• Ian Evans

Bonsai Watering. The first bonsai lesson, and also the last bonsai lesson

Water. Water. Water.


Just saying it makes me want to take a drink. A fundamental need, the slaking of thirst. From the parched lips of a person emerging from the desert: “water”. For mammals as well as bonsai, water is the most basic of needs after oxygen. Some bonsai need water more urgently than sunlight during certain times of the year. For humans, thirst is a sensation we feel to help us ward off dehydration. If you are thirsty, chances are you are already dehydrated. And chances are, your bonsai are probably ready for a drink, too.


Bonsai live in small containers, all cramped in there with severe restrictions placed on them above and below the soil surface. Minimal soil exists around the rootball to offer water droplets to the tree’s root system. The shallow depth allows very little water to remain for long. Fast-draining soil substrate allows water to run freely from the drainage holes, creating pockets of humid air that dry out quickly on a hot day. Roots cannot grow downward into the earth to find those naturally occurring pockets of cool moisture. The sun’s warmth causes transpiration of the water from the foliage, and also from the soil in the pot. In winter, the drying cold sucks the moisture out in a similar fashion. Bonsai trees simply cannot survive without enough water. How much? and when? are the unanswerable questions, at least to any very specific degree. I would never say “once a day”, or, “at 9am and 3pm”. When to water your bonsai and how often depend on many factors, like species of tree, age of the tree, size of the pot, the color of the pot, season, temperature, UV exposure, angle/direction the tree faces, time of day, humidity/dew point, precipitation, wind intensity, wind-exposure, etc.


So, watering bonsai is the first lesson and the last.


General guidelines for watering bonsai

  • Check your bonsai tree at several points throughout the day. Morning, afternoon, and evening, if possible. Do this year-round. Make it a habit, like your coffee or tea. Have coffee, and check your trees. Keep your personal schedule in mind. Are you away for long periods each day? If so, never skip your checking and watering routine before you leave the house for a long time.

  • Determine the exact path of the sun throughout the year over your garden or bonsai bench. This is critical. Knowing exactly when your trees will receive the most and least sun is important. This, combined with an accurate weather forecast a day or two out, will help inform your watering choices, as well as the actual placement and location for different species of bonsai. Northern exposures offer less direct sun and often more winter weather threats in North America. Southern exposure gets a lot of sun, perhaps too much for certain trees. Knowing these facts of life in your area is critical to finding the sweet spot of water, sunlight, and shade.

  • Know from which direction(s) the wind typically blows. Wind can dry your bonsai out any time of year, especially during periods of low humidity. Storms tend to arrive from a certain direction or directions, so being aware of this can lessen the impact of dangerous weather on your bonsai year-round. Make sure you know what kind of weather you can expect in the next couple of days.

  • Consider the temperature and remember that even ambient heat radiating from buildings, concrete, asphalt, and even bench surface materials (Metal heats up more than wood. Seriously, don’t use metal!) can affect how often you need to water your trees. Even outdoor A/C units blow hot, dry air from the side. We can’t always have our trees in a cool area away from buildings or paved surfaces. You can certainly use humidity trays outside to provide ambient moisture among your trees if they are exposed to a lot of radiant heat, or during a summer heatwave. Also, misting your trees’ foliage and trunks is great for a cool-down, but beware of harsh sun and misting. Heatwaves and droughts can be deadly for bonsai; very hot weather with no rain is terrible, even for well-cared-for trees. Extreme heat is dangerous for bonsai and can result in sunburn and even dieback of whole branches or areas on the tree, even when well-watered. Weak trees (from disease/pathogens, poor care, or bonsai potted that same year) can easily die. Climate change is happening, and it does affect our bonsai.


How often should I water my bonsai?

This is a question I get asked all of the time.


When we check our trees, we inform ourselves as to the amount of moisture in the soil. Whether we should add moisture by watering, will depend largely on season, amount of new growth, soil components, as well as the age and species of the bonsai.


Let’s start with the soil, and what it means to appropriately drain the soil.


