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Watering Basics

Watering is the most important task you do to care for your bonsai. It seems easy – just pour some water on top of the plant or immerse it in a pail of water and you’re done, right? Unfortunately it’s a bit more complicated, but this article will help you understand the basics.

Because bonsai are planted in shallow pots they don’t drain as well as you might expect. That’s one of the reasons why bonsai soil is course – to ensure good drainage. We are trying to have enough moisture in the soil mix for the roots to pick up but not so much that the roots rot.

Your bonsai soil should not be allowed to become completely dried out, nor should it be kept overly wet and soggy. Its drying cycle is variable, dependent upon weather, position in your yard, and the season (dormant or in leaf). In the winter, without rain, you will find that you don’t need to water until several days or a week after the previous rain. In hot summer, your Bonsai may need to be watered daily, or at least every-other-day.

If you bought your bonsai from a store then some of the following guidelines may not apply to you. Please read the Store-Bought Bonsai section at the end of this article for more details.

When to Water

The best time to water is in the morning if you can. Some plants grow better when the evenings are warmer so if you water in the late afternoon then you’re cooling it off and not getting as much growth as you can.You may also attract more bugs to your tree if you water in the evening. On especially warm days (over 90 degrees) you may need to water in the morning and the evening. We’ll discuss that later.

Determining if Your Tree Needs to Be Watered

Whether it’s morning or evening, you need to assess each tree to decide if it needs to be watered.

  1. Touch the soil -The surface dries out faster than the rest of the pot, so press your finger into the soil 1/4-inch or deeper somewhere along the edge of the pot. If you feel moisture then you don’t need to water now. If the soil is very firm (full of roots) then you can use a chopstick – push it in, pull it out and inspect for moisture. Using a chopstick isn’t as sensitive as your finger, but can be easier.
  2. Look at the soil – It will be a darker color than when it’s dry, but each tree is different. A tree that looks wet may simply have moss or something else on the surface. But if you learn by touch when the tree is wet or dry and also look at the soil, with a bit of practice you can tell when it’s probably wet or dry. Here are a couple of photos to illustrate what we mean:
Wet Soil
Wet Soil
Dry Soil
Dry Soil

Don’t wait too long

When you have several trees it can be difficult to check the soil of every tree every day. And, if you skip some trees because they are still moist while others are dry then you run the risk of letting those dry out too much before you get back to them. Here are some rules of thumb to help make sure you don’t wait too long between waterings.

  1. Winter – Unless it is raining at least once a week you should water at least every four days.4 parts Lava
  2. Spring / Fall – Unless it’s raining at least once a week you should water at least every second day
  3. Summer – Once temperatures are above 80 degrees water every day. Once temperatures are above 95 degrees water twice a day, morning and evening
  4. Windy – When it is very windy soil and trees dry out quickly, so double check your trees and water if necessary.
  5. Very cold temperatures – It is rare for our area to have freezing temperatures but when that happens the humidity in the air is very low and soil and trees will dry out quickly. Double check your trees and water if necessary.

How to Water

Trees should be watered from the top with a gentle spray to avoid washing away the soil in your pot. The easiest way is by using a watering can or a watering wand with very fine holes.

There are different opinions as to whiter it’s better to water with the water arching up and then back down or directed down. As long as the water flow is gentle, it doesn’t seem to matter.

Arched Spray
Arched Spray
Spray Directed Downwards
Spray Directed Downwards

As you see the water is directed at the soil, not the foliage, or needles in this case. There are a couple of reasons for this:

  • Watering the leaves and needles of a tree is a good way to spread any fungus infection to the rest of your tree.
  • When water evaporates any dissolved salts will be left behind (see Dissolved Salts later). Those will be visible as white spots on the foliage and can cause “burning” of leaves and needles. The leaves of maples seem to be especially suspectible to burning. However, junipers benefit from a bit of water on their needles. Just minimize watering the tops of decidious trees all the time.

Occasionally it’s a good idea to spray maple foliage to knock off aphids which appear mostly in early spring. As shown below, spray the water up to wash the undersides of the leaves. Bugs do come back so after washing them off one day you may need to do it again next week. Red Spider Mites often invade pines and shimpaku juniper during hot weather. Occasionally washing these trees can remove these bugs.

Washing underside of leaves to remove aphids
Washing underside of leaves to remove aphids

How Much to Water

You want all of the soil to be moistened so move the shower of water around. Continue watering until you’re sure water is coming out of the bottom of the pot. By making sure water comes out the bottom. As the water drains out it draws air into the roots from the top. This is important because roots also exchange some oxygen with the air.

On healthy trees the roots will grow rapidly and will fill up the pot. When this happens the water may not soak in as easily. In this case water the tree a bit, wait a few seconds and then water it a bit more. You can also poke holes into the root ball using a chopstick, or even an electric drill.

