How to evaluate bonsai water needs

Once you’ve determined that a given species can thrive in your area and you’ve found a spot in the garden well-suited to its development, the next order of business is determining the tree’s water needs.

At a basic level, determining water needs is simple. Dig down into the soil to where roots are growing and see if the soil is wet or dry. If it is dry, the tree likely needs water. If the soil is wet, you might be able to wait a while before watering.

Watering well, however, can be tricky, and no published guides will tell you exactly when and how often to water the trees in your garden as conditions can differ wildly depending on the species the grow and the climate in which you live.

Some research can be useful here as some varieties appreciate more water than others. Consulting online resources like Wikipedia or gardening guides like Dirr’s Encyclopedia of Trees & Shrubs, the New Sunset Western Garden Book or Hortus Third can be a good place to start. Better yet, consult with local nurseries or bonsai clubs for watering advice based on local experience.

If you decide to reach out and ask someone in a bonsai club for watering advice or post a question to a forum, be prepared to offer a few details. Instead of, “How much should I water my maple?” try something more specific, like: “How much should I water a young, healthy, Japanese maple growing in course, inorganic soil growing in a shallow wooden box in full sun?”

Once you’re armed with some information about a given variety’s water needs, file it away in your memory for when you’re holding a hose or watering can and deciding whether watering will help the tree or hurt it at that moment.

As the topic of watering is large, I’ve included a laundry list of basic watering tips and tricks.

General watering tips

• Water bonsai that is dry or wilting. Bonsai soil should never be completely dry, even when repotting. The small roots responsible for water update die when the soil dries too much – better to water when the soil is slightly damp.
• It’s impossible to give a tree too much water during any given watering. There is no need to worry that you’re letting the hose linger too long over a tree that is due for a watering as excess water will drain away. A far more common problem is watering too frequently as keeping soil too wet can stress and eventually kill bonsai.

Watering a pine thouroughly

• It’s not necessary to water bonsai by dunking them in a tub or basin. That said, letting a tree with very poor drainage soak in a tub is a good way to ensure water penetrates into the rootball.
• Learn to distinguish wet and dry akadama. Lava and pumice look the same wet as they do dry, whereas akadama lightens as it dries. Spotting dry akadama can be a good way to tell if a tree needs water at a glance.

Dry akadama and wet akadama

Dry soil mix with akadama, pumice and lava on the left – mostly wet mix on the right

• In general, it’s good to water the foliage when you water your bonsai as this can wash away dust and insects. Proceed with caution if fungus is a problem in your area. It could be that it makes sense to overhead water in the morning but not in the evening as moist foliage is less likely to dry at night in humid climates.

Responding to each tree’s needs

• The trees in your garden won’t necessarily dry out at the same time. Even if they are all the same size and age and planted in similar soil and containers, trees have individual needs and can dry out at quite different rates. Don’t be afraid to pass over trees that are wet from the previous watering.
• Deciduous varieties typically require more water than conifers.
• Sick trees generally need less water than healthy trees. Sick trees often benefit, however, from overhead watering.
• Trees growing in soil mixes with large grain size will dry out faster than those growing soil mixes with small grain size.
• Root-bound trees dry out faster than trees whose roots still have room to grow.

Trees that dry out too quickly or slowly

• If the pot drains slowly, consider replacing the surface soil with fresh soil (see “Summer Soji“).
• If a tree tends to stay too wet – or incessant rain is a problem – try raising one end of the pot with a scrap of wood. This will increase the amount of air in the pot and help the tree dry out faster.
• If a tree dries out too quickly, it might be due for a repotting. In the meantime, try moving it to a shadier corner of the garden, using shade cloth, or covering the surface of the soil with a layer of New Zealand sphagnum moss. Filling a tray with water and gravel can also help keep trees well-watered (see “Keep bonsai from drying out in summer“).

That covers some of the basics. Have your own suggestions? Add them below in the comments!

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