The best bonsai soil is fast-draining and is composed mostly or entirely of inorganic particles of different kinds of volcanic substrate. These can include Akadama, Lava, different varieties of Pumice, and Kanuma, or other kinds of porous or otherwise tiny and absorptive rock. Many bonsai practitioners do not use any dirt at all for mature trees, as dirt tends to compact or turn to mud, which inhibits the draining speed of the soil, leading to root-rot and dieback. Bonsai rarely like “wet feet”, which is why we want the water to drain out quickly. When the soil is properly poured and gently packed into the pot around the roots, the introduction of water allows the hard porous substrate particles to create tiny pockets of humid air within the pot. These pockets of moisture, along with the oxygen from air molecules trapped via the watering process, allow the roots to access enough water and oxygen to grow and nourish the tree without being soaked to death. Side note: If you notice that your bonsai doesn’t drain, you will need to figure out why, and fast! Because bonsai soil is fast-draining, transpiration of moisture happens more quickly, depending on the size of the pot, its location, and the weather conditions.


Basic bonsai transpiration guide

More heat=faster transpiration.

More humidity= slower transpiration.

Less heat, more humidity=slower transpiration.

More cold, less humidity= faster transpiration.

More wind + any other factor= more transpiration.

Very cold, dry = most transpiration.

Very hot, dry= most transpiration.


Hot weather bonsai watering

During hot weather, most of the water is used just for cooling the tree down. Additional applications of water during a heatwave will continually cool down the entire planting: The soil, the pot, the roots, the trunk, branches, and the foliage. This reduces stress on the tree’s vascular system as it can more easily move water up and down if it isn’t having to fight off excess heat. Lowering heat-related stress, using any-and-all methods, will help prevent branch dieback, and sunburned leaves or needles.


Is your bonsai dry? Look for these signs.

One important visual indicator is the relative dryness of the soil particles close to the surface. When dry, Akadama is sort of tan. When wet, it is dark brown. Black lava when dry is grayish, but dark black when wet. Pumice is chalk-white when dry, and a darker gray when wet. Sphagnum moss top-dressing, whether dyed black or not, changes color too, when moist. I don’t advise poking your finger down into the soil, or using the ol’ chopstick method to check for moisture. No point in disturbing the soil or risking damage to the roots: These are small pots, some as shallow as an inch or less, so it’s a good bet that if the surface is dry, it's all dry, and high-time to water.


How you'll actually water your bonsai

When we water our trees, the first thing is to do it in a way that does not overly disturb the surface of the soil or cause erosion of the soil out of the pot. A small one-handed watering can should suffice, even if you have to fill it again and again.


This is what I use to water the trees at Bebop Bonsai.

Nothing special or expensive is required. It can have a showerhead or a spout. All that matters is that you don’t disturb the soil too much, while also making sure the water is evenly distributed around the pot. When water is coming out the bottom of the pot, if it’s a large bonsai, just keep on pouring. If it’s small, or in a very small pot, you can stop. Wait a few minutes, and use the time to examine your tree for pests, blight, hard-water marks, etc. When the water has stopped coming out of the pot, big or small, water it again. The first watering has pushed out all the old moisture and any impurities and replaced it will fresh droplets. Pouring the second time will ensure there is enough moisture in the pot until it is time to water again. If it is a very hot day or a sunny day where you will be gone for a while, you can water a third time once it stops dripping. This will fill all the nooks and crannies with droplets and air molecules.


Your bonsai pot will affect how you water your tree

Some things about pot shapes and sizes. Some are obvious, some not so obvious. Taller pots drain faster than shallow ones. Deeper or larger pots need more water than smaller ones. An outward-curved sidewall pot can retain more moisture and drain much more slowly than a flat-walled pot of the same depth, length, and width. Understand the Venturi effect and use it when visualizing drainage from a bonsai pot. Gravity acts on the water, and the shape of the pot affects the downward flow of the water.


There is an old Japanese saying “water bonsai three times: once for the tree, once for the pot, and once for the soil”. This makes sense to me, especially when I think of watering for cooling during hot weather or even just making sure a bonsai is thoroughly watered.


