A commercial grade gas string trimmer is a another must have tool if you're serious about your lawn care service.
The string trimmer, pictured above, is one I have had for a while now. It gets a lot of use every day in our lawn care service. I don't know what I would do without it.
If you have never tried a commercial grade string trimmer you will be shocked at the power and how fast it cuts down even very thick grass.
Line trimmers often pull double duty, mine does.
I use mine for the usual trimming grass but I also use it to make a clean edger around planting beds and trees. An edger does a good job but for most areas I choose to use my string trimmer.
You can get a top string trimmer from any number of manufacturers including (in no particular order) Stihl, Echo, Husqvarna, and Honda just to name a few. String trimmers use two stroke engines so you can use the same gas / oil mixture you use for your lawn edger and your blower.
When choosing a gas string trimmer you can get a couple of different options. The main one being your choice of one of the newer four stroke models or the conventional two strokes. The trimmer pictured above is a hybrid model. It is called a 4 stroke (it has four strokes per compression) but still takes 2 cycle oil. I have never had a minutes trouble out of it.
As with most power tools for lawn care everyone has an opinion on which one is better. Some string trimmers will feel different than others so here again, I suggest you try one out for a while at your local lawn care equipment dealer and then decide.