September Zone 7/8
Buttonwood, Cotoneaster, Pyracantha, Quince, Spruce.
Hornbeam, Pine - cork bark, Yew. 

Trim/Pinch New Growth: 
Buttonwood, Cedar, Hemlock, Juniper, Willow.
Azalea, Cotoneaster, Hemlock, Holly - new growth only - old wood too brittle, Jasmine - winter, Mountain Laurel, Podacarpus, Pomegranate, Privet, Quince,  
Sasanqua, Yew.
Autumn starts this month. However, there could be some days as hot as in summer. This variable cool and hot weather during this month means close attention should be paid to watering, particularly to over-watering during cool spells. There is also the danger of trees re-sprouting due to early leaf drop and new growth not hardening off (maturing) before the onset of cooler temperatures. It is best to keep the soil most and cool, and, if possible, move bonsai to a shady area. If the recently styled and potted bonsai should develop long, new growth, don't cut it off completely. A slight trimming when the new growth has hardened can be done without injury to the trees. 
It is not unusual at this time of year for trunks and branches of many species of tree to swell dramatically. Check trees often during this month and October for wires that could be biting into the bark and cut them off before any damage occurs. If necessary, continue light trimming of your trees to maintain the desired outline for your bonsai style. Some trees can put out new growth this month and you should be alert to maintain your bonsai style. Check air layers which may have been started earlier. If sufficient roots have developed, cut the air layer from the parent plant and pot up. Treat these severed air layers as large cuttings and give them winter protection in a cool greenhouse. 
Check any cuttings you may have made earlier an, if roots have formed, pot them into individual containers. These should also be given winter protection the first year in a cool greenhouse.