Cold weather bonsai watering

Generally speaking, you will water more when it’s hot and less when it’s cold. Winter can be tricky because bonsai will hardly be consuming much water at all, and the water you pour in will likely freeze. That’s ok. The idea during winter is to prevent the roots from being frozen AND dry. Water freezing in the pot shouldn’t be an issue as long as the pot is properly fired. If the soil freezes with water in it, that is not the same thing as dry soil freezing around the sensitive roots. That is certain doom and is to be avoided at all costs. Dry winter winds will do more to dry the soil and the tree than summer heat. Freezing winds can desiccate and kill a bonsai, no matter how much water is in the pot. Anything above the surface of the soil can withstand deep freezes, down to -40F or so, but anything below the soil line can die off if not properly protected, no matter how precisely you water your bonsai.


During the cold months, I still check my bonsai several times every day, and I still try to be proactive regarding weather and temperature changes. I make sure to water well on days when it’s above freezing, and again before the freeze returns. Below freezing, there isn’t much point in watering a tree that isn’t inside a greenhouse. The idea is to make sure it doesn’t freeze when it needs to be watered. If you are checking your trees several times a day, it might be strange at first to go many days (a week or more- even up to a month for some deciduous bonsai) without watering them. Just wait it out. When the thermometer gets above freezing, even for a couple of hours, pour that water.


During winter as well as summer, it’s about knowing your local climate, solar patterns, species, and soil composition. Same with spring and autumn, which we will discuss now.


Transitional season bonsai watering

Spring and Autumn are both major transitional seasons for our bonsai. In spring, trees are awakening from winter dormancy (even though they aren’t truly “dormant” - there are tons of vascular things that happen during winter) and beginning to show signs of growth. Trees are photosensitive, which is to say their spring growth and their autumn change are both triggered by daylight length. It has nothing to do with temperature, let alone an arbitrary date on the calendar like Mother’s Day.


When the days get long enough, the trees begin to grow. They shift from storage mode and energy-loading into foliar-mass development needed for photosynthesis. When the days get short enough, trees start to shift back into the vascular growth period and begin the accumulation of sugars they need to gain cold-hardiness. They then build off of the energy reserves accumulated via photosynthesis all summer long to compensate for winter’s lack of sunlight and warmth. During these transitions, often the weather is unpredictable at best, and chaotic at worst. Temperatures can easily fluctuate 50F or more in a single day where I live in southwestern PA, so my watering practice has to account for those swings. In late spring here, it can snow one day and be 70F the next. We had a freeze on May 31st, 2020, if memory serves…In this case, just stick to the guidelines, pay attention to the forecast, then plan and act accordingly.


The bottom line of bonsai tree watering: there are SO many variables

Don’t underestimate the importance of being aware of your environment. Know your local atmospheric and solar conditions, and the patterns of weather the way a farmer would. That alone goes a long way toward knowing when to water.


Know the challenges of your geographic location, be it lots of ambient heat in a city (been there, done that!), the likelihood of sudden and powerful storms, or cold winter wind, ice, and sleet. Having a reliable weather app is a great place to start. (If I know the weather and my specific microclimate info, I already know when and how often I will have to water that day before I step outside. This comes with some experience, and many mistakes!)


Develop and understanding of the individual needs of each species of bonsai you have, and treat them all accordingly. Move them if need be, out of high winds, hot or cold. If it’s 100F at 2 pm, put them in the shade and hose them down thoroughly, but gently. Move them out of long and hard summer rains or cold/freezing winter rain. Water them when it’s not freezing, and definitely before it freezes again. Water when it’s warm and sunny. Water two, three, or four times a day if the weather and other conditions so dictate. Don’t ever rely on the rain to water your bonsai; rain won’t do a more thorough and painstaking job than you, trust me. Don’t wait, either. Plan ahead, plan for the seasons, plan for tomorrow and the next day, and check on your trees at least three times every day. We are dealing with nature, so there is a cycle and cycles within cycles. There is always something to be done now and something else to plan for.

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