by Ross Clark

The biology of roots is a complex and incompletely understood subject.  No matter what anyone might write or say on the subject will be incomplete, and vulnerable to contradiction and differences of opinion.  So by all means, let’s address it head-on!

How roots function is of paramount importance to bonsai.  However, open any bonsai book you choose, and you will find that the subject of root function usually is discussed only in passing, mostly with reference to nebari, repotting and root pruning.  When we look at a tree, our impression is heavily weighted toward what we see, but a tree can fulfill its potential only if its root system is healthy.  Although it is important to artistic design, the surface root buttress bears little relationship to the health of a bonsai.

All parts of a bonsai exist under unnatural conditions, but the environment of bonsai roots is perhaps the most extremely artificial part of bonsai cultivation.  Another significant point to keep in mind is that there are very significant differences in the ways roots and stems function.  Most bonsai roots (with the exception of Ficus/fig prop roots, etc.) are adapted to being underground and are physiologically adjusted to an environment where changes are more gradual and extremes are less intense than the environment above ground.  In underground natural environments, extremes in moisture and temperature usually develop gradually, and interactions with soil organisms are incredibly complex.  When we grow a tree in a bonsai pot, roots are subjected to a much more artificial and extreme environment that than they would experience in nature, and they are exposed to those extremes day after day after day and year after year.

Maybe a closer look at some aspects of the bonsai root environment will give us some insights into growing better bonsai.  We will begin with temperature.

Bonsai Roots and Temperature

Roots cannot tolerate the temperature extremes that stems can tolerate.  In places where trees grow naturally, soil temperature usually changes gradually from season to season.  In forests, temperature changes occur with the seasons, as the photoperiod or day length, angle of the sun, and changes in the overhead foliage shadow.  In spring, longer days cause the soil temperature to gradually increase, and in fall, photoperiod and soil temperature gradually become lower.  A few days of unseasonably cool or hot weather will hardly affect the soil temperature at all a few cm. below the surface, because soil is a good insulator.  Organic litter and vegetation shade and insulate the soil itself.  Obviously, air temperatures fluctuate much more quickly and are more extreme, so stems and leaves are adapted to greater and much more rapid temperature changes, compared to roots.

Above-ground parts of fully hardy plants will resist frost damage to very low temperatures if they are dormant and protected from direct winter sunlight and drying wind.  However, roots are not nearly as tough.  The roots of fully hardy woody plants (such as spruces, temperate rhododendrons/azaleas, and ponderosa pines) begin to die when the soil temperature goes lower than the low 20s (oF).  For that reason, winter root protection from cold is necessary for all hardy woody plants when they are container-grown in cold regions.  To minimize risk to their roots, cold-hardy plants in unprotected pots should never be exposed to air temperatures lower than the mid-20s.  Only a brief time below the killing temperature will kill roots.  The roots of semi-hardy plants such as olives, pomegranates and yaupon holly should not be exposed to freezing temperatures, but they do benefit from a cooler winter period.  Some tropical plant roots will not tolerate soil temperatures below 50-55o, especially if the temperature remains low for long.  The roots will die, and if they remain moist, they will begin to rot.  Only heroic measures and good luck can save the plants then.  To be on the safe side, the soil of fully tropical plants should be kept above 60o year-round.

It may surprise you to learn that the roots of most temperate species grow best and function most efficiently when soil temperature is in the 60s and 70s.  In fall and winter, roots of temperate plants grow more slowly as temperature declines, stopping growth only when the soil temperature approaches freezing.  In the spring, rising soil temperature triggers the roots to begin absorbing and growing again.  About 36-38oF. is considered to be an ideal temperature for fully dormant temperate woody plant roots.  Once the roots break dormancy in spring (the signal for broken root dormancy is that soil dries out more often), you should not expose pots to temperatures below freezing.  (Remember, the temperature in unprotected pots will be roughly the same as the air temperature.)  A common symptom of winter root kill is that the plant will not come out of dormancy in the spring, even though the cambium and buds appear to be alive.  When the roots have been killed, the buds simply won’t swell and open, even though the stem may remain temporarily alive.  If the plant has been killed in winter from becoming too dry instead of from root freezing, the cambium in spring will be dead and dry instead of alive.  The cambium of root-killed plants will die when the food (starch) stored in the stem is used up.

Wintering soil temperature is a commonly discussed subject, so we will not rehash it here.  But what about summer soil temperature?  The subject is usually omitted from bonsai discussions.  Maybe we don’t hear as much about it, because we’re watching our plants more carefully during the growing season and we can catch potential problems earlier.  Or maybe, a certain amount of root stress aids dwarfing.  (It is worth noting that you can produce beautiful bonsai without subjecting your plants to high stress.)  At any rate, what is the effect of high summer temperatures on roots?

Roots do die, without our help.  It is natural for smaller roots to die and regrow.  There is much more natural turnover in root systems than in shoot systems.  (The shoot system includes the stems and everything stems produce.)  However, when you unpot a container-grown plant and discover that the container contains mainly dead roots, it is a clear signal that there definitely is–or was–a problem.  Often, the problem is related to temperature.  (Underwatering also is often to blame.)  Experienced nurserymen (yes, and nurserywomen) are aware that direct sunlight on nursery containers easily produces temperatures high enough to kill roots.  Informal experiments have shown that the roots of common landscape woody plants are under stress when soil temperature reaches 90, and roots die as the temperature rises through the 90s and beyond.  So, the moral is, try to keep the soil temperature in all bonsai pots below 90o, if possible.  Some genera, such as spruces and hemlocks, are adapted to cooler summer soil temperatures than most other trees.  If you want those genera to stay vigorous, you should take special precautions to keep the pots they live in on the cool side, by not exposing the pots to any more sun that is absolutely necessary.  If you have had trouble keeping spruce and hemlock bonsai healthy over the long haul, perhaps you need to try harder to duplicate the conditions under which they grow naturally; that is, moist acid soil with excellent drainage, cool summer soil, and long, cold winters (but always with root protection).  Or perhaps you simply live in a climate where spruces and hemlocks will not do well as bonsai because the summer temperatures are too high to maintain healthy roots.

