It is a very common thing you hear among Bonsai Enthusiasts when they say " it has rained every day this week, i haven't had to water". Believe it or not but this could be bad for your trees. Lets find out why.
Now this article will be short as there isn't too much technically that goes into this but it doesn't make its value any less worth your time reading this. In fact this could actually help the health of your trees. We all know when we start out in Bonsai that we freak out when it rains too much that our trees are getting over watered, then we have 1 hot day and we freak out that we are under watering. We have learned that this mostly comes down to your soil substrate and species type as to what is too much and not enough. But why can not watering your trees on a rainy day be bad while people are freaking out that their trees might be receiving too much water.
First of all for the tree to get a proper watering you will need a downfall that produces at least 5ml of rain to ensure the tree gets a proper soaking and the entire root ball has received water, If you only have light showers through out the day not only will the tree not receive enough water to get all the way through the root ball but it may actually get taken up or evaporate before it gets down to the bottom of the pot. Think about when you water your trees how you ( or at least you should ) do 3 solid passes or soakings of the soil surface and allow that water to fully work through the roots system until it is pouring out the bottom of the pot, Not only does this completely soak the root ball providing moisture to the whole root system but the large amount of water applied to the soil surface in 1 soaking also pulls through a good amount of fresh oxygen into the root system replenishing the stagnant oxygen that already exists. For rain to do a good job watering you would need to get that 5mls in a relatively small period especially in summer, Even though its raining in summer it doesn't mean your trees might not need a watering once or twice through out the day, Yes the humidity in the air will slow the process down but it still pays to check on how well your trees have been watered.
The second thing to take notice of is the drip line and canopy of your tree. If your tree has a very full canopy like a juniper for example chances are not much water at all is getting through that canopy and if your drip line is outside of the pot then chances are even less water is actually making it into the pot. Wet your trees from overhead next time you are watering and watch where all that water drips off the tree and how much of that water actually made it into the pot and how much longer it would take you water the tree like that then if you simply just soaked the soil surface directly. This will give you an indication of how much constant rain you would need through out the day to get a proper watering on your trees.
Obviously like anything in Bonsai there are different factors that control how much of an affect this has on your trees such as pot depth, soil type, tree type, the season you are in and the current weather conditions, humidity, how full the canopy is and the trees drip lines.
I hope this has given you something to think over and maybe you have noticed some leaf curl after not watering for a week because you had a small amount of rain each day. It is always worth checking your trees even on the days that have had some rain.
Until Next Time, Enjoy Your Bonsai Journey.