I'm not sure what you mean by a flex pipe, but I assume you mean a plastic, corrugated perforated drain pipe. How deep is this pipe buried? If it is within 12-18" of the surface, it might be within the reach of essentially any reasonably vigorous woody plant. In that case, it might be better to consider an herbaceous plant instead. Let me know.

The information below was for a response to tree roots in pipes and should provide some guidance on potential remedies. This is from Kristin Ramstad at the Department of Forestry. Feel free to write with questions.

The best way to avoid root-killing chemicals is to have the invasive roots professionally and regularly cut out and removed from the pipes (in my house, we do it almost annually). This involves having a professional come in and using a long spinning snake-like thing that cuts the roots out of the pipe. Depending on the circumstances, there is also the option of adding a sleeve inside of a damaged pipe that will prevent the roots from encroaching as well.

As for the system-wide situation, (this also applies to the homeowner situation too), there are a few things to keep in mind. (1) Tree roots rarely create cracks in pipes, but the finest of root hairs can invade an existing hairline crack and over time make it larger, and ultimately cause great damage. (2) The other way tree roots damage pipes is by sending a fine root under or around the pipe(s); the root then gets larger as the tree grows, finally lifting or crushing the pipe(s). So the first thing to ask is whether the pipes in the system may already be cracked and easily exploited by roots, or whether they were installed poorly, and tree roots have lifted the pipes... If either of these is true, the pipes themselves need to be fixed.

Another thing to consider is the trees and soils near the pipes. Some trees have more “aggressive” roots than others, and some soil conditions contribute to that “aggressiveness.” Roots don’t “seek” moisture (like heat-seeking missiles), rather if roots simply grow into soil that has both oxygen and moisture in its pores, they will “hang out” and proliferate there. So, leaking pipes in an otherwise dry soil will create conditions for root growth and aggregation near the pipes. One way to minimize this situation is to make sure the entire area around the tree(s) is irrigated properly.

Several trees are notorious for having “invasive roots,” which tends to suggest that these trees create “mats” of fine roots along rapidly growing horizontal or vertical roots. These trees should be planted at least 30ft away from pipes. The Worst Offenders include tree-of-heaven, weeping willow, sweetgum (Liquidambar), poplars, elms, silver maple, and sycamores. Generally, smaller trees have less “aggressive” roots, but they do not provide the other environmental benefits (shade, stormwater mitigation, air-cleaning) that larger trees provide.

OK – so this brings me to copper and chemical root killers. Usually, it is probably better to err on adding a natural element such as copper into the system, rather than using harsh chemicals, especially if the pipes in question do not allow any way for the chemical to enter the soil. Root-killing chemicals will have very specific guidance on their labels as to the types of situations the chemicals can be used in, primarily to try to minimize environmental damage. Not following these label instructions – even using more chemical than directed -- is against the law. Another issue, apart from the environmental damage issues, is that often – especially in large diameter pipes – the chemical (or copper sulfate) is found washed or pooled along the bottom third of the pipe, but the roots may be coming in from the top of the pipe, so the chemical doesn’t do much to kill or stop the roots!

So what are my suggestions to deal with roots in pipes…?

(1) Call the city Public Works AND the local watershed council to see what they recommend for dealing with roots-in-pipes in your area, especially in consideration of how close your pipes are to a major waterway or fish-bearing stream.

(2) If possible, hire a professional to remove invasive roots annually with a coil-type device, and make sure the area is irrigated properly to encourage better distribution of roots.

(3) If possible, fix pipe defects that may be contributing to root invasivity.

(4) If using chemicals is OK where you live, I would try something like copper sulfate first, but personally, I would avoid using most other chemicals. I don’t know how these chemicals will negatively affect “good bacteria,” but I think there is some truth to this assertion.

(5) Finally, I hate to say this … but if the invasive roots – after cleaning-out, fixing/rerouting the pipes, using copper sulfate, etc – still create difficult, expensive, ongoing problems with the pipes, you may need to consider removing and replacing the offending tree(s).