Ask Mr. Smarty Plants

Well, I have some bad news for you—plants require water, lateral lines have water, so plant roots are going to grow towards the water.  The roots of woody plants will seek a way to get as much water as possible and will make their way into the lateral lines and clog them.  The further away the plants are from the lateral lines, the longer it will take them to get there and begin growing in—but they will eventually get there.  A brochure from the Metropolitan Sewer District of Cincinnati recommends rodding (inserting a flexible cable with a cutting tool on the end into the line) once a year to clear the lines of any roots that have grown into it. 

Your septic tank man was exactly right when he told you not to plant any photinia, but it doesn't have anything to do with whether or not it would plug up the lateral lines. Photinia is a non-native shrub that has been vastly overused and now is falling victim to a fungal disease that is simply wiping some species out. You don't want any woody plants close to your lateral lines, as in trees and shrubs. They are the ones that send roots out far beyond their driplines, sneaking up on you. But you don't want to leave that area completely bare, because you know it wouldn't stay bare, it would get weedy and unless it was mowed, some of those "weeds" would grow up to be woody plants. You are located in Comal County, which means you are sharing in the drought conditions that Central Texas has been experiencing, making any kind of a water source even more attractive to plant roots.

We finally located a website that made specific recommendations on what should be planted above lateral lines-grass! The Ford County Kansas Septic System Information site gives a lot of good information on how to manage a septic system, but the piece of information we were most interested in was that perennial short grasses should be planted above the lines. Apparently, the shorter the grass, the shallower the roots. Furthermore, the grasses will assist in evaporation from those lines, and should thrive with that unaccustomed dose of moisture. We found three grasses native to Central Texas that are less than 1 ft. tall, ordinarily, spread by stolons, and are attractive year-round. Although buffalograss is often planted by plugs or sodding, these grasses can all be seeded. The Native American Seed online catalog features a good selection of grasses, including mixes, and do mail order. Here are three possibilities for grasses for your situation.

Hilaria belangeri var. belangeri (curly-mesquite) - tufted perennial, grows 6" to l' tall.

Bouteloua gracilis (blue grama) - 12 to 14" tall in flower, perennial, turns tan when dormant

Bouteloua dactyloides (buffalograss) - perennial warm-season grass, 3 to 12" tall.

Hilaria belangeri var. belangeri

Bouteloua gracilis

Bouteloua dactyloides