Now is the Time: Hot Weather & Bonsai by Ken Wassum

A sustained spell of hot weather has prompted requests for an article on caring for bonsai when the temperature climbs up into the high 80’s and 90’s.  What follows is how I take care of my trees when the weather heats up.

High temperatures can sometimes be as brutal to the health of bonsai as sub-freezing temperatures can.  The recent run of hot weather is unusual in that it has lasted so long.  These sustained temps can put a lot of strain on our trees.

There are some things that you can do to help your trees endure these temperatures, even thrive in this hot weather.

1. Move those trees that are smaller or more sensitive to hot sun, such as maples, to more shady areas.  The direct sun is often more punishing than the actual heat.  For those yards that may have little shade there is the option of using shade cloth.  While not very attractive it can be useful for short periods in reducing the intensity of the sun depending on the strength of the shade cloth, with 30% being a commonly used density.
2. Cover the surface of the soil with shredded sphagnum moss.  This helps to protect the roots near the top of the pot from the intense sun, and helps the pot retain moisture when watered.
3. Water enough but do not over-water. I had a conversation with David at the Potluck and he stated that heavy watering for a couple weeks usually does not harm bonsai, it is important to recognize that bonsai soil in a pot may be dry on the surface but still wet deeper in the pot.  Watering heavily for a long period may result in some unhealthy roots.   When the weather is hot I often water twice, even three times, per day.  These are usually much lighter than the morning watering, and are primarily intended to protect the surface roots rather than soaking the whole root ball.
4. Keep your roots healthy by using inorganic mixes.  The root growth and health using these soils is far superior to the roots that grow in denser mediums that often inhibit root growth.  Since it is the roots that carry the moisture to the branches and leaves, healthy roots are a must for bonsai health during hot weather.  My experience has shown that when the roots are in optimum health, my trees are much better equipped to deal with extreme temperatures.

I hope you find a lot of enjoyment with your bonsai this summer.