When should you repot? Generally, trees should be repotted in the spring. This includes taking plants from nursery pots and from the ground, as well as repotting in bonsai pots. Root pruning and then repotting in the same size pot will give the roots room to grow. Only new roots absorb water and nutrients; so when the roots have filled the pot, you have to prune out some of the old roots to make room for new growth. In addition, the organic material in the soil should be replenished to maintain its nutrient value. For this reason, do not re-use old soil. Replenishing the soil makes a tree grow more vigorously, and thus can cause foliage to increase in size and internodes to lengthen. With deciduous trees, you can prune the vigorous growth; and they will bud back, thus staying compact. Evergreens, however, should perhaps be repotted less often if you want to keep their growth slow. Young trees should be repotted once every year or two and older bonsai every three or four years, depending on the type of tree and the root growth. Bonsai need repotting if they are root-bound and the top of the soil begins rising in the pot, if the roots start growing through the drainage hole because they lack room, or if drainage is slow.

Trees that seem to be declining and growing too slowly may be invigorated by replenishing their soil. Repot deciduous trees in the early spring when buds start to swell but before leaves start to unfold. This indicates the tree is waking up and new roots will be stimulated to grow. Before the leaves unfold, they won't lose water; as a result, the tree will not dry out when you disturb the roots. Repot conifers after deciduous trees, but still in the spring (before the end of May). Some hardy plants such as junipers can be potted in June. Flowering plants, like azaleas, should have their flowers removed after they bloom. They can then be pruned and repotted (usually near the end of May). If you cannot repot at the right time, wait until the following year, unless it's an emergency.


Roots need air, water and chemicals. This determines what you use for soil. The most common problem is not giving roots enough air. Tree roots need oxygen just as much as water; otherwise, we would grow them in mason jars full of water. The soil has to have spaces for water to drain out and air to replace it. The ability of the soil to drain properly is its most important characteristic. The soil should, however, hold some moisture to be gradually released into the air spaces between waterings. Roots absorb moisture from the vapor created in the air spaces. Finally, roots need food in the form of chemicals (nitrogen, phosphorus, potassium, and others). Some chemicals come from the air and water; others are provided by organic materials and clay particles in the soil. We also use fertilizer, but more about that later. To summarize, then, soil needs air spaces, some water retention and organic material for food. These needs are filled by sand, loam and peat.

Sand can be any type of rocky hard, non-absorbant material. The main consideration is particle size. The larger the particles, the larger the air spaces, the better the drainage and the faster it will dry out. Good materials include river sand, volcanic sand or crushed granite (poultry grit). The type of particles (smooth or sharp) may also be important. Sharp particles create irregular spaces in which the roots must divide or grow at angles. Theoretically, this root growth is reflected in angular and irregular growth of the branches. On the other hand, smooth sand allows roots to grow straight, and this causes the branches to flow. This type of growth may be desirable for maples, for instance. If it is too fine, smooth sand will pack down and choke the roots. Therefore, use coarse river sand. By the way, never use ocean sand because the salt is toxic. A favorite material to use is hadite, which is somewhat like volcanic rock in that it is porous but doesn't break down. It can replace both sand and loam or clay.

Loam consists of particles of clay, garden soil, potting soil or some other clay-like material that absorbs and holds water. Loam also holds dissolved chemical fertilizer and releases it slowly. One favorite material is baked clay particles which will not break up into mush. Terra Green is a popular brand marketed as a soil amendment for turf. It resembles kitty litter without chemicals. Try to find a product with a particle size of 1/16 to 1/4 inch diameter. Sift out any dust before using.

Peat is a generic term for materials that provide organic compounds to the soil. Peat moss of various kinds can be used in bonsai soil, but it tends to compact, dry out and be hard to wet properly. A better material is ground bark, which must be aged or rotted, not fresh. "Soil-less" mixtures, such as PRO-MIX may be used. It contains peat moss and other organic materials, along with Perlite and Vermiculite, which partially replace sand and loam. This kind of mix is easy to come by, but it is expensive and the particle size is so fine that you lose half of it when you sift out the dust. I used to use a mixture of 1/3 PRO-MIX BX, 1/3 Medium crushed granite (turkey grit), and 1/3 Terra Green, all sifted before mixing. Currently I use about 1/2 hadite and 1/2 bark in 1/8 to 1/4 inch particle size. For smaller pots or moisture-loving trees I mix in some Terra Green with a finer particle size.

Sifting soil. All soil components should be screened to get out the small particles unless it is pre-screened or washed. If you don't sift, the small particles will fill up the spaces, prevent drainage and not allow air to enter the soil. You can buy a set of screens with a holder from a bonsai supply vendor. Before mixing the components, sift each material through the finest screen, which is about 1/16 inch or window screen; only use what doesn't go through. You can use the larger size screens to adjust the size of the particles, especially with bark. Use the big bark chunks for mulch in your yard.

