I would rip the grass out and leave at least a 2 foot gap between the trunk and the grass line all around the tree and maybe get some pine bark mulch, pine needle mulch or any mulch as long as it's not rubber mulch or colored mulch. During dry weather I would water your mameys very well every two days during the hot months. If it rains a bit, water them anyway, unless it's a long good downpour. Sandy soil doesn't tend to hold moisture too well.