Mesquites make wonderful landscape trees and provide coveted shade. Eventually they will reach 30 feet tall and 30 feet wide. They can grow extremely fast in our urban yards due the the excess water they receive. This causes a couple of issues. The first is that the canopies outgrow the roots, which contributes to 'wind throw' or blowing over in winds. Second, the wood produced from fast growing trees tend to be weak. To prevent this, move the emitters to the drip line (edge of the branches) and only water once every two weeks in the summer tapering to once a month in the winter.

As far as pruning your trees, it sounds like they have been pruned too much already. If the canopy is 6 feet off the ground and they are only 9 feet tall, that only leaves 3 feet of foliage or 1/3 of the tree. A better balance would be 50/50. You won't be able to 'force' your trees to grow taller by pruning the branches. This is a misconception. Genetics will determine how tall your tree grows. Removing the branches (and leaves) can actually stunt your tree. Every green leaf is a food factory that the tree uses as its energy source. Pruning too much can basically starve the tree. The rule of thumb is to never remove more than 25 percent of the living tissue in any year. Exceeding this can cause stress and make your tree(s) vulnerable to pathogens or damage from insects.

A Mesquites natural growth form is with multiple 'trunks' or low branches. While you can prune to create a single trunk, usually this is done when the trees are small. Removing one of the low branches now may impact your trees health.

Wait until next May before doing any major pruning on your trees. This has been found to be the best time by the experts at the Desert Botanical Garden. A little minor pruning can be done nearly any time except the middle of summer.

Here is a great reference for pruning trees and shrubs that was written for our region in the southwest. You can find it at local bookstores or on line.

Here is a link that might be useful: Pruning, Planting and Care

Great info as always treelover. Thanks.