When You Have More Trees

If you have more than a few trees a watering wand can be more efficient. The ones you find at a nursery often have holes that are a bit too large. That’s why we recommend using one designed specifically for bonsai. You can order these online or buy them at the Mammoth Bazaar in February. They’re usually about $30, but they’re heavier duty than the ones you find at the nursery. The one in the photo below is 12-15 years old and continues to work well even though it has gotten banged up.

Notice that on the left there’s a brass water control valve. This valve makes it easy to adjust exactly how much water is coming out of the nozzle

Watering Wand
Watering Wand

Hot Summer Days

When the temperature is especially warm the soil will dry out more quickly. On those days you’ll want to water in the morning and possibly again in the evening. You don’t want to water during the hottest part of the day if the pots have been setting in full sunlight and gotten hot. There is some concern that watering a tree in a hot pot will kill the roots. However, you can gently spray the outside of the pots to cool them down and then water. You should also consider moving your trees to areas with afternoon shade – or draping shade cloth over the trees. This is especially important for maples and other deciduous trees.

Always assess the dryness of each tree’s soil before watering.

Symptoms of Over-Watering

Over-watering means that the root system stays wet. That can cause several problems:

  1. Lack of oxygen – When the roots stay too wet the roots don’t get oxygen. That can cause the fine roots, the ones that actually absorb the water and nutrients, to suffocate and die. Your tree will begin to look “unhealthy.” This is when most of us think we’re not watering enough and make the problem worse.
  2. Dead roots begin to rot – When rotting begins naturally occurring bacteria and fungus will break down the dead tissue. This can spread through the root system and kill off the rest of the roots. As this is happening the root system can no longer support all of the foliage. Rotted roots will turn black and break apart when touched. We sometimes see this during repotting
  3. Foliage beings to yellow – Without enough roots to support the trees the leaves or needles will drop. Smaller branches will die back. At some point, if you’re lucky, the live portion of the tree will be small enough for the roots to support. But if you continue to over water then the whole tree will die.

The only reliable way of stopping root-rot is to cut away all dead areas of root. That means repotting and having a smaller sized tree afterwards. We want to avoid this. So, check your tree before watering and only water when needed.

If you see mushrooms above the soil line then you have a fungus, probably phytophthora cinnamomi. By the time you discover you see mushrooms it is too late. It is best to throw the tree away than risk transferring the fungus to other trees. Put it in the garbage, not the recycling bin.

Advanced Topics

Root-bound Trees

Occasionally we have a tree in poor soil or that has become root-bound. That is, it should have been repotted last year and wasn’t! In these circumstances the water will not flow through the soil as it should. You can use a chopstick to make several holes in the soil so that water won’t just run off. For tough soil you may even use an electric drill. But you may also need to immerse the tree in water as described below.

Store-Bought Bonsai

When you buy a commercially made bonsai the top layer of the soil is often treated with a glue-like substance to keep the soil, or decorative pebbles, from coming out of the pot during shipment. You can tell by touching the top of the soil. If it doesn’t move then you have one of these trees. That doesn’t mean they are bad bonsai, but it does make watering more difficult. The recommended way to water a bonsai (from the top) won’t work well because the water will mostly run off the top and the soil doesn’t get wet. That’s why many of these trees die soon after purchase.

Some people recommend immersing the pot in a larger container of water. That can work, but it’s difficult to judge when you need to water so you either water too often or not often enough. Instead of immersion we recommend removing the top layer of pebbles / soil. Unfortunately that usually means you need some more soil to replace the bit that you’ve removed, but nurseries don’t sell bonsai soil. If you are a member of our club we’ll sell you a small amount of soil mix and teach you how to make more when the tree needs to be repotted.

Dissolved Salts

All tap water has some dissolved salts, but some have a high levels. Maples, and azaleas tend to be more sensitive than junipers and pines to high levels of dissolved salts. If you get your water from the Hetch Hetchy reservoir, San Francisco and much of the Peninsula down to Sunnyvale, then you’ve got great water without a lot of salts. However, a lot of water in cities south of Sunnyvale comes from wells and sometimes have a lot of dissolved salts.

During the rainy season a lot of salts will be washed out of your soil mix. But during the non-rainy season you may see a bit of “crust” on top of the soil. This is salt left behind when the water evaporates. When you fertilize you are adding different kinds of salts, for example, potash. So fertilizing can also contribute to this “crust.” The problem with this crust is that it make it more difficult for water to penetrate the top of the soil and cause it to simply run off. Also, too much salt of any kind can be bad for the roots. So when you see this developing we recommend replacing the top 1/4 to 1/2-inch of soil. Not only does that remove the crust, it also removes weeds.

You can also collect rain water during the rainy months to use on your most sensitive trees.

Related – Alkali Water

Water with a lot of disolved salts are usually more alkali – which is a problem for “acid loving plants” such as pines, junipers and azaleas. You can counteract this by adding a small amount granulated or powered sulphur to the top of the soil. The sulphur make the water more neutral or slightly acidic depending on the amount used. A tablespoon of sulphur for a medium-to-large pot is a reasonable amount to begin with.

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