This is the crux of the art of bonsai. Most trees that die are lost to dehydration, either from lack of watering or from being kept in a low humidity environment like indoors too long.
Different soils dry at different rates, trees differ from species to species in water requirements and even different styles of pots dry out at varying rates, so each pot must be checked regularly until you become more familiar with the plant in question. The other end of the spectrum, over watering can damage plants nearly as quickly, it is in finding the proper balance of soil, water and air that you will develop a healthy root system and thus a healthy bonsai.
Watering accomplishes three things for your bonsai.
- Frist, and most obviously, it provides water for your tree.
- Secondly, the water that flows through the soil carries nutrients your plant needs and washes out the excess salts that might otherwise build up.
- Third, and least obvious, the flow of water pushes out the old, spent gasses in the soil and pulls in new, fresh air.
People are conditioned to believe all the plants transpiration takes place in the leaves, but if that was true, how would a plant that was totally cut down spring back from the roots. The first few inches of soil are crucial to all of a plant's functions, even a mighty oak does most of its feeding and drinking and much of it's breathing in the top foot of soil. It is this reliance on a shallow soil profile that allows us to grow trees in such shallow pots in the first place. The porous quality of bonsai soil allows for quicker water flows and better gas exchange, this is why bonsai soil is so granular, it helps develop a dense mat of roots to support a dense mass of foliage.
Bonsai watering basics
Some quick watering tips:
The old Japanese adage is to water three times, once for the pot, once for the soil, and once for the tree. By going back and forth over your collection three times, it allows the water to soak into the soil and the pot and leave water for the tree to take in.
Don't just water the soil. All parts of a plant absorb water to some degree. 35 percent of the water intake for a plant doesn't involve the root system at all. Washing off the foliage also keeps dust and pollutants from clogging stomata or breathing holes in the plan't leaves.
Sure it's raining, but is it enough to get good flow through? Better safe than sorry, water anyway. Been raining all week? Prop up one end of the pot a few inches to increase drainage.
Using the hose to water? Too much water pressure can blast soil out of the pots, be sure to get an adjustable spray nozzle to allow for different needs. Get one with a mist setting, it's great for occasional wash downs of the foliage. Keep in mind that if that hose has been lying in the sun for a few days, or hours, that first blast can boil a tree, literally.
Once a week or so, water by totally immersing the pot in water until the bubble trail quits. This assure top to bottom watering and lets the pot and soil soak up their maximum holding capacity, making it easier to keep them watered during the rest of the week. I think this is the best tip in here, especially for novices.
Water early in the day. If you must, water late in the day, but be aware that leaves your plants more vulnerable to fungus and slugs. Plants don't transirate, breathe above 85 degrees, so when you water in midday, you temporarily cool the leaf enough to start transpiration, which allows the moisture inside the leaf to escape in the exhalation, kind of counterproductive, AND the water sitting on a leaf in the midday sun can act as a lens, burning leaves. So watering early morning beats all these problems and gives you a few uninterrupted minutes with your trees, and that can be a real stress beater. If you have flowering bonsai, don't water the flower, it will make them pass almost instantly.
Watering is a learned skill. In Japan an apprentice is given pruning shears on his first day, but he won't touch a watering can for another four years.