum. So far, these varieties have all been old varieties, that also have been grown unexposed as landscape plants for many decades. But most new satsuki hybrids have only been cultured in pots and often given winter protection. Additionally, many new varieties include genetics from both Northern and Southern groups, as well as genetics from varieties that are not satsuki. These varieties may have been cultivated in sheltered pots for several decades. A good example of a varieties with very complex heritages, including Belgian indica azaleas, are 'Iwaiboshi' and 'Asuka'. Both of these varieties have grandparents that were likely all grown by satsuki enthusiasts with some level of winter protection. This makes them different from some of the older varieties, that survived during periods of economic crisis or war, as neglected garden plants. Newer cultivars that are products of deliberate breeding can be more delicate. They will get propagated, spread around and survive for an initial time period. But age acts as a filter, weeding out unreliable or weak varieties. Therefore, older varieties are usually stronger and more robust than newer varieties. Including having experienced and survived several unusually cold winters.

A final conclusion is that as mature and healthy garden plants in the full ground, the older Northern narrow leaf satsuki are hardy up to -12 to -20°C, but probably not beyond that. Southern round-leaf satsuki are less hardy up to about -12°C. The newer varieties that have narrow leaves might be hardy enough to survive -12 to -20°C, but maybe -12 to -15°Cis their limit. For newer varieties high in genetics from tender azaleas like Belgian Indica or Southern round-leaf satsuki, their limit may be -8 to -12°C, as they were never selected for their cold-hardiness. These are very rough estimates for healthy and established garden plants, that experienced good summer leading to mature new growth, went into dormancy as winter gradually became colder, and are experiencing a short cold spell without unusually strong winter winds.

Satsuki and other tender evergreen azaleas generally do not require winter protection, even as potted plants. However, when temperatures drop below freezing before the plants are dormant, or in late spring when the plants are actively growing, then refer to the guidelines for zone 7 and 8. When winters get unusually cold, in a once in a few decade occurance, then also refer to the section for zone 7 and 8.

Oceanic climates with zone 7 and 8 USDA winters are temperate climates that cover most of western Europe, all of New Zealand, parts of Southern Australie and South Africa, as well as the coastal Northwest of North America. During ordinary winters, these regions can experience winter cold of -5°C to -10°C, usually during only a couple of days. However, every so many years these regions will experience a seriously cold winter, where a cold spell can last for 3 weeks and bring temperatures as low as -15°C. While mild winters in these regions may see very little frost at all. Note that most satsuki varieties will be hardy as mature garden plants in these oceanic zone 7 climates.

Potted plants are are more susceptible to the cold than plants in the funn soil. Generally, they are rated 1 hardiness zone lower. Since these are temperate regions that often have a shorter growing season that Japan, the first step to winter hardiness is to make sure the satsuki gets sufficient direct sunlight during autumn. While potted satsuki require shade during the hottest days of summer, they are more sun-loving than other vareities of azaleas. And they require suffient sun hours to mature their foliage and set flower buds. Therefore, when the weather is cool, it is best to give them as much sunlight as possible. This will improve their overall health and winter hardiness. A second step in winter management is to seize fertilizing in late winter or early autumn. Highly available nitrogen can drive elongated and immature growth.

Once winter starts, it is important that the satsuki goes dormant. These signs should be observed before frost occurs. If the satsuki is not dormant and frost is predicted, it should be stored for the night in an unheated shed. Such a shed should be just a few degrees above 0°C. Once dormant, the previously discussed cold-acclimation-strategy should be employed. Once the satsuki is fully dormant, cold-acclimation is underway, and the -5°Cto -10°Cwinter cold of an ordinary winter arrives, the satsuki generally do not need to be moved indoors. The first step in protecting these plants is to put them in a more sheltered part of the garden. Plants in urban gardens near heated buildings generally experience way less cold than say plants in open fields in rural areas, far away from any heated buildings. Moving the potted satsuki from their display benches to the ground helps prevent the pots from freezing as rapidly. During winter, the ground is a source of heat. Especially if the soil is still moist from earlier rain. By placing pots in a spot on the ground, packing them closer together, and sheltering them from the wind, the satsuki will experioence less cold than when spaced out on display benches, surrounded by the cold air. Make sure the root ball isn't almost dry when a cold spell starts. Water inside the soil or substrate is the best way to prevent the water around the roots from freezing solid. Additionally, mulch can be packed around the pots to trap soem of the heat provided by the ground. A final consideration when storing satsuki outside is direct sunlight on cold sunny winter days. This type of sunlight will increase the rate of water evaporation from the leaves. Water evaporation should be minimized while the roots are frozen solid. In Europe, a dry strong eastern or north-eastern wind during moderately cold temperatures can provide an additional risk. In such cases, it may be wise to move the satsuki indoors into the unheated shed.

When the winter is exceptionally cold for a zone 6 or 7 zone, with temperatures below -10°C, moving the satsuki into an unheated shed is ideal. The same should be done for cold spells that last for more than a week. Some plants may have no trouble dealing with this type of winter cold, but there are no guarantees. To prevent dessication, the satsuki should be able to take up liquid water at least once every week. Therefore, moving it indoors so it thaws earlier could be required. Once inside and with the roots thawed, it is important that the satsuki are watered. The temperature in such a shed should be below 10°C. If the temperature is around 0°C, the satsuki can be kept there in total darkness. However, if it is warmer, the satsuki will require light. Higher temperates means more metabolism, which means more energy is consumed. Without any light, there is no photosynthesis and the plants will slowly starve.

