I agree with a lot of what is being said on this post especially the fact if these guys are bothering you b/c you feel threatened by them? These low-ballers do shotty work at best, still work hard (just not working smart), and apparently have their clients fooled. There are always going to be the trucks with a magnet on the side and two schlubs attempting to make a living. Some people just don't care about quality, only price. These pennypinchers are hard to satisfy b/c their budget is never realistic for a professional contractor.
We are in a mixed market where I work and I always tell customers when getting bids from a few companies to run down a check list. By informing YOUR potential customer, you're already ahead of the other guy. Offer references, make sure they're comparing apples to apples (plant sizes, good design, warranties, reputation!), and most of all I say "Mrs. Jones, there's always going to be someone out there that will do the job for less than me, but there's a reason why our prices are at that level...Our nursery has been in the industry for over 20 years years now... All of our workers have at least 7 years of experience."
Bottom line is, you need to let your market know what sets you apart for the competition. Is it experience? or are you a better BS'er? You decide
"Well, I am certainly wiser than this man. It is only too likely that neither of us has any knowledge to boast of, but he thinks that he knows something which he does not know, whereas I am quite conscious of my ignorance. At any rate it seems that I am wiser than he is to this small extent, that I do not think that I know what I do not know."-Socrates