Bonsai books and discussions contain recommendations about growing various species in shade, partial shade, or sun.  Those discussions are informative, as far as they go.  The problem is, information of this type is often copied from one author to another, without conveying anything about the actual natural habitat of the plants.  Another problem with such advice is that people who write bonsai books and those who read them often live in quite different climates.  The climate of Japan and much of the U.S. west coast is a maritime climate, greatly influenced by proximity to oceans.  As a result, the climate is more humid and has more moderate summers than we have in Kentucky.  Growing instructions for Japanese or German bonsai are not especially helpful for the interior of North America.  Also, Japanese bonsai nurseries typically are set up so that there is someone whose primary responsibility is watering.  If you are a typical American bonsai enthusiast, you may not spend much of every day at home with your bonsai, may live in the continent interior, and may not have time to mist and water at the optimum times.  Therefore, perhaps it might be logical to keep your bonsai in slightly larger pots.  Is a healthy tree in a slightly oversized pot as beautiful as a stressed tree in a minimum-sized pot?  What works best in Japan or England or Seattle does not always work best in continental climates.  What is considered to be over-potted there may be necessary here.  Health and artistic design and balance are what make a bonsai beautiful; pot dimension is important but not primary.   (For example, do you use Naka’s Fibonacci method to calculate pot dimensions, or a more informal, less precise method?)

Here are some methods of trying to keep summer soil temperatures within bounds.  Almost everyone uses a combination of some of these methods, but perhaps there is an idea or two here that you might want to consider trying.  These techniques are not listed in order of their effectiveness, and some of them obviously are very elementary.

• Monitor the temperature near your bonsai.  The microenvironment of the actual place where your bonsai grow is vastly more significant than the temperature at the airport or in a neighboring town or city.
• Water more often when weather is hot and dry.  Evapotranspiration (the combination of water loss from the soil surface and leaves) cools plants and their roots, because water must absorb heat to change from a liquid to a gas.
• Mist plants and soil surfaces during hot weather.  Do it in the middle of the day, when the cooling effect will be greatest.  Have no fear:  it is an absolute myth that watering plants in the sunlight will damage their leaves.  Don’t mist in the evening, because it will encourage pathogenic fungi to infect the foliage.
• Give all of your plants shade protection from overhead whenever the temperature is predicted to be in the mid-90s or above.  Remember, officially predicted or reported temperatures are the shade temperatures—it will be even hotter in the sun.  Full sun does more harm than good to plants when the soil temperature becomes high.
• Know the natural tolerances of the plants you grow as bonsai.  Plants that are physiologically adapted to high light intensity do not need unfiltered sunlight the entire day.  Plants that can do well in Japan with full sun usually benefit from from some shade protection during the summers of continental climates.  (Have you noticed that some bonsai nurseries use shade cloth, even for junipers and pines?)
• Vary the exposure of your bonsai from season to season.  A full sun location may be shaded during certain parts of the growing season and very sunny at other times.  If the temperature is cool, some species can tolerate more light.
• Pots of a lighter color will stay cooler than traditional dark brown training pots.
• Consider modestly overpotting your plants.  You (or your designated waterer) will be less nervous about the well-being of bonsai if you are away for a few more hours than you planned to be.  Don’t greatly overpot though, because permanently wet soil can encourage roots to rot.
• Position your growing benches strategically.  For instance, you could position a bonsai bench below a deciduous landscape tree so that the bonsai receive morning and late afternoon sun, but not midday sun.
• Position bonsai so that the foliage of a bonsai shades its own pot during the heat of the day, or so that smaller, low value plants  (such as one-gallon mugo pines) shade the soil (but not the foliage) of larger plants.
• Use moss to help insulate the soil surface.  Moss also increases the evaporative surface area, which increases the cooling effect.  Trays also increase the functional surface area, but keep in mind that faster evaporation will dry the soil faster.
• Mulch the soil in your pots to insulate it from the sun.  Materials such as pine bark or pine needles can also help maintain higher acidity in the soil, which benefits some plants, such as azaleas and pines.  Please don’t use cypress mulch, because most of it is made by chipping up whole, healthy cypress trees.
• In an extremely hot spell, consider an inexpensive, reflective cover for pots, such as cheesecloth.  Has anyone tried this?  I haven’t, but it should work.  Not only is it reflective, but it also is an effective evaporative surface.  Don’t use aluminum foil to protect pots; it can cause other problems.
• Position bricks or empty pots strategically among your bonsai to intercept heat (act as heat sinks) that otherwise would heat the soil of your bonsai.
• Under extreme conditions, water with refrigerated water?  Hey, extreme conditions may call for extreme remedies.
• If I have left anything out, doubtless you have noticed.  Please let me know how you think this list of suggestions could be improved.

ILLUSTRATION OF SOME OF THE TECHNIQUES MENTIONED ABOVE:
Note the brick-in-can heat sink in foreground, sand-colored drum pot in right rear, small mugo pines on the left to shade soil and provide heat sinks for the pots to the right, moss in the pot behind the can, and pine bark mulch in the sand-colored drum pot.  Pots have been carefully positioned so that foliage does not shade the foliage of other pots, except those whose plants require more shade.  South (direction of maximum solar radiation) is to the left.  Picture was taken in early morning.  One of the problems created by close spacing of plants on benches is that pests can spread more easily.  (Photo by Ross Clark)

 . . . to be continued in a future issue . . .