Another consideration is that the soil in a pot should be uniform in particle size. The reason is that soil with larger air spaces will dry out faster than heavier, finer soil. If the two soils are next to each other in a pot, one soil will dry out while the other will still be moist. Since roots will only grow in the moist soil, this reduces the effective size of the pot. Soil uniformity is important in two cases. First, if you keep a large root ball and only fill in soil around the edges, use soil that is similar (as far as particle size and water retention) to that in the root ball. Second, use the same soil in the whole pot. To maintain drainage channels, put a little coarse gravel over the screen so the finer soil won't escape, rather than using a thick layer of coarse gravel in the bottom of the pot. A thick layer of coarse gravel will quickly lose water and not allow roots to grow into it. Thus the roots will be confined to the moist soil above this layer. This is called a "perched water table."


Before disturbing the tree roots, have the tree and pot ready. Select a pot with plenty of room for a tree in training. Only use pots with drainage holes. They may be clay, plastic or wooden. Put 1/8 inch screen inside the pot over the holes, making each piece of screen just a little larger than the hole. Secure it there with a piece of copper wire that goes through the screen in two places, out through the hole, and is bent over to hold the screen against the hole. Also put copper wires through the holes to tie the tree in the pot. Put one end through each hole if there are two, or secure the wire to a larger copper wire outside the pot if there is one hole. You will later twist the ends of this wire around the root ball to hold the tree in the pot. You should prune and wire the tree before repotting, because once you pot it, the tree should not be disturbed for a while until new roots grow. When the tree, the pot and the soil are ready you may work on the roots.

Take the tree out of the container. Use your fingers to loosen and spread the bottom of the root ball. Use a chopstick sharpened in a pencil sharpener to comb out the roots and remove the old soil, starting at the bottom outside rim of the root ball and working upward and inward. Leave the center portion of the root ball near the trunk intact without removing the soil. Look at the pot you are going to use. You need to trim the roots to fit; they should not curve around inside the pot, nor should they stick up out of it. Mentally place the tree in the pot. Take the distance from the trunk to the edge of the pot on each side of the tree. Comb the roots out to within 1/3 of this distance from the trunk. Then trim the roots with sharp scissors at a distance of another third from the trunk. Thus when the tree is in the pot, there should be 1/3 undisturbed root ball, 1/3 combed out roots, and 1/3 space on each side of the trunk. Also remove dead roots or larger roots that do not have a lot of fine roots on them.

The tree is now ready to pot. Put a layer of soil at least 1/2 inch thick in the bottom of the pot. Make a mound of soil where the trunk will be, so the tree will sit up a little bit and the roots will be aimed slightly downward from the trunk. The tree should be positioned off center and slightly behind the middle of the pot. Position the tree so it looks balanced the way you want it and so the front is facing the exact front of the pot. The exposed base of the tree should be just above the rim of the pot. Now put the wires which you threaded though the drainage holes around the root ball and twist them together. Use pliers to pull on this wire and twist until it is snug. Do not strangle the roots, just make it snug enough to hold the tree. Sometimes two wires are used, one behind and one in front of the trunk. Cut the excess off and bend the twisted ends down into the soil. Add dry soil mixture over the roots. With a pointed chopstick, eliminate large air pockets in the soil. Do this by inserting the chopstick through the roots to the bottom of the pot and moving the top of the stick back and forth. The dry soil will flow down along the stick and fill the air pockets. Do this all around the pot, from the center outward. Then raise the pot a couple inches and tap it on the table to further settle the soil. You also may press the soil down while it is dry; but once you water it, do not push on the soil surface because wet soil will pack down too much and may eliminate air spaces. Mound the soil up slightly near the trunk leaving the top surface of the large roots exposed. The soil should be slightly below the rim at the edge of the pot.

Then water. It is best to soak the newly potted roots and soil in water for about ten minutes. Add 10 drops of Superthrive (vitamin B1, a root stimulant) per gallon of water. Set the pot in a pan and then pour the water around the pot up to its rim. Let the water soak up from the bottom holes into the soil all the way to the top. After soaking for 10 minutes, take the pot out of the water bath and let it drain. Handle the pot gently from then on so you do not pack the soil. Do not fertilize for a month.


After potting a tree, it needs special care for two reasons. First, the fine roots growing into the new soil are very easily broken. Therefore, absolutely avoid moving the tree around in the soil. Don't touch it; don't try to work on it; don't even leave it where it might be bumped or disturbed until the roots grow and strengthen (about 2-4 weeks). Second, the tree will have trouble absorbing water until the roots recover and begin growing. In fact, after the initial soaking you should let the soil get relatively dry before the first watering to stimulate the roots to grow. The main thing is to prevent the tree from losing too much water through transpiration. Some water is lost through evaporation from the leaf surface itself; but even more may be lost through the stomata, which are pores on the leaf surface. When these are open the tree can lose up to fifty times more water than when they are closed. What opens them? Sunlight. Therefore, after potting, keep the tree in a place that is shaded, humid and out of the wind. This will prevent much of the evaporation and transpiration so the tree will not wilt. There are two additional tips, especially if you are repotting late when there are leaves on the tree. One is to cut off most of the leaves so the tree won't lose water. This can be risky, however. The other method is to spray the leaves, top and bottom, with Wilt-Pruf, a waxy substance that prevents evaporation. This is probably safe for most trees. As mentioned before, it is better for deciduous trees to be potted before the leaves unfold in the spring, so they won't lose much water.

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