Mixing bonsai soil for Pacific Northwest winters
Most bonsai soil guides are written with a Japanese or a dry-climate American context in mind. If you are growing trees in Portland, Seattle, or anywhere west of the Cascades, the standard 6:3:1 akadama-pumice-grit ratio will hold too much moisture through the wet season and your roots will suffer for it. This is what we have learned over six winters of adjusting.
What the standard mix assumes
The 6:3:1 ratio (akadama, pumice, decomposed granite) is designed for a climate with a dry summer and a cold but not waterlogged winter. It works well in most of Japan and in the drier parts of California and the Southwest. West of the Cascades, winter rainfall averages 35 to 50 inches between October and March, and trees kept outdoors will sit in wet soil for weeks at a time if the mix does not drain fast enough.
Akadama, which is a fired clay granule, absorbs water and holds it. That is useful in a dry climate. In a wet one, it means the root zone stays saturated longer than the roots can tolerate, particularly for junipers, which are susceptible to root rot even in otherwise healthy trees.
The adjustment we use
For junipers kept outdoors in USDA zones 8 and 9, we now use a 4:4:2 ratio: akadama, pumice, and decomposed granite. The higher pumice content improves drainage without sacrificing the structural stability that akadama provides. The extra granite adds weight, which matters for trees in shallow pots that can tip in winter wind.
For Japanese maples, which tolerate slightly more moisture, we stay closer to 5:3:2. Trident maples are more drought-tolerant than Japanese maples and do well with the same 4:4:2 mix we use for junipers.
- Junipers (zones 8-9): 4 parts akadama, 4 parts pumice, 2 parts decomposed granite
- Japanese maple: 5 parts akadama, 3 parts pumice, 2 parts decomposed granite
- Trident maple: 4 parts akadama, 4 parts pumice, 2 parts decomposed granite
- Shimpaku juniper: same as standard juniper mix
When to repot and refresh the mix
Repotting in the Pacific Northwest is best done in late March to mid-April, when nighttime temperatures are reliably above 40°F and the buds are just beginning to swell. Repotting too early, when the soil is still cold and wet, slows root recovery. Repotting too late, after the leaves have fully opened, puts the tree under stress during its most active growth period.
After repotting, keep the tree in a sheltered spot out of direct rain for two to three weeks. A cold frame or a covered porch works well. This gives the cut roots time to callus before they are exposed to the wet again.
The mix we use is available pre-blended in the shop, but the ratios above are easy to assemble yourself if you have access to the individual components. The important thing is to test your drainage before the wet season arrives, not during it.