Bonsai roots require a balance of water and oxygen to survive. This is not negotiable. Overwatering- water without oxygen will kill them. Underwatering- oxygen without water will kill them, faster.
I could launch into a short (or long) lecture about shallow pots and the movement of water through different sized particles, but honestly, I’d put you to sleep. Suffice to say that dedicated bonsai growers use a “soil” mix that has nothing in it that looks like dirt, because over the long haul, overwatering is the biggest danger. But I have found that a soil mix that drains really well but holds more moisture than the traditional substrate gives beginners a better chance at success. Underwatering can lead to a very quick death without giving the beginner a chance to uncover and correct the mistake.
Most people want to know how often they should water their bonsai tree and the answer they are looking for will always be something like – every other day or twice a week. But- You Cannot Water a Bonsai on A Schedule.
Do you want to know how often to water your bonsai? There is only one honest answer and that's "When it needs more water", and depends on many factors. How big is the pot? What kind of tree? What time of year? Where is it placed? How big is it? What type of soil is it in?
But still it's not a tough question. At first, you'll need to stick your finger in the pot and feel the soil to see if it needs water. I still do this every time with every tree. Water when the soil about an inch beneath the surface feels dry. For the majority of trees, if you're not sure err slightly to the dry side. But some trees like Serissa, Sageretia theezens and Formosa Firethorn (pyracantha) really hate to dry out, so if you're not certain, err on the side of more moisture.
Never water a tree that is obviously already wet and never, ever allow a bonsai to sit in a drip tray filled with water. You'll kill it faster by not watering at all, but overwatering will kill a tree just as surely.
Some people have been told that ideally you give a bonsai just a little water every day. Bad idea. Drench it. Let it dry out (as much as that tree will allow). Drench it again.
Don't Give Up Too Soon
Just about everyone at one point or another will allow a tree to go too long without water. Almost for sure you're going to drop a lot of leaves. Maybe they'll go crispy first – a sure sign that you waited too long to water. If it's flowering, the flowers will fall. If it has fruit, they'll drop. It’s a natural stress reaction, but it doesn't necessarily mean your tree is dead. Many bonsai will recover so just keep caring for it and give the leaves a few weeks to grow back. It's worth a try.
Observe Your Tree
When you first water your tree, take notice- How heavy is the pot? How does the soil look when wet? If you have moss on the soil or the tree, how does moisture affect the appearance? What about the leaves on your bonsai? Look at your tree carefully after you water and you'll see that the leaves seem a little firmer, stand a little more upright. Once you've been tending your tree for long enough, you'll be able to tell by looking at it from across the room if it needs water.
Inspect your bonsai on a regular basis. That's how you'll really get to know the tree – does that sound crazy? Maybe it will to many people, but with caring observation you can see what's growing, how the shape is evolving, where new branches are starting out, or dying. You’ll spot problems early, before they threaten the health of your bonsai. You'll spot the aphids and mealy bugs and notice the fine webs of spider mites and get rid of them before they ever become a problem.
And best of all, you'll really learn to appreciate how marvelously beautiful your bonsai is.
How to Water
Water from the top or from the bottom. It's up to you. Regardless of whether you water from the top or the bottom, make you’re your tree gets a thorough soaking so that the full root zone benefits. Rain water is good. Tap water is ok, if possible, let it sit for twenty-four hours for the chlorine to disperse.
Moss- I have a love/hate relationship with moss and the hate part comes in because while the moss does shelter the soil from summer heat and overall has a moderating effect, once it dries out it repels water and it irritates the heck out of me to pour water on a tree only to watch it roll off the top.
If the moss on your tree dries out, spraying it with water a few minutes before you water the tree reduce the run off. Even then, after you’ve watered, lift up a corner and make sure that the soil under the moss is wet. You will often find it isn’t wet. So, either sit the pot in water until it soaks up from the bottom, or water several times or do both.
Dry Core – By core, I mean the root ball immediately under the tree. Bonsai pots tend to the shallow side and it is not unusual for the base of the tree to sit above the level of the rim of the pot, which makes it very easy for that core area to stay perpetually dry. The cure? Pay attention when you’re watering. Mix up top down and bottom up watering. Drip water slowly at the base of the tree.
Super Dry – It happens. The weather got hot. You got distracted. You went away. You missed a watering (or three) and the soil is so dry that it’s pulling away from the side of the pot. You water and it first runs off the top and then simply runs down the inside of the pot and out the drainage holes.
Pot bound- You’re tree is healthy and growing well, but you can’t help but notice that the water sits on the top of the soil and takes forever to percolate into the soil. Um, when did you last repot that tree, because this is pretty much the classic indication that your tree is due for a repot.
Overwatering – A classic sign of needing water is wilted shoots, so what gives when the shoots (branch ends) are wilted but the soil is wet. Root rot. You have overwatered this tree. First – stop watering it. Tilt the pot up on its side to help the water drain. Do not feed it. If that doesn’t work you might need to take more extreme steps.
Remove the tree from the pot. Trim the rotten roots, put it back in the (washed) pot with new soil (in a pinch you can use something like cactus soil from a hardware or garden centre). Now grow it on the dry side and cross you fingers. Good luck.