You can read the following article online at: http://guzmansgreenhouse.com/seasons/septembergardentips.htm

Well it is now September, and the mornings are great aren't they? It is now beginning to be the best time to plant trees and shrubs. It is a great time because the above ground temperatures are dropping and the below ground temps are still warm.

This helps plants get a jump on next spring because the roots still develope in the warm soil while the tops slow down in growth. Also, there are less winds and typically more moisture during this time of year, which of course reduces stress on newly planted shrubs and trees. You should be able to plant just about anything, including pansies, (which will last thru next May) mums, winter veggies (starts), most trees and shrubs.

There are a few exceptions: Palm trees might be better planted in the spring and summer, in order to get a good root establishment before the winter sets in. (For the lower elevations of the southwestern U.S. States)

Oleanders may experience some winter kill if planted too late in the season, especially the 1 gallon size. The larger sizes seem to suffer the least winter kill. This is usually for the the first winter, after that they will have even less winter or no winter kill as they age. Using a winterizer fertilizer with plenty of potash will help increase winter hardiness just about all shrubs and trees.

It is recommended for valuable plants that may suffer from winter damage. Use this product before October 30th. Don't forget to use compost, peat moss, or soil builder and root stimulator on all plantings, and make sure plants have adequate water.