Bonsai watering is the single most important factor in keeping your trees alive. It is also the hardest skill to learn. This is because various species of trees have different water demands.
Some trees tolerate dry conditions better than others. Some trees will “tell you" they are thirsty when their leaves look limp. For other trees limp leaves mean the tree has already died.
So where do we start? All bonsai trees are grown in special bonsai potting mix - not dirt, not potting soil, and not garden soil. This special bonsai soils coarse and granular, and should allow for water to freely run through it.
All bonsai pots have holes for drainage. When bonsai are watered, most of the water runs out the bottom of the pot, and only a small amount is retained in the pot. If a bonsai is growing in a free draining soil mix, during the active growing period (spring to fall) they are usually watered once a day.
Water your bonsai with a watering can, or with an attachment to your garden hose that will deliver a gentle spray of water. Be careful you do not blast the soil from the pot with too strong a spray. Water until water runs from the bottom of the pot.
Bonsai watering is best done in the morning so the tree and soil are fully hydrated during the hottest part of the day. It is often said that it is impossible to ower-water a bonsai if it is in a good potting mix.
However, as the roots grows, and as the potting mix starts to break down and become more compacted, the pot will not drain as freely as it once did. So, some trees may require water today and others not.
You should check to see if your bonsai need water by gently probing with your finger slightly below the soil surface to check for moisture. Do not water if the soil is still moist.
However, do not let the soil become bone dry. Some bonsai growers insert a short section of wooden chopstick near the edge of the pot. To check moisture simply withdraw the chopstick and examine it for moisture. Then replace it again.
Most conifers generally don't like their roots to be wet all the time and tolerate dry conditions better than broadleaf trees. Smaller pots will require more frequent watering than larger pots.
Wind, humidity, pot size, temperature, and soil compaction all are factors in how often you should water. When you go on vacation have a reliable neighbor water your trees.