Watering is the most difficult part of bonsai care. You can kill your bonsai by overwatering or under watering. The only difference in the end is that generally, under watering will kill them faster.
I think most of the trees that die are compelled to shirk off their mortal coils because they have been methodically drowned. This drowning causes root rot and prevents them from drawing moisture into the plant. Ironically, a tree with root rot caused by overwatering usually has the droopy look of a tree that desperately needs water.
If you think that watering bonsai correctly is harder than watering regular houseplants, you’re right. It is. A big part of that is the pot. Those small, shallow bonsai pots will hang onto water far longer than a taller pot. It's a gravity thing. I’ll bore you to death with the details later.
Most trees will easily tolerate a little drying out (Serissa’s don’t care for it and a Formosa firethorn that dries out will punish you).
The only way you will really know when a tree needs water is to pay attention every day:
- Look at the colour of the soil, as it dries out it will look lighter.
- Pick up the pot, as it dries out your Bonsai will weigh less.
- Work your finger into the soil and feel. Dry soil is warmer and harsher to the touch, and I think we are mostly familiar with wet soil!
Follow these steps consistently, because the differences are all relative.
- Don’t water a tree that already feels wet.
- Never let your bonsai sit in water for any length of time.
- When you water, be sure that you’re soaking the entire root ball. It is not unknown for the outside to be soaking wet and the middle of the root ball to be dry and hard as a rock.
- If you tilt your bonsai to the side, does water run out the drainage hole? If so, let it drain completely before you put it back.
Imagine a washcloth. Soak it. Now, wring it out.Twist every bit of water out if it that you can. How wet does it feel? One to two days after your bonsai soil feels like that, water it. And watch out for the exceptions. If your tree is outside on a hot day it will probably need to be watered every day.