The most commonly asked question at the National Bonsai & Penjing Museum is “How often do you water the bonsai?” The person asking the question usually expects a simple, straightforward answer, like “Every 3 days,” even though the more complete answer is more involved and can be applied to any potted plant, not just bonsai.
When the bonsai are actively growing during the summer months, they are watered about once a day. They are planted in a soil mix that drains quickly and they are outside getting plenty of light. During the fall they are watered less frequently since they are going into dormancy. In the dead of winter they are moved to winter storage where they are watered even less, perhaps once every couple of weeks.
In addition to the time of year, there are other variables to weigh when deciding to water or not to water a bonsai or any potted plant.
For starters, it helps if the same person waters the bonsai every day. That person knows when the bonsai was last watered. They know if the tree was watered yesterday morning or yesterday afternoon. For trees in large pots, knowing what was done yesterday is very helpful since the moisture on the top of the soil is often very different than the level of moisture near the bottom of the pot. That is one of the reasons why museum staff members water the trees rather than volunteers that come in once a week.
The location of the bonsai in the museum is another factor that affects the watering of the tree. There are many microclimates in each outdoor pavilion. Some areas get morning light and afternoon shade; some get sun most of the day, and some only get indirect light. Selecting the right tree for the right location is always important, not only for sunlight, but for how it affects watering of the tree. Each of those locations has an effect on when trees are watered, and how much water the trees will need. The trees that get morning light are the first trees we check in the morning to make sure that they are not too dry.
Within these microclimates there are still other factors that influence how much water is given to each tree. Our sunniest locations might be fine for the crapemyrtle, black pines, elms, and California junipers, but not all of them will be watered the same because certain species demand more water than others. The crapemyrtle would be happy getting watered twice on a warm sunny day in July, while the California juniper might only get watered every other day. Keep in mind the natural growing conditions of the tree or potted plant. If the plant thrived in wet conditions in nature, they will want wet conditions as a potted plant also. The opposite is true of plants that thrived with little water in nature.
In addition to location and species, the soil mix also affects watering greatly. Over the years, different soil mixes have been used at the museum. Some retain water more than others. Many of our soil mixes use akadama and kanuma, two soils imported from Japan. Our watering practices have had to be altered accordingly because these ingredients are a form of clay which drains well but also holds moisture well. A plant growing in a “light”, sandy soil mix will dry out more quickly than a plant that is growing in a heavier, clay, or loam mix.
Wind and humidity levels are two more factors to keep in mind when watering a potted plant. If it is a very windy day, the plant will need more water. If that cool dry Canadian front passes through during the summer, be prepared to water the potted plant a little extra. On the other hand, ease up on watering a bit during those extremely hot, calm days that are very humid.
When a bonsai or potted plant gets fully leafed out during the summer, all that lush growth needs a steady supply of water to stay hydrated. When a bonsai is pruned back during the summer to keep its size in check, watering is reduced. When bonsai begin to flower or develop fruit, a steady supply of water must be provided if the flowers and fruit are going to remain viable.
All of these factors will help determine when to water a bonsai or potted plant. The ultimate answer to the question “How often do you water a bonsai?” is “Whenever they need it.” Knowing when they need it is the key.
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