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Watering bonsai is a delicate art. Alas, there is no foolproof method for bonsai watering which can be followed formulaically for healthy plants. Each bonsai is different, with unique needs which can vary depending on the tree, the time of year, the potting medium, and the pot itself. It can take some trial and error to learn about the preferences of a particular bonsai, and many experts have tales of woe about bonsai they accidentally killed when they were just getting started.
The first thing to consider when watering bonsai is the stated water needs of the tree. Some trees, for example, prefer dry conditions, while others may like moist soil, or may like to stay damp with humidity, with relatively dry soil. These needs become especially complicated in a pot, because in nature, a tree can control the amount of water uptake by spreading its roots accordingly and adapting to the environment. In a bonsai pot, the tree is at the mercy of the cultivator, which means that people need to be attentive when watering bonsai.
As a general rule, watering bonsai should occur whenever the tree is dry. Many people can tell whether or not a bonsai is dry just by looking at it, as the color and texture of the soil will change. Gardeners can sometimes also tell by lifting the pot and feeling for a change in weight; for individuals with less experience, a finger can be stuck into the soil to determine whether or not the upper portion is dry. Bonsai will dry out at different rates, depending on relative humidity and temperature, so people should not count on watering on a schedule.
If a plant is dry, it should ideally be watered in the morning. There are varying recommendations about watering method. Some people suggest that watering bonsai should be done in a two-cycle system. During the first round, water can be sprinkled with a watering can onto the pot until the drainage holes start to seep. Then, the plant should be allowed to rest for 15 to 20 minutes before a second watering cycle. Others recommend that the bonsai be watered three times, “once for the pot, once for the soil, and once for the tree.” People should take note of how a particular bonsai adjusts to watering practices and make adjustments accordingly. Some plants, for example, like to be watered twice a day, while others may go a week or more without water.
While watering bonsai, it is important to avoid getting water on the leaves and flowers, as it can cause damage if it occurs routinely. However, plants do periodically appreciate being misted or gently sprayed to remove dust from the leaves.
When bonsai do not get enough water, they tend to develop withered and yellowing leaves, and flowering and leafing out may be suppressed. Trees getting too much water may droop, yellow, or brown, and if root rot is allowed to set in, they can start to list to the side. Bonsai care can also be complicated by moving the pot or repotting; both of these events can shock the tree and cause it to react by dropping leaves or yellowing.
I think that another thing that people who grow bonsai trees need to keep in mind is the amount of sun exposure the plants get each day. For example, if you have a bonsai tree that you keep by a window that gets full sun each day, you will probably need to water your plant more than someone who keeps theirs in a shady location.
I have found that watering bonsai trees involves becoming aware of the specific needs of your plant. Basically, if the bonsai tree is doing well and doesn't show any signs of problems, you should water it when the soil begins to feel dry.
On the other hand, if your plant seems to be failing, consider the moisture level that you have been keeping in the soil. If it always feels wet to the touch, cut back on the amount and frequency of times you water it. If the soil is usually dry, you should try giving your bonsai tree more water, more often.
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