Answer: The best solution for the grass problem is to pull it up, roots and all, and then place a 3-4 inch layer of mulch material in the basin. You'll still be able to water, and weedy grass may still grow, but it will be easier to remove because the roots will grow into the mulch rather than down into the soil. Use organic material, such as compost or peat moss, or use bark chips or rocks in the basin. Chemical herbicides are not a good choice for around your tree - many weed and grass killers will penetrate the soil and can harm the roots of the tree. Any overspray can damage the bark and trunk of the tree. Once the weeds have been removed by hand, you should be able to nip the problem in the bud by removing any supsequent sprouts that appear.
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