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1927 Map of the Arboretum

Bonsai & Penjing Collection Care

Following is an outline of the procedures used by the staff of the Arnold Arboretum to maintain the health of the Bonsai & Penjing Collection.


Loosening bonsai soil from the roots
Loosening bonsai soil from the roots

The smaller the pot, the more frequent the need for repotting. This procedure is best done in early spring mid to late March before the plant shows signs of growth. After the plant is removed from its container, 2 to 3 centimeters (one inch, more or less) of roots plus their attached soil are removed from the sides and bottom of the rootball. Any roots thicker than a pencil are cut off to encourage small feeder roots to develop. This rejuvenates the root system of the plant and prevents lethal girdling roots from forming. After the rootball is trimmed, the plant is returned to its original container, surrounded by fresh soil. The large hinokis are repotted every 4 to 5 years, while the smaller plants are repotted every 2 to 3 years.

Soil Mixes

Plant roots are so intimately involved with soil particles that it is best to think of the soil as part of the plant itself. Therefore, time and care needs to go into its preparation. The potting mix should provide for both water retention and air circulation. Our repotting mixes consist of coarse sand (particle size 1 to 3 mm), peat moss or leaf mold, and screened loam, with proportions varying for different plants. In general, we use one-half sand, one-quarter loam, and one-quarter peat for the conifers; and one-third sand, one-third peat, and one-third loam for deciduous trees. In either case, small amounts of superphosphate and organic nitrogen fertilizer are added.


Pruning a Hinoki Cypress bonsai
Pruning a Hinoki Cypress bonsai
Root pruning bonsai
Root pruning

There are no universal rules about how much to prune a bonsai; the techniques vary according to the species. The best time to prune is when the plants are producing new growth in early spring for deciduous plants, such as the cherries and japanese maples, in mid spring for pines and spruces, and in early to mid summer for the junipers and hinokis. At least 50 percent of the new growth is removed at the time of pruning. If the plant produces a second flush of leaves later in the growing season, these also must be pruned.

Additional pruning procedures are used for specific groups of plants.

  • Pines The number of candles is thinned by 1/2 to 2/3, and those that remain are shortened.
  • Spruces and firs The newly flushing shoots are pinched back to half their length, inducing replacement buds to form at the base of the new growth rather than at the tip.
  • Maples The new shoots are pinched back to a maximum of two pairs of leaves and sometimes only one pair. Any vertical-growing shoots are removed or wired into a horizontal position.
  • Hinokis and juniper Because these plants produce new growth throughout the growing season, the new growth is pinched back several times. If it is not rigorously thinned, the new growth becomes excessively congested and subject to death by self-shading.


In young, vigorous bonsai, shaping the branches with copper or aluminum wire is an extremely important part of the training process. We generally wire branches in a horizontal position to achieve the effect of age. It is important to remember that wire should not be left on the tree more than a year since the branch can easily be girdled by it.

Wire on a bonsai branch
Wiring holds a branch in place

On plants as old as the hinoki cypresses in the Larz Anderson Collection, reorienting their twisted branches with wire is very difficult. These branches thicken so slowly that it may take two or three years for them to produce enough wood to overcome their old orientation. We have found that tying them down with nylon fishing line is more effective than wiring.


Because the Larz Anderson Collection consists of large plants in small pots, their water requirements are high. During spring growth, they need watering at least once a day. In summer, daily watering is the minimum on days when no rain falls, and often they require more.

Watering bonsai

To determine if a plant needs water, we place the palm of the hand on the soil surface. If we detect any moisture, we do not water the plant. If it is dry to the touch, we water it. The palm is less heavily calloused than the fingertips and therefore more sensitive. The rootball of a healthy bonsai behaves like a sponge, that is, water is uniformly distributed throughout its mass at all times, so the moisture content of the surface mirrors that of the base.

When watering, we take care not to wet the foliage, especially on sunny days when water drops can magnify the sun’s energy enough to produce burn spots. At watering time, we fill the pot to the top and allow the water to drain through; then we fill the pot a second time. This double dousing ensures that we wet the entire rootball; any excess will percolate out the drainage holes. If only the top part of the rootball is moistened, the bottom will become excessively dry and the plant could be seriously injured. Less frequent, thorough watering is always preferable to frequent light watering for any containerized plant.


While the instructions provided by the Yokohama Nursery call for fertilizing the plants with powdered oil cake (consisting of soybean or rapeseed after the oil has been pressed out) or bonemeal, we use a chemical fertilizer solution diluted to a concentration of approximately 0.01 percent nitrogen, phosphorus, and potassium. When growth begins in the spring, we water the plants with this solution every one to two weeks until mid July, after which point we fertilize only once every two to three weeks through October. In November, the plants are going dormant, and we stop fertilizing them altogether.

Winter Storage

In the milder parts of the United States, as in much of Japan, bonsai can be left out-of-doors all winter with only minimal protection from the elements. New England’s more severe winter weather requires us to protect the plants. A plant that is perfectly hardy growing in the ground is not as hardy when grown in a container above ground; the soil, which has great insulating power, never gets as cold as the air, which has no insulating value at all.

The Arboretum bonsai are stored in a concrete-block structure with the temperature between 33 and 36 degrees Fahrenheit. The plants are checked for water once a week. Generally they need watering about once a month. If the plants become too dry during storage, they become difficult to rewet in spring. On the other hand, if the plants are kept too wet in storage, they become susceptible to fungal infections.

Bonsai spend the winter the cold storage building
Bonsai spend the winter in cold storage

As long as the temperatures remain below 36 degrees, the plants survive even in total darkness. Storage that dark will not work at higher temperatures. The key to successful winter storage is to make sure that the plants are fully dormant before they go in and that they come out before they show any signs of growth. Typically, our plants go into cold storage on Veterans Day (November 11) and come out on Patriots Day (April 19), although a week either way makes little difference.

Excerpted from “Early American Bonsai: The Larz Anderson Collection of the Arnold Arboretum” by Peter Del Tredici, which was published in Arnoldia (Summer 1989) 49(3): 2-27.