Willow trees cause some of the worst root clogs in home septic systems, but any nearby tree's roots will invade sewer pipes in search of water. Roots follow buried pipes from joint to joint, inserting feeder roots through any tiny opening. Once inside, the roots find perfect growing conditions. Sewers provide water, oxygen and abundant nutrients. Masses of roots could completely fill small pipes or cause backups in large sewer mains. Physically removing the roots does not stop the problem.
If sewers back up from root clogs, sewer systems have already suffered permanent damage. As roots grow inside the pipes their diameter expands, forcing joints apart and cracking the pipe walls. Any woody plant with a tap root could attack the pipes, but willows cause some of the most severe problems, since these trees grow vigorously in wet soil. Even when cut down, a willow tree's roots sometimes live for several years. Grasses, flowers and any other small plants with shallow rooting habits cause no problems in drainage fields, but don't plant woody plants in these areas.
Treating the sewer system regularly with chemicals kills tree roots before they grow large enough to cause severe damage. Copper sulphate crystals, a common treatment applied by both homeowners and professional plumbers, affects the entire tree. The massive dose of copper poisons roots on contact. Copper prevents trees from leafing out properly and could kill invading trees. Copper sulphate sometimes only damages trees. Test treatments of eucalyptus roots in the late 1960s killed feeder roots quickly, but trees recovered within one month and established a new root mass within four months, reported botanist O.A. Leonard of the University of California.
One the most effective root-killing treatments combines a poison that kills roots on contact with a persistent aquatic herbicide that prevents new growth. Metam-sodium, the fast-acting agent, creates a toxic gas called methylisothiocyanate when in contact with water. MITC and sulphides formed by this chemical reaction affect humans as well. Anyone breathing MITC could suffer severe lung damage. Both metam-sodium and dichlobenil, the residual herbicide in the treatment, combine with foaming agents to fill sealed pipes completely and penetrate any cracks or burst joints. Metam-sodium requires professional application. However, homeowners can use safer root killers combining foaming action with dichlobenil for slower but effective sewer treatment.
Root killers only kill roots. In clogged pipes, application leaves a mass of dead roots that could require months or years to decay. Without mechanically breaking up the root clog first, foaming chemicals can't reach all parts of the pipe. Chemical effects linger long after application. Dichlobenil persists in treated soil for up to three years. Copper sulphate leaves toxic copper residue in the septic system, which eventually escapes into the environment. Homeowners need to know the sewer pipe diameter and length before using root killers, to ensure thorough treatment without overapplication. Two pounds of a typical foaming root killer treats 50 feet of 4-inch sewer line, according to NewTechBio.