The correct method for repotting and changing the soil of stump bonsai

  After a stump bonsai has grown for some time, its roots fill the potting soil, crisscrossing to the point that their ability to absorb fertilizer and water weakens; at the same time, the nutrient elements in the potting soil become fewer and fewer, which is unfavorable to the growth of the stump, so repotting and changing the soil are necessary.

  Repotting is generally carried out in September in autumn or from early spring to Qingming. For tree species with fast root growth, such as pomegranate and trident maple, repot once every 1-2 years; for slower-growing species, such as pines and cypresses, repot once every 3-4 years.

  The raw material for potting soil is generally leaf mold, which can be made by composting mountain soil, garden soil and fallen leaves. The specific method is: after the trees shed their leaves in late autumn, dig a pit 1 m deep in an open and convenient place, pile the fallen leaves and mountain soil or garden soil in layers, and pour on some human feces and urine. Pile it 20 cm high, seal it around with mud, and let it ferment. Turn the pile once about every 1 month; after 3 months the leaves have rotted and it can be sifted for use as potting soil.

  When repotting, generally only the potting soil is changed, not the pot. Take the stump bonsai out of the pot, remove the old soil around the root ball; for tree species that root easily, such as Vitex negundo var. heterophylla and elm, about 50% of the old soil can be removed; for pines and cypresses, which are difficult to root, only 20%-30% of the old soil should be removed. If the root system is not well developed, the new roots are growing poorly, and the fibrous roots have not covered the bottom surface of the soil ball, remove little or none of the old soil, and cut off part of the old roots and rotten roots, with the cuts kept smooth.

  After repotting, when putting it into the pot, use the "deep pot, shallow planting method" (if using a shallow pot, this method can better shorten the recovery period). That is, add a layer of hard plastic ring 4-6 cm high at the pot rim. In this way, more broken tiles and coarse potting soil can be added at the bottom of the pot, raising the pot bottom and prompting the positioned tree roots to rise. Then use the conventional potting method to fill in new potting soil up to 1-2 cm from the opening of the plastic ring, and after new roots grow and the growth vigor recovers, remove the plastic ring. Later, through watering and rain washing, the soil above the pot surface gradually falls away, achieving the purpose of naturally raising the roots.

  After repotting is finished, water immediately. Because the newly repotted soil is relatively loose, you can water with a watering can, but the first watering must be sufficient and thorough (for home flower growing, the "pot soaking method" can be used for watering), and place it in a semi-shaded place; afterward, do not water until dry. Because at this time the potted tree's water absorption capacity is weak, watering too much easily causes root rot. For evergreen species, spray water on the leaves more often to increase the relative air humidity and reduce evaporation of water from the leaf surface. Wait until the potted tree sprouts new roots and its growth vigor recovers (about 20 days), then switch to normal watering.

  Stump bonsai pots hold little soil, and after 2-3 years in the pot, the nutrients in the soil will be exhausted and the soil texture will become hard, which is unfavorable to plant growth, so repotting and changing the soil are necessary.

  1. Timing of repotting

  Whether repotting and changing the soil or moving it into a larger pot, it is best to do it during the dormant period of deciduous species or the slow-growth period of evergreen species; if done during the vigorous growth season, wilting or leaf drop will inevitably occur.

  In some special circumstances, for example if underground pests appear in the potting soil, or because of commercial sale or the need for flower display, and repotting and changing the soil or changing the pot must be done during the growing season, then shading and moisture retention work must be done well, spraying more water on the leaf surface and keeping the potting soil well aerated.

  2. Method of repotting and changing the soil

  After taking the stump out of the pot, first use a bamboo strip to clear away about half of the surrounding old soil, then properly prune the root system. For some dish-shaped shallow pots, there is no need to use broken tiles to line the drainage bottom hole before adding soil; especially for some miniature bonsai, whose height is not much greater than that of a tile, plastic window screen should instead be used to cover the drainage hole to prevent the potting soil from leaking out.

  3. Interval between repottings

  The interval between repottings should be flexibly controlled according to the growth habits of the tree species and the size of the original pot. Flower and fruit stump bonsai such as pomegranate and Chinese wolfberry consume a lot of nutrients, so it is best to repot and change the soil once a year, but do not move them into a larger pot, otherwise the plant will expand year by year and cannot be controlled, so that it grows into a large tree and loses the meaning of a stump bonsai.

  Broadleaf species such as Serissa japonica can have their soil changed once every two years. Pines and cypresses grow relatively slowly, so their soil can be changed once every 3 years. Large stump bonsai are mostly given a soil change once every 3-5 years, and medium-sized ones once every 2-3 years. The older the stump, the longer the interval between soil changes. Stump bonsai become more precious the older they are, but they cannot be allowed to grow into large trees; the trunk can be allowed to thicken, while the crown should be controlled at an appropriate size and height. Besides achieving the above purpose through pruning, controlling fertilizer and water and reducing the nutritional area are also quite important, so do not rashly move them into a larger pot.

  Stump bonsai pots are generally both small and shallow, hold very little soil, and will use up the nutrients in the potting soil in a relatively short time. The root system will also fill the whole pot and push the potting soil out of the pot surface, to the point that watering becomes impossible, while the lateral roots and fibrous roots located at the bottom and sides of the pot often wither and die, causing thin branches and yellow leaves with no vitality at all. Therefore, repotting and changing the soil should be done in good time according to the situation, combined with pruning the root system.

  Repotting and changing the soil should be carried out before new buds sprout in spring; during mass production, it is best to finish all of it before Qingming. If repotting and changing the soil during the peak growing season, branch tips often wilt because of root pruning, unless underground pests have appeared in the potting soil or it is necessary for sale or participation in a bonsai exhibition, when the old soil must be cleared or the plant must be moved into an exquisite bonsai pot; when it is unavoidable to change the pot during the peak growing season, shading and moisture retention work should be done well, spraying more water on the leaf surface and watering less into the pot, otherwise not only will wilting occur, but root rot will also result.

  After taking the stump out of the original pot, first use a bamboo strip to clear away about 1/2-2/3 of the root soil, then prune the root system. For coniferous species, the main roots growing downward are often more numerous than the lateral roots growing sideways, and there are also relatively few fibrous roots; these main roots grow into a ring at the bottom of the pot each year, and after several years without repotting they grow into several rings. However, their ability to absorb water and nutrients is not strong, and they also easily push the potting soil out of the pot surface, so these main roots should be heavily pruned; on the one hand this frees up area to add new potting soil, and after heavy pruning it can also prompt them to sprout lateral roots and fibrous roots to increase absorption capacity.

  Stump bonsai that have already taken shape should as far as possible not be moved into larger pots, so as to control their growth and maintain the original tree shape. If the pot body is too large and the nutritional area is too much, once leggy growth occurs, the tree shape will be very difficult to control, and at the same time it will look top-heavy and out of proportion. When using dish-shaped or boat-shaped shallow pots, the drainage bottom hole in the pot should no longer be lined with broken tiles; plastic window screen can be used instead to prevent the potting soil from leaking out. When planting the stump, if using a round or cylindrical flowerpot, the trunk should be planted in the center of the pot; if using a rectangular or oval flowerpot, the trunk should be offset to one side, and the empty pot surface can be used to place companion stones and ornaments.