A 10 foot tree isn't very big as trees go, so the root system of that tree may be comparable to the root system of a medium shrub. What kind of tree was it?

Many shrubs sold as shrubs eventually can become small trees--good old Ligustrum japonicum comes to mind--it's usually seen as a clipped hedge 4-6' tall but will become a 25' tree given time and opportunity.

You might consider a citrus on true dwarf rootstock. They'll usually range between 6-10' at maturity, but the real solution would probably be to put your height somewhere else in the area, away from the pipeline.

Simply not planting anything near your main line pipe larger than a lavender or some other sub-shrub might be best. Or what about some kind of structure to provide height without a root system, such as a decorative freestanding trellis or tuteur with a mannerly flowering vine growing on it? Plumbing repair is expensive!

Can you recommend a lavender sub-shrub?

What about planting a California lilac, like the Dark Star variety?

Citrus is actually prohibited in my area for now because of a spreading citrus virus.

This post was edited by gardenweb88 on Sat, Apr 20, 13 at 0:29

It is a shame that the pipe could not be replaced with a modern one that is not prone to root incursion. We lived once in San Francisco sand dune area where Joint Grass was a constant problem in the drain pipe. The real solution is to replace the pipe. Al

Any kind of shrub will have roots that will invade your pipe, I think. Maybe you should consider cactus of some sort, or you could put a draping plant up on a pillar there for height.

There is a barrier you can install between the plant and your pipe made by Villa Root Barrier, Inc. It comes as a set of thick plastic panels 18x24 or 24x24. There are little raised deflectors to turn back the roots and the thick plastic keeps roots from getting into pipes, bricks and cement work.

I'm going to second what Al said, the only real solution is to rip the clay pipe up and install a modern sanitary line. Personally, I would have done that before ripping out an established tree unless of course the tree is directly above the line.

Any lavender would work, they don't have much of a root system. When I said "sub shrub" I meant a shrub-like plant that grows and looks good for a few years then has to come out and be replaced. Usually a very modest root system in comparison with a woody shrub. Lavender is an example. As opposed to a long-lived shrub like Boxwood, Myrtle, or Ligustrum.

Check with a nursery and see if any dwarf palms are recommended in your area with your particular site requirements. Palms have very shallow root systems.

I do have Garrya 'James Roof' I grew as a small tree. It has never really looked like a tree. Suckers a lot and looks unkempt if they are not constantly removed. A nearby Clematis has partially covered it now and we have just let it go. Al