Answer: Round-Up's main ingredient is called glyphosate, which is a systemic. This means when it is sprayed on a plant, the plant absorbs and distributes it throughout its system. Eventually, it kills the entire plant, including the roots. The spray can drift to other plants, particularly if there is any breeze. It's possible the magnolia roots can absorb some of it, also, although it is not supposed to last in the soil. If the volunteers are bedding plants, you should be able to eliminate by pulling. Soak the soil area slowly and deeply to make it easier to pull them out by the roots. They may also be coming back from seed; pull them as soon as possible before roots get established.
Another nontoxic way to get rid of them would be to apply a thick layer of mulch to preclude them from sprouting. I hope this info helps!
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