At least two species of the magnolia genus of flowering trees are commonly known as “Chinese” magnolias. Not all magnolias have development habits or roots that are suitable for growing near water lines. The most appropriate trees of any species are the ones that are small and slow growing. As a rule of thumb, the bigger the magnolia, the greater the spread of its origins and its potential for invading and disrupting water lines.
Saucer or Oriental Magnolia
University of Tennessee Extension urge the deciduous saucer magnolia (Magnolia x soulangiana), sometimes called the Chinese magnolia, for planting close to a sewer or water line. The saucer magnolia, grown because of its spectacular white flowers that are flushed with purple, grows from 20 to 25 feet high and 20 to 25 feet broad. It grows in U.S. Department of Agriculture plant hardiness zones 4 through 9.
Dwarf Yulan Magnolia
The “Elizabeth” magnolia (M. denudata “Elizabeth”) grows up to 15 feet tall in a decade, a magnitude that suitable for growing close a water line, and it can be grown in a Mediterranean climate (See Reference 6). The “Elizabeth” magnolia is a dwarf hybrid Yulan magnolia (M. denudata), sometimes called the Chinese magnolia, that grows 30 to 40 feet tall with an equal spread, a magnitude that’s not appropriate for growing close a water line.
“Little Gem” Southern Magnolia
“Little Gem” Southern magnolia (M. grandiflora “Little Gem”) could be planted near sewage and water lines, advises University of Tennessee Extension. “Little Gem” is a slow-growing magnolia, reaching a height of 30 to 35 feet with a spread of 8 to 12 feet. This magnolia, in the edge of acceptable dimensions, is better suited to warmer climates, growing in USDA zones 7 through 10s. The U.S. Forest Service recommends it for planting on lawns and close decks and patios.
Star and Oyama Magnolias
The deciduous “Royal Star” magnolia (M. stellata “Royal Star”) grows from 10 to 20 feet high and 8 to 15 feet wide, making it a candidate for planting close to a water line. It yields star-shaped white flowers and can be grown in USDA zones 5 through 8. The Oyama magnolia (Magnolia sieboldii) yields white, cup-shaped flowers that are pinkish to climbed crimson in the center. It rises in 10 to 20 feet tall in USDA zones 6 through 8.