Question
I have a spot in my yard where the septic tank is buried.  I have tried for years to grow grass, it comes up and then dies in the heat of the summer.  Could I plant something else that wouldn't damage the septic system underneath?  I have thought a tree's root system would damage the septic, maybe something else.
I live in southern Illinois.

Answer
Hi Jeana,
There is usually much truth in the statement "The grass is always greener over the septic tank" so no pun intended, but I think we need to dig a little deeper to find out what's happening. From your question, I am assuming the grass is only dying over the septic tank. First, have your soil tested for pH, nitrogen, phosphorous and potash. Your local agricultural extention office may due this for a nominal fee or you can purchase a soil test kit for under $20. The pH for grass should be around 6.5.  Phosphorus and potash are critical for plants to develop heat and drought tolerance, so if they are deficient, it may be a contributing factor. Next, are you planting a grass suitable for your area?  I often see grass mixes sold in chain stores that are inappropriate for where I live.
Next, we need to know what water is doing in your soil.  Dig a hole 12-15" wide and deep and fill it full of water.
How long does it take to drain away?  If it's gone in an hour, then we know your soil isn't holding enough moisture and either increased waterings or soil improvement will fix it. If it's there tomorrow, then decreased waterings and soil improvement may fix it.
I would not plant a tree over it.  The roots may indeed damage your field lines and if the septic tank ever needs work, you lose the tree.  I have no problem putting a small raised perennial/annual bed over it.
If this is a sunny area that just dries too quickly to easily maintain grass, consider the groundcover sedums as an option.  They are evergreen, very drought tolerant and come in a huge array of foliage texture and flower colors.
If this area gets less than 6 hours of sun, then try Vinca minor. It's a great evergreen groundcover, blue spring flowers and very rapid growing.
If we rule out the above then we look to disease or toxins and put in a call to your county agent.
Have a great gardening day.
Jim Crowder