Ask Mr. Smarty Plants

We very recently answered a similar question, having to do with roots getting into the lateral lines from a septic tank. You really have to consider the whole system when planting over it, because of the damage roots can cause. We don't know too much about septic systems, but we're assuming that the "septic mound" you refer to is the earth over the tank. The tank should be sealed to prevent roots invading it, but you don't want to take any chances. 

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.

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 could find no specific information on Berkshire County, nor any advisories concerning septic systems there. So, we're going to assume that the use of native grasses over the entire system is the safest, most economical way to maintain that system. 

We're going to go to our Native Plant Database, go down to COMBINATION SEARCH, select on Massachusetts, and then "Grasses/Grasslike Plants" under Habit. From this list, we'll select some shorter grasses suitable for your purposes. These plants are all commercially available, and if you have difficulty locating sources for plugs, sod or seeds, go to our Native Plant Suppliers section, type in your town and state in the "Enter Search Location" box and you will get a list of native plant nurseries, seed suppliers and landscape and environmental consultants in your general area.  Because our information on plants for the Northeast is somewhat limited, we would suggest you contact the University of Massachusetts Extension Service, which may have material available that will help you with your project.

GRASSES AND GRASS-LIKE PLANTS FOR MASSACHUSETTS

Bouteloua gracilis (blue grama) - 12 to 14 inches tall

Carex blanda (eastern woodland sedge) -1 to 3 ft. tall

Carex pensylvanica (Pennsylvania sedge) - 6 to 12 inches tall

Elymus hystrix var. hystrix (eastern bottlebrush grass) - 1 to 3 ft. tall

Hierochloe odorata (sweetgrass) - 12 to 20 in. tall

Hordeum jubatum (foxtail barley) - 1 to 3 ft. tall

Muhlenbergia schreberi (nimblewill) - 1 to 2 ft. tall

Bouteloua gracilis

Carex blanda

Carex pensylvanica

Elymus hystrix var. hystrix

Hierochloe odorata

Hordeum jubatum

Muhlenbergia schreberi