One of the easiest ways I have found to create holes for plants is to use a battery powered drill with an auger.
I have tried two sizes, a 1.75″ diameter with a short shaft and a 2.5″ diameter with a two foot shaft. The smaller shaft works well if you are sitting down, but does not leave a clean hole. You need to use one hand to clean it out before putting in the plant. The advantage is that the hole is smaller. The drill does have a tendency to get dirty as a result of having a shaft which is so short.
The 2.5″ diameter auger makes a clean hole, but the hole is larger. This is my favorite since it is easier to make a clean hole and easy to just throw in the plant and fill it in.
I have had good luck using an auger in good soil and bad. The auger will break up the soil into small pieces and will make it easy to fill in when the plant is in the hole.
Butterfly Gardening 