Moving the satsuki indoors into a heated space should be avoided. Once a satsuki is moved into a warm, spring-like enviroment, it will leave dormancy and start to bud out. Such a satsuki no longer has any protection against the winter cold but will also require sufficient light for active growth. The light levels that satsuki require are between 400 to 600 µmol/m₂/s. Growing satsuki indoors during winter is possible, but it is not a good overwintering strategy, as satsuki require seasonality and a cold period. Additionally, growing satsuki indoors is much more challenging than keeping them outdoors.

Once the coldest part of winter has passed and the days are getting longer, the main way of protecting your satsuki against the cold is by keeping them dormant for as long as possible. Direct sunlight from sunny weather can warm up the pots and thus the roots of your satsuki. And this may end winter dormancy prematurely. Once a satsuki starts to actively grow, all new growth is highly suceptible to even just a few degrees of frost. Such plants should also be moved into an unheated shed during these cold clear nights with frost risk during early to mid spring. An additional trick that might be used is to spray them wet with water. This technique is widely used in agriculture, especially to protect fruit trees and vineyards. This technique does not seem very popular with bonsai enthusiasts. Likely because the sight of a prized bonsai encased in ice is unsettling. But the laws of thermodynamics are quite clear, and the formation of ice releases a lot of heat, which protects the plant tissue inside the ice. The tender leaves and buds of satsuki are better off encased inside ice than to be exposed directly to the air cold enough to cause all that liquid water to freeze. This technique can be used most effectively when in spring, temperatures drop below zero 0°C just for the last few hours of the night. During these spring early mornings, temperatures often drop to their lowest just after sunset. It should not be used in the middle of winter where temperatures drop below freezing all night.

In regions with a zone 6 winter, most satsuki varieties will not be hardy even as healthy large and mature landscape plants. Eventually, a very cold winter will come and the plant will suffer significant winter damage. For these areas, it is very challenging to overwinter satsuki bonsai or potted satsuki. Minimum templeratures become too low and usually for too long of a period. For most zone 6 regions, periods of cold last too long for temporarily storing a satsuki inside a unheated building to be sufficient. These places usually lack sufficient light for healthy overwintering of valuable satsuki bonsai for a periods of weeks, or longer. In these winter hardiness zones, potted satsuki require a greenhouse. However, it should be noted that while a greenhouse has a significant effect on ambient temperatures, during periods of deep freezes, temperatures inside a greenhouse may drop. The big advantage of a greenhouse is that during the day, when the sun is out, temperatuers inside the greenhouse can rise significantly. Ideally for the satsuki, temperatures are fairly stable around 0C. But, for greenhouses that are not insulated, nighttime temperatures will eventually approach outdoor temperatures as heat leaves the greeenhouse during the night. A greenhouse without temperature control will make temperatures fluctuate more, rather than less. And in prolonged periods of daytime frost, the greenhouse effect of a greenhouse will eventually be nullified. To protect satsuki against the cold properly inside a zone 6 area, the greenhouse will need to be heated or insulated. But a greenhouse can also be a disadvantage. Because it is essential for the satsuki to stay dormant, the greenhouse should not warm up during sunny days and make the satsuki inside experience a false spring. A greenhouse that gives the satsuki spring-like temperatures during midwinter, but then does not warm or insulate these same satsuki during a cold spell, will actually be worse than no protection at all. Temperature control is essential. Both cooling and heating. If there is no mechanism to prevent the greenhouse from getting warmer than 10C, the satsuki should be taken out of the greenhouse. This may actually mean that the satsuki need to be moved in and out of the greenhouse many time during the coarse of winter. Which also introduces the possiblilty of mistakes or stressful situations. Ideal would be a large heated greenhouse with automatic ventiation and cooling. Such a greenhouse can be quite an investment.

One more factor to consider when keeping satsuki, or other plants, inside a greenhouse for most of the months of winter, are pests. In an enclosed envoriment an at higher temperatures, harmful insects may get free reign to harm your satsuki. Even more of an issue during the winter period, are fungal infections. UV light, moving air, and low humidity can keep fungal infections in check. During summer, UV light is the main killer of fungi. But inside a greenhouse, most of the UV light is blocked out by the glass or plastic walls. The enclosure also blocks the wind and captures inside moisture, resulting in stale humid air. In such an enclosed, moist, and cool enviroment, fungal growth can become unchecked and harm your satsuki. So when storing satsuki for weeks or even moths inside a greenhouse, it is important to be on the lookout for symptoms and be ready to treat plants with an appropriate fungicide.

Winters in zone 5 are too cold even for the most cold-hardy evergreen azaleas. Only a few evergreen azaleas and some decidious azaleas can survive winters in these regions. Overwintering inside a heated greenhouse may still be possible, but will be even more challenging. The satsuki likely need to be inside the greenhouse for all of winter. This requires