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Dormancy and Indoor Bonsai

by Brent Walston


The most difficult barrier to growing bonsai indoors is the need for a cold dormant period in temperate climate woody plants. The following article discusses the problems associated with dormancy and how one may overcome them.

Outdoor versus Indoor

All trees are outdoor trees. The terms 'indoor bonsai' and 'outdoor bonsai' are meaningless except to describe where you keep your trees. There are no plants that cannot be grown 'indoors' if you can supply them with what they need. In most cases, keeping temperate climate woody plants indoors is very difficult.

The factors involved in keeping plants indoors are light, temperature, humidity, watering, and most importantly, dormancy requirements. We have had intensive discussions about the role of dormancy in bonsai in the Internet Bonsai Club. The last round was just recently with some excellent information and research from Andy Walsh and Anton Nijuis. The archives of the IBC may be found at

What is Dormancy?

Dormancy is a survival strategy that temperate climate species have evolved to stay alive over the winter. These species have a biological clock that tells them to slow activity and prepare soft tissues for an onslaught of freezing temperatures.

Species that have well developed dormancy needs cannot be tricked out of them. If you attempt to give a such as species, for instance Japanese maple, Acer palmatum, an eternal summer by bringing it in the house, it will grow continuously for as long as two years. After a maximum period of sustained growth, a temperate climate plant will automatically go dormant no matter what the season or condition. Deciduous plants will lose their leaves, evergreens will curtail all new growth. This is very stressful to the plant and usually fatal. It will be 100% fatal if the plant does not receive the necessary period of cold temperatures required to break the dormancy.

To summarize, temperate climate plants require a cold dormant period. They have internal clocks that tell them when to go dormant. The clocks can be tricked to some degree. After a normal growing season, dormancy can be brought on by decreasing temperatures and shortened daylength, or delayed by maintaining summer temperatures and daylength.

Cold Hardiness

Cold hardiness acquisition is also a necessary part of dormancy in temperate climates. Plants begin entering dormancy by setting buds in mid to late summer. Stem tissues begin increasing levels of sugars and carbohydrates in response to lowering temperatures in the fall. By the time freezing temperatures arrive, they have developed enough natural antifreeze to survive freezes. Different species develop different degrees of cold hardiness according to their natural environment. The degree of cold hardiness they can acquire is genetically determined. Roots do not develop cold hardiness in the same fashion and must be protected to a greater extent than top growth in container plants.

Breaking Dormancy

In order for these species to break dormancy and begin growing again they must acquire the requisite number of hours of cold temperatures. For most of these species it is 1000 hours of temperatures below 40F. Once this requirement has been satisfied the plant may begin growing again immediately. The new growth is triggered by temperature alone. If temperatures rise much above 40F for any extended period of time, say a week or so, the buds will break and the plant will begin growing. This can happen outside in January if there is a freak warm spell, or it can be artificially manipulated if plants are brought indoors. A return to cold weather will of course kill the new growth and buds.

Dormancy in Tropicals

Tropical and subtropical plants that have evolved under milder conditions have modest or no dormancy requirements. They are capable of continuous growth at 70F+ temperatures. In fact most tropical species will grow more slowly or not at all at certain times of the year, but this is not related to dormancy. Andy Walsh refers to this phenomenon as 'quiescence'. Temperate climate plants also exhibit this phenomenon, most notably during the hot dry part of summer for desert plants. Growth resumes when favorable conditions returns.

Treatment of Subtropicals

Subtropicals such as Chinese elms, Ulmus parvifolia, have little if any dormancy requirements. In colder areas they drop their leaves, go dormant and act like deciduous trees. In milder, non freezing environments, they are evergreen and exhibit continuous growth except for occasional 'quiescence'. They require fairly high light levels and that will be the most difficult factor to maintain. A sunny window is usually insufficient and supplemental light, such as a fluorescent lamp six inches above the plant, is strongly recommended. Most subtropical plants that do not have strict dormancy requirements, still seem to perform better if they have a brief cold dormant period that allows them to lose their foliage. Both Chinese elm and Pomegranate, Punica granatum, fall into this category.

Determining Which Plants Need Dormancy

When determining whether or not a plant can be grown indoors, the strongest clue will come from its natural environment. If the species is native to a temperate climate area that receives regular freezing winter temperatures, it will be impossible to grow this plant continuously indoors. It can only be an indoor plant if you can also satisfy its dormancy requirement by providing it with the requisite number of hours of temperatures under 40F.

How to Give Plants a Dormant Period

It is not easy, but some people have become adept at growing temperate plants indoors by giving them a dormant period each year. This can be done by keeping plants in the refrigerator, in a cold garage, or outside until the dormancy requirements are met. The plants are then brought back into the house and growth is reinitiated by providing warmer temperatures and increased daylength with grow lights. This is not a procedure for beginners, and if you wish to try it, expect failures until you learn the proper techniques and the eccentricities of each species.

If, for some reason, you cannot keep your temperate plants outside all winter to give them a dormant period, here is how you can do it can do it in the refrigerator: First (if possible), keep them outside and let them enjoy a few light frosts. Ideally, four to six weeks of decreasing day length and mild cool weather where the temps are around 25 to 35F at night, will adequately prepare them. If this is not possible, just keep them as cool as possible as late as possible in the fall, and then put them in the fridge. The above preparation is not strictly necesary, but it does keep them healthier and minimizes the refrigerator period. Going directly from a growing state (AFTER a full season of growth) into cold storage will not adversely affect any temperate climate plant. They will just go dormant in the fridge, drop their leaves, etc.

Some precaution against drying out in the fridge must be taken, especially in modern frost free refrigerators. You can wrap them loosely with plastic, but do allow some circulation. Take them out weekly and check to see if they need watering. They still must be watered normally when they begin to dry out. Light is not necessary as long as the temperature is low, about 35F or lower. If you have the option, keep the temperature hovering just above freezing, it will minimize fungal problems.

As a minimum, keep them in the fridge for six weeks, longer is fine. After six weeks, they will have the 1000 hours of chill considered necessary for most temperate climate plants. You can then take them out and return them to growing conditions. This may be inside, but please read the articles on growing indoors. This will almost certainly mean good air circulation, grow lights, and added humidity such as a growing chamber or small greenhouse.

In the beginning, it is far more important to learn how to properly water, prune, fertilize, and repot your tropical bonsai than it is try to manipulate the dormant period of temperate climate species.

And finally

Why is there so much apparent conflict in the advice of individuals and books on which plants can be grown indoors? The key goes back to my opening statement: All plants are outdoor plants, but any plant may be grown indoors if you give it what it needs. Some people have discovered what a particular temperate species needs, others have not.

As a beginner, stick to tropical plants, such as Ficusspecies, that have no dormancy requirements for indoor growing. Match their natural growing conditions as closely as possible. As you gain experience you may want to try to grow some temperate species indoors by providing them with a yearly dormant period.

Related Articles:
Growing Indoors
Overwintering Bonsai

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