Andrew Edge

As we established a few editions ago overwatering your trees at certain stages post repot or at bud burst can have ramifications for your ramification little bonsai pun there!

We understand that correct watering is the key to health for a bonsai with the balance of water and oxygen in the pot the number one priority for us as growers of trees. This balance is found with a good coarse substrate mix which allows for free drainage of excess water and ample oxygen space.

With most of us maintaining an outdoor collection, the rain can come into play when deciding to water or not. We need to understandhow rain affect our trees and what we need to do about it. As a bonsai artist I am also an amateur weatherman, constantly checking the sky and the Bureau of Meteorology (BOM) app on my phone to see when and how much rain if any is forecasted. Consistent overwatering of our trees will lead to issues in the root mass such as root rot, lack of root growth and a lack lustre and unhealthy tree due to the saturation of the substrate and lack of oxygen to maintain balance. The wet/ dry action in the soil is imperative in allowing roots to ‘search’ for moisture in the pot. So what if there is consistent rain?

Heavy consistent rain for a few weeks will not kill your bonsai or cause undue stress to the root system unless the tree had root problems before that may be exacerbated by too much water. A healthy root system will easily cope with excess water for a short time. But should the rain continue then you may need to provide shelter for some trees by moving them inside or under a pergola or eve of your house. Large garden trees can provide enough of a cover to prevent the bulk of the rain getting to the bonsai. For trees that can’t be moved, tilt the bonsai pot by placing a block under on side, allowing for extra drainage. (Be mindful of this when the pot only has holes in the middle of the base. Tilt this and you’ll create a ‘pool’ in the bottom of the pot, worsening the situation for the tree).

The biggest misconception for the rain is that it is enough to water bonsai. Generally it is not. Unless the rain has been reasonably heavy and at least greater than 5mm, then only the very surface of the bonsai will have taken in any water. Add to this that the canopy of our bonsai covers the pot and the drip line is outside the pot rim. When we water using a hose or watering can we douse the tree with a lot of water, completely soaking the substrate each time. Summer is the big time to be wary of rain, as the core of the pot may be bone dry whilst the surface of the soil is still holding moisture. This can be a disaster for your trees.

With this in mind, you should water your bonsai as per usual unless the downpour has convinced you that the tree has been properly drenched. A tree will suffer greatly from any period of underwatering, but will survive short periods of overwatering as long as it isn’t chronic. Always better to err on the side of caution. When I water, I will totally drench the soil substrate. I water the soil surface 3 times, drenching it each time. This allows for the water to fully penetrate the substrate as each pass over with the hose allows the water to work its way deeper into the soil. Capillary action will also help to make sure the core of the tree is watered too which is where soil particle size consistency becomes important.
Bonsai substrates will react differently to watering and how they dry out over time. Inorganic mixes (akadama/ pumice/ scoria/zeolite mixes) are very hard to overwater as water passes easily through the mix with only the required water held by the substrate. Organic (peat/ pine bark based mix) breaks down far quicker and retains more water in the mix. Far less oxygen space allows for fungal issue to spawn more easily if the water/ oxygen balance is lost. So next time it rains, be sure to double check your trees as it could be crucial to their ongoing existence.