Roots of Magnolia can be rising above the soil surface at it ages over time. However Little Gem has a more limited root sytem and thus would not be a problem as far out from the plant as standard types of southern magnolia. If the patio was about 6 feet from the trunk it should be fine. Plus it takes many years for any roots to become very large and your landscape design may adjust as time goes on. I am not aware of Magnolia producing root toxins but their dense shade is a challenge for most plant. Monkeygrass seems to do okay beneath the outer canopy however.
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