Bonsai Growing Tips: Clippings from September 2025

Technically, summer ends on September 22nd this year, but it’s not all fall color, cool evenings, and apple cider from here to the end of the year. Average temperatures are headed down, but there are still hot days coming, and our infamous Santa Ana Winds are right around the corner, bringing some of the lowest humidity of the year . . .

Pay attention to the weather, not the season: We often talk about bonsai timing in terms of “seasons” (e.g. “repotting season”), but this time of year, the ten-day forecast is more important to me. Our Mediterranean Climate is one of the rarest climate types in the world, and consequently, lots of bonsai advice needs to be localized. Some bonsai practitioners in other climates may advocate fall repotting, but they may not be picturing windy 85˚ days with 5% humidity. I would localize that advice by suggesting that fall repotting may be fine for some species, as long as the subsequent two weeks are cooler and more humid — I would always check the forecast before doing the work. Also, because of our long growing season, “fall” for my plants starts a lot later than “fall” on the calendar. Be patient.

Junipers: One of the most frequent bonsai jobs I’m doing this month is pruning/cleaning junipers. Many junipers have put on a ton of mass over the warm months, and now I’m removing runners, cleaning bottom sides and branch crotches, and thinning out dense areas, leaving what John Naka called “room for birds to fly through.” I’m also doing some light wiring following the cleanup, but I will wait for slightly cooler weather before making larger bends.

Tropicals: Nighttime low temps are headed down, but should still be above 50˚ for at least the rest of the month. Given that, I’m still pruning my ficus, but I’m no longer defoliating. Keep an eye on the weather in the coming month or two. I like to let my tropicals grow a bit of a “winter coat” before they stop growing for the year — that means I stop pruning them a couple of weeks before those sub-50˚ nights start.

Fall Color? Lots of my students are interested in deciduous bonsai that display fall color. If you have maples in coastal so-cal, you may have noticed that we don’t always get cold enough in the early fall to stimulate that change, and even if we do, leaves are often battered and browned from the hot dry weather preceding. I suggest growing hardy species that reliably change color in our climate; my favorites are Ginkgo biloba, Liquidambar (both species), and Crape Myrtle (Lagerstroemia).

Flowering Bonsai: Species that flower on bare branches before the spring foliage emerges, like Cherry Blossom, Ume, and Wisteria, set those flower buds on the previous year’s wood during late summer and fall. That means that if we prune them later in fall, we are cutting off next year’s flower buds. I would finish any pruning on these trees soon to get the best show in 2026, even if that means they get a tiny bit unruly — we can always cut them back hard after flowering and before the new leaves emerge.

Plan for next year: I like to avoid spring panic by planning for repotting season in advance. As 2025 winds down, I’ll start considering which trees I will repot in 2026, and if I have the appropriate pot. If I don’t, then I start thinking about doing a little holiday pot shopping for myself ;)

I’ll finish with one final reminder that the Santa Ana Winds are coming. Keep an eye on the weather projections, and remember to put tall or tippy trees on the ground ahead of wind events, and check all of your shade structures, greenhouses, etc. Flying shade cloth can be just as dangerous as the wind itself! Happy and safe growing to you all.