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Watering a bonsai is the most common and most important measure in bonsai care. The water requirement of bonsai is influenced by many factors (e.g. wind, temperature, pot size). What is to be considered ? In this guide, we have summarized the factors that are important when watering bonsai.

What should you watch out for when watering a bonsai ?

  • Check the water requirements before watering the bonsai. Bonsai should only be watered when they need water.
  • Water the bonsai moderately, not excessively. Trees don't want to be drowned.
  • Do not irrigate all bonsai trees all at once and just as every bonsai needs.
  • Never let a bonsai tree dry out completely. Especially not in midsummer.
  • Check the water requirements of your trees several times on windy summer days.
  • If possible, do not water bonsai trees with cold water.
  • Repot the bonsai when the bonsai pot is deeply rooted. Otherwise waterlogging occurs.
  • Use a well-drained substrate when repotting. Fine soil is unsuitable.
  • When repotting, only use bonsai pots with water drainage holes in the bottom of the pots.
  • Immediately after repotting, the bonsai soil should be heavily watered.
  • Do not forget to water your bonsai in the winter quarters.
  • Holiday supervisors should be thoroughly instructed in watering bonsai.
  • Avoid irrigation systems if possible and water the bonsai by hand.
  • It is best to water indoor bonsai with a ball shower.

The right supply of water and the nutrients it contains is crucial for the health of the bonsai. The need for water depends on many factors and can vary greatly from day to day. The different requirements for water must be taken into account otherwise the quality of the bonsai can quickly deteriorate or it can even die.

How is the water requirement of a bonsai affected ?

  • Weather

    The weather, which changes at short notice, has the greatest influence on the water requirements of a bonsai. After a strong thunderstorm on a midsummer night, there is no need to water bonsai outdoors in the morning.

    If the temperature rises to 30°C at noon, it looks different. Now you should check if a bonsai needs to be watered. If there is still a little wind in the afternoon, especially small bonsai trees can dry out quickly.

    On such windy midsummer days, small bonsai should be checked at least twice to see whether they need watering. If a hot midsummer day is to be expected, a small bonsai should be checked in the morning and watered if necessary.

    If you are absent during the day, it is best to place small bonsai in partial shade to protect them from the midday heat. Do not underestimate the sun and wind in midsummer. In the afternoon, dark bonsai pots can quickly reach temperatures of 50°C and more in the sun.

  • Season

    The influence of the season is also a very strong factor. But it changes much slower than the weather. Here you can approach it a little more relaxed.

    What the situation looks like in midsummer was already described in the part above (weather). It looks completely different in autumn. The leaves fall off many deciduous trees. This greatly reduces evaporation. At the same time, the temperatures are milder, in many areas it rains more frequently, and the humidity is higher at night.

    In October at the latest there are the first days when especially bonsai in larger pots do not have to be watered on a daily basis. In many northern European countries, it often rains for days in November. Sometimes you even have to put some trees under so that they don't get too wet. This is especially true for bonsai that have not been repotted for a long time (e.g. imported bonsai such as Japanes white pines) and where the substrate is quickly and completely rooted through (e.g. Spindle tree).

    When overwintering the bonsai in a greenhouse or foil tent, watering can often be paused for weeks. Our bonsai in the bonsai nursery come to the foil tent at the beginning of November. With a few exceptions, we usually no longer water until the beginning of February of the following year.

    In the tent or greenhouse the humidity is so high that there is hardly any need for watering. In some cases, humidity is even a problem. E.g. privet bonsai quickly develop rust fungi in the high humidity. At the same time, however, you cannot ventilate on cold days because it would be too cold for this indoor bonsai. It is better to reduce the watering during that time significantly.

    In the middle of winter, there is also frost. Even if you wanted to, watering the bonsai now makes no sense. The soil is frozen and does not absorb water. Now the high humidity in the greenhouse is very desirable. So hardly anything evaporates and the bonsai are not damaged.

    When bonsai friends talk about winter protection, they usually don't mean protection from low temperatures. They think of protecting against dehydration when the ground is frozen. This is the only reason why we put the outdoor bonsai in a foil tent. The temperatures there are only 1-2°C higher than outside. But the high humidity protects against dehydration.

  • Location

    The location is also very important when watering bonsai trees. And above all - we can influence it ourselves.

    In short: a sunny location increases the water requirement, a partially shaded location reduces it. Protected from the wind (e.g. in a corner or as an indoor bonsai in the apartment), a tree consumes significantly less water than exposed to the wind in the middle of the garden. Moist locations (e.g. in our forest or in a greenhouse, foil tent) reduce the water requirement.

    All indoor bonsai need less water. Regardless of the tree species. Maintained outdoors in summer, the water requirements of these bonsai then increase massively.

  • Climate

    Since we send our bonsai all over Europe, the climatic conditions in the home country also play a major role. Winter in Spain is in some ways comparable to summer in Great Britain. The pouring behavior must be adapted accordingly. The influence of the seasons increases depending on the region. Winter in northern Sweden and summer in southern Greece require more attention to watering bonsai than in more temperate areas.

