This sounds like a stupid question: “How do I water my bonsai?” Um, you put water into the pot.
This is not a dumb question. Over and under watering is a prime cause of dead bonsai trees. It’s the prime cause of death of potted plants in general, and bonsai have more specific needs than other potted plants. Anyone who criticizes you asking about watering or criticizes watering instructions as being too basic doesn’t know much about bonsai themselves. Or they do and they’re just being a jerk. Or they’ve forgotten what it’s like to be new.
FREQUENCY OF WATERING
A bonsai tree needs to be watered when it needs water. It’s completely true, but generally unhelpful as understanding when a tree needs to be watered and how much takes time to learn. That’s why books, experts, and instructions for trees will frequently list easy rules of thumb in an attempt to help new bonsai caretakers. For example, “water once daily, but as little as once a week in winter.” Here in Florida you’d probably more commonly hear the “water once daily, but twice daily in the peak of summer.”
Bonsai are little trees in little pots. That means by design generally bonsai will need watering more frequently than other potted plants. If you’re just starting and you have just one or two bonsai don’t lump them into the same watering schedule as other potted plants and figure watering twice a week does fine for everything else, it will be fine for your bonsai too.
Understanding when a plant needs water depends on what growing mix it’s in. Seedlings, and many pre-bonsai may be in purely organic mixes or even regular potting soil. That makes their watering needs much like that of a house plant. Bonsai soil mixes generally drain well because they incorporate a large amount of inorganic material. This is increasingly true the older the bonsai is. Bonsai planted in a purely inorganic mix are fairly well-protected against overwatering but will dry out relatively quickly.
Some indicators on if a bonsai needs watering are what color the soil is, what the top of the soil feels like, and how heavy the pot is. Consider then how sunny, hot, cold, windy, wet, dry, or humid it has been and what growing conditions the plant in question prefers. Overall, touch the top of your growing medium and water if it has begun to feel dry.
If none of that works for you, consider using a moisture meter.
TOP DOWN OR BOTTOM UP?
At it’s most basic you can water in two different ways, from the top down or bottom up. Top down is the obvious way you think of as you would water from a watering can or garden hose. Bottom up involves submerging the plant in a larger container of water and leaving it to soak. For most plants most of the time, watering top down is the way to go. But there are some times with some plants when bottom up watering makes sense.
Bottom up watering helps ensure that the entire soil mix is thoroughly wetted. That is something else about watering that sounds obvious, but isn’t. Many people think that applying a generous amount of water means that everything in the pot is wet. But if the soil mix is very dry or the root ball is very compact it will take some time to absorb water. Much longer than the 15 or 30 seconds that someone watered thinking they gave a generous amount.
Some planting mediums also take awhile to fully take in as much water as they can hold. Slight watering only gets them somewhat wet. One example of this is orchids. Orchids frequently use sphagnum moss as a potting medium. Though sphagnum moss holds moisture, it takes awhile for it to fully absorb, especially if there are compact bits within the roots. That’s why you might hear advice to water once a week by submerging the plant’s pot into water and leaving it to soak for a set amount of time. In bonsai, some practitioners will bottom up water after repotting to ensure all the new growing mix is thoroughly wetted.
To bottom up water, fill a basin larger than your bonsai pot with water and put your bonsai into it. You can either have the water level to sit slightly below the rim so as to not have your soil mix wash away. Or just go ahead and submerge it and touch up any displaced soil after you’ve finished. The whole medium will get quite thoroughly saturated this way. If you’ve submerged the whole tree, you’ll see bubbles rise to the surface as air escapes from the soil. The quantity of bubbles and speed they come out will give you an idea of how dry the soil was. Once there are no more bubbles, the tree can be removed from the bath and set aside to thoroughly drain.
Before bonsai, I had no idea that it was possible to spend $100 on a watering can. Now I know that it’s not only possible, it’s easy to do. Bonsai people don’t talk about their watering equipment frequently, but if asked they’ll all agree it’s vitally important.
The goal is to water thoroughly, evenly, and gently. You may hear the advice to water, wait, and water again. I was once told to water once for the pot, once for the soil, and once for the roots. These statements are all driving at the same point as above with the bottom up watering. Such advice is all driving at the same thing. It can take some time to get the medium completely wet. For this reason you should still water even if it’s rained (lightly. If you’ve had monsoons this is obviously unnecessary).
When watering you might hear advice like use a “gentle” stream to water. For top watering you want some kind of soft water setting.
If you’re watering with a hose, shopping in garden stores will frequently give you a hose attachment that has several settings. Some of those (jet, flat, conical) are problematic for watering bonsai. Watering from a plain spigot is also problematic for watering bonsai. Too much water coming from one stream will make a hole in your soil or push soil out of your pot. Too much water pressure is also an issue. I’ve found the “shower” option to be the best for bonsai in the most popular attachments on the market right now. Your water stream should evenly wet and not disrupt the soil. Water until water runs out the holes at the bottom.
If you’re looking at a watering can, you may want to consider one made for bonsai. A watering can rose or rosette (the end part with the dots on it where the water comes out) should have a lot of holes in it. Having more holes lets the same amount of water be channeled or distributed out over a larger surface area, making a larger softer, more natural shower like effect. The long spout gives the force needed for the spray. The long spout and upward turned rosette also help control the water and allow a gentler stream out onto your tree.
Water should be poured over the soil and into the pot. If the water pools on top of the soil, wait and let it soak in and then water again. Know too that water that pools on top of the soil is likely a sign of either a pot or potting medium in need of better drainage (or possibly that your soil is bone dry, or that your tree desperately needs repotting).
Keep in mind when watering from a can or hose that you want tepid (room temperature) water. I live in Florida, so for me in August when you turn on a garden hose the first stream of water is hot. Really surprisingly hot. Wait for that water to clear. It’s the same as when you shower you might stick your hand under the water stream to wait to jump in at the right temperature. Wait until the hot water cycles out of your garden hose before turning it on your bonsai.
TIME OF DAY TO WATER
Many prefer to water their bonsai in the morning, when the bonsai has time to soak in the water and use it for the day. I’ve also heard some water at night, to give maximum time for the bonsai to absorb as much as possible. Yet others believe night watering invites mold or bacteria. Some of this will depend on the weather where you are. Many have said not water at mid-day when the sun is strong and full due to the concern that water on the leaves, in full sun, may burn the leaves (much like a magnifying glass frying an ant on the sidewalk). Others have said this is a myth. You may hear different opinions on this topic depending on who you ask. Choose what works best for you and your bonsai.
IS MISTING THE SAME AS WATERING?
Misting and watering are the same in that they’re both ways to get water into your plant, but they’re different in that misting generally provides less water and it creates humidity. This is especially helpful in dry climates and in very hot weather.
Bonsai should be misted if it is very dry. Indoor bonsai will generally need misting in the winter if their growing environment uses central heating, which dries the air.
Those looking for more moisture might consider using a humidity tray, or shallow tray filled with pebbles for the bonsai to sit on. Humidity trays raise the tree out of any standing water and act as a drip tray, protecting indoor furniture or other surface the bonsai might be sitting on.