Autumn is a busy time, many different tasks at this time.
- important- for all items on this list, remember that trees must be able to go dormant for the winter rest. A little of everything is good. Drastic work in autumn is not good.
Pretty much for every species of tree used for bonsai, these tasks are appropriate.
-Autumn clean up, remove dead leaves, pull weeds, scrape back excess moss growth, especially off of trunks, and if moss is too dense on potting media surface. If you used organic fertilizer, often a caked layer of fine particles forms on the potting media surface, scrape this off. Wipe down pots, get rid of dirt and detritus that can get caked on the pot.
-Fall fertilization - for most species the autumn dose of fertilizer is very important, as this sets up for the vigor of the spring flush of growth. Key is the fall dose(s) of fertilizer should be dilute, but frequent that the tree has time to get a nice amount stored in the tree's storage system. Chemical soluble fertilizers should be dosed at no stronger than half label strength, organic applied at about half strength. Too concentrated and fertilizer can trigger another flush of growth that won't have time to harden off for winter. So autumn fertilizer needs to happen, but not too strong. This is especially important for pines. For some pines, such as Japanese white pines, autumn is the only time you fertilize. Fertilize at least 3 times, twice in early autumn, once just before your average first hard freeze. The 3rd application needs to be dilute.
-Autumn is a good time to assess each individual tree, make your plan for the next year. Take photos.
- Prune back - most conifers, and most deciduous benefit from light pruning for returning silhouette to design and some structural pruning. Excess pruning can lower winter hardiness, so in general, autumn pruning should never be more than 25% of the total volume of foliage or branches. Pines, this is when the current year's growth the whorls of new branches are reduced from multiples to just 2 or 3, the continuation of the main branch or trunk and one, or at most 2 branches. For deciduous a similar light pruning is done. Note: for flowering bonsai be aware of where your species of tree forms flower buds. New flowers at the ends of branches? if so, leave branches without pruning for branches where you want flowers. Buds in Leaf Axils? then leave several new internodes for branches where you want flowers, but okay to cut back a little. Some like Wisteria and apples, Malus - flower buds form on sprurs, or spur like buds, recognize what these are and be sure to keep them when you do your pruning. Diospyros, kaki and other persimmons, flower on the previous years growth, usually in the leave axils between the 3rd internode thru the 7th internode, so pruning leave at least 5 to 7 internodes for future fruit. Key with pruning, in autumn, it should total less than 25% of the total foliage mass. More than that the tree will need protection from extreme cold over the winter.
-wiring, the early half of autumn is an excellent time to wire most conifers and deciduous. Remove wire that is biting in, give the tree a week to "relax" then re-wire using care to not place new wire into the old wire scars. Wiring is the least stressful of many bonsai practices, you can do very extensive wiring in autumn. Second ideal time for wiring is late winter just before spring.
Repotting should have been done, finished for the year, at least 8 to 12 weeks before your average first frost. Most potting is recommended for spring, but a number of species do well with summer, late summer repotting. All repotting should have been finished at least 8 weeks before average first frost date. Autumn is a good time to plan your repotting for spring. Mark which trees will need early spring repotting, and pick out the pot you will be moving the trees to. Plan ahead. Over winter get your potting media and supplies in order, so everything is on hand during that week in spring when everything is waking up and needs repotting.
So with this short and incomplete list, you can see there is plenty to do in autumn. You should spend some time with every tree, even every ''stick in a pot''. You might not have to do much, but you do need to at least do a little weed removal and clean up. Take pictures. We often forget that.