  • Bonsai tree species

    There are big differences in water requirements between the different tree species. These were created by adapting to different conditions in their natural locations. An olive bonsai adapted to southern Spain has different demands on the water supply than a ficus bonsai from the rainy subtropics.

    Particularly thirsty are: redwood bonsai, hornbeam bonsai and larch bonsai. With these species you have to be careful in midsummer. It is best to place it in partial shade (especially in the afternoon). For these species even maybe it is a good idea to have an automatic sprinkler system installed.

    The following species are more tolerant of temporary water shortages: pines, olives, pistachios, money tree (jade tree).

  • Size of the bonsai tree

    The size of the bonsai tree has an impact on the water demand. But this is not very big. This is mainly due to the fact that large bonsai are usually in large pots and small bonsai in small pots. This means that the water requirement is larger with large bonsai, but the supply of water in the pot is also larger and usually suitable for the size of the bonsai tree.

    Nevertheless, one can say in general: Small bonsai need more attention when watering than large ones. But this is less due to the size of the tree and more to the lower supply of water in the pot. If this supply is used up in the midday heat (small pots get hot much faster) a small bonsai dries up much faster.

  • Size of the bonsai pot

    The size of the bonsai pot is very important for the irrigation and water requirements of a bonsai. Small pots get hot quickly in summer. When the bonsai soil begins to dry it is often completely dry within 2-3 hours.

    In other words, with a small bonsai pot, you have to check much more often whether the bonsai soil is still moist enough. Successfully caring for small bonsai is therefore much more difficult than large garden bonsai.

    If large bonsai are kept in a small bonsai pot (e.g. in the literati style), more attention must also be paid to watering the bonsai. Essentially, it depends on how much foliage the bonsai has.

  • Permeability of the bonsai soil

    The permeability of the soil is a very important point when watering the bonsai. About 90-95% of the bonsai offered in Europe were imported. They are usually sold without repotting and were often tended in the bonsai soil for 2-3 years before import.

    This soil has decomposed over the years and the bonsai also completely rooted it. As a result, it is almost no longer permeable to water and, above all, to air.

    If too much is poured now, the excess water can no longer run off. But what is even worse: where there is water in the ground, there can be no air. However, like every cell, root cells need oxygen to live. In summer at high temperatures, the microorganisms multiply explosively and use up the remaining oxygen. The root cells and with them the entire tree suffer. This waterlogging is dangerous for the bonsai.

    After buying it, however, often repotting is not possible, especially in summer. What helps is: to pour the bonsai tree moderately. Then there is not to much water in the bonsai pot and at least a little oxygen comes to the roots.

    This problem should be remedied at the next suitable opportunity by repotting with a root cut. Use a well-drained substrate for this. Akadama bonsai soil is well suited for most bonsai, pine and juniper bonsai feel comfortable in Kiryu, azalea bonsai are potted in Kanuma.

  • Carried out maintenance measures

    Certain maintenance measures can change the water requirement of a bonsai suddenly. For example, a bonsai needs significantly less water in the weeks after repotting with root pruning. Even if you have chosen a very coarse substrate. The fine root hairs that absorb the water were partly removed during repotting or have died. The bonsai can now only absorb the water poorly. Water the bonsai more sparingly after repotting. If you water too much, the soil will constantly cool down. This significantly slows down the formation of new roots.

    Even a strong pruning of the bonsai during the growing season suddenly changes the need for water. If many leaves are missing, the tree can evaporate less and accordingly needs less water.

Now we've covered everything related to water needs of bonsai. Sounds very complicated. But it is not. Can be summarized in a nutshell:

  • Only water a bonsai when the soil has dried slightly
  • Beware of high temperatures and wind
  • If the soil is frozen, the tree should be protected from drying out
  • Use a well-drained substrate for bonsai soil

Bonsai and humidity

Does humidity have any influence on our bonsai ? Clear answer: Yes.

Bonsai, like all plants, have stomata (closable pores). If these are open, water evaporates. The water loss is compensated for by the roots.

If the evaporation is high due to dry air and the water supply is insufficient due to dryness in the soil, the bonsai closes the stomata. This reduces water loss.

If the stomata are closed for a long time, the tree has two problems:

  • Without water flow from the roots, there is a lack of nutrients for cell growth
  • Carbon dioxide from the air to build up sugars can no longer enter the plant

If the stomata are closed, the bonsai would stop growing. Therefore, sufficient air humidity as well as sufficient soil moisture is vital for the bonsai.

When is the humidity low ?

  • On sunny, windy days. Not only in summer. Also on sunny frosty days
  • For indoor bonsai in summer with the windows open
  • For indoor bonsai in winter with a location close to the heating

How can the humidity be increased ?

  • Outdoor bonsai: Above all, choose a location that is protected from the wind
  • Indoor bonsai: Do not place them above the heating or in the draught

One way to support an indoor bonsai with low humidity is to place it in a drip tray filled with water and expanded shale / pumice gravel. It raises the humidity, extends the watering intervals and also looks quite decorative.

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