One of the easiest ways to slow down the rate at which deciduous bonsai dry out is to apply a top dressing to the surface of the soil.
A top dressing is a thin layer of soil or related material that is applied with the aim of retaining moisture and/or improving the look of the soil.
There are many recipes for creating top dressings, but most share the following characteristics.
Top dressing particle size is usually small as small particle mixes preserve more moisture than large particle mixes. The particles themselves are usually heavier than water as particles that float can easily wash away during watering. Finally, top dressings often contain organic components like moss that retain moisture and encourage roots to grow near the surface of the soil.
This last point may come as a surprise to growers who avoid using organic ingredients in their mixes. Although I don’t typically use moss or bark in my soil, I frequently apply it as a top dressing.
The most common approach is to apply a layer of white sphagnum moss (aka “New Zealand moss” or “orchid moss”) to the surface of the soil. Depending on how much moisture you want to preserve, you can apply a thin layer or a thick layer.
Recently, I’ve been using a combination of moss and small soil particles as a top dressing. Here’s what the process looks like.
After noticing that the soil near the surface of the Chinese quince below had washed away, I wanted to apply a top dressing that would stay put. I began by applying a layer of moist white sphagnum moss.
Chinese quince – the top 1/4″ of soil had washed away
After applying a layer of white sphagnum moss
Next, I sprinkled fine akadama particles over the moss and gently mixed the two together with a chopstick.
After sprinkling fine akadama particles over the moss
Next, I took some dry sphagnum moss and ran it through a sieve. Using these finer particles, I sprinkled a thin layer over the moss and akadama.
Fine layer of sphagnum moss on top
I carefully watered the top dressing and then patted it into place just below the lip of the pot.
After watering in the new top dressing
Top dressing complete
By adding this top dressing, I effectively made the pot 1/4″ deeper which will preserve moisture and provide room for roots to grow near the surface.
If the top dressing stays wet enough, I can expect moss to start growing. This happened with a Korean hornbeam in my garden that was repotted earlier this year.
Volunteer moss
Healthy green moss is a good sign that the surface of the soil is just moist enough to keep surface roots healthy. If, however, the surface stays too wet, liverworts will appear.
Liverwort – the surface of the soil is staying too wet
Over the last few years, I’ve seen moss develop on a surprising number of trees in both shady and sunny spots in the garden. The moss on the hornbeam above grew under 30% shade cloth, and the silver moss below grew on the surface of a pine that receives full sun.
Volunteer silver moss
In an effort to encourage moss to develop more consistently, I sometimes mix dried moss into the sphagnum-based mixture applied as a top dressing.
Sphagnum moss mixed with green and silver mosses
These moss mixtures can show signs of growth within a matter of weeks when conditions are right, but even if they don’t grow, the white sphagnum moss does a great job of preserving moisture near the surface.
Up next: a top dressing for azaleas – mountain moss.
Podcast Interview Published Last Month
I recently sat down with The Iowa Idea Podcast for a conversation about bonsai with host Matt Arnold. Matt has long had an interest in the pursuit and has recently started some trees of his own. We cover a variety of topics ranging from basic bonsai care to how people teach and learn bonsai.
You can listen to the episode (#20) at the Iowa Idea Podcast website or download it to your favorite podcast player.
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