Watering deciduous bonsai – part 1 – Bonsai Tonight

The easiest way to tell if a deciduous tree needs watering is to look at the soil. For most deciduous species, I want the soil to start drying out before watering. Here’s what this looks like.

Moss covering the surface of the soil

Wet soil – no need to water

Later the same day

Soil is starting to dry out – time to water

Different species have different water requirements, but in general, deciduous species need more frequent watering than conifers, and some species like Japanese maples and stewartia need more frequent watering than other deciduous species.

Over the years, I’ve found that the most important factor in watering deciduous bonsai is finding the best spot in the garden for each tree to grow. Because deciduous species show signs of damage after a short time without adequate water, the better suited a tree is to its environment, the easier it will be to keep it healthy.

When deciduous bonsai get too much or too little water, they typically let us know about it. The most common sign of underwatering is wilted foliage.

Wilted foliage on Chinese quince

If a tree goes too long without adequate water, the tips can turn brown, and in some cases, foliage will show signs of sunburn by discoloring as in the example below.

Sunburned leaves with dried tips on Japanese maple

Healthy foliage from the same tree

Overwatering can be hard to diagnose as the symptoms are similar to those caused by underwatering. The best guide to whether the problem is caused by a tree staying too wet or too dry is to think back on your watering habits and determine if the soil was usually wet when you watered or whether it dried out between waterings.

Likely signs of overwatering on a shishigashira Japanese maple

Keeping deciduous bonsai too wet can also lead to fungus problems. One of the most common fungi is powdery mildew. The best way to prevent it is to avoid overhead watering trees for which it is a problem.

Powdery mildew on Japanese maple

Once you’re familiar with the signs of over- or underwatering, it’s easier to know what changes need to be made to keep your trees healthy.

Of course, making changes can lead to new challenges. This spring was warmer than usual, and as a result, some of my azaleas started showing signs of stress (more yellow leaves, fewer new shoots). To reduce the stress on these trees, I switched my shade structure from 30% shade cloth to 40%. (The number refers to the percentage of the sun that is blocked by the cloth; 40% shade cloth lets 60% of the light through and blocks the rest).

30% shade cloth on the left, 40% on the right

Many of the azaleas in the shade structure started producing new growth within a few weeks of making the switch, but I started seeing less desirable changes in some of my deciduous trees.

Species like trident maple and Korean hornbeam don’t need as much shade as Japanese maple or stewartia, yet all of these species grow under the same shade structure in my garden. After making the switch from 30% to 40%, the new leaves started growing much larger on my hornbeams.

Large leaves on Korean hornbeam

To keep the leaf – and intenode – size in check, I need to make sure these trees start receiving more light.

There’s far more to say about the water needs of deciduous bonsai, so tune for Friday’s post: Watering deciduous bonsai part 2 – top dressings.

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