Protect Trees from Bark Damage
Unplanted mulched areas around trees protect them from bark damage by mowers and trimmers. Tree bark is much more than a protective layer. The inner bark layers contain tubes which transfer food made by the leaves to the roots. When bark is damaged by mowers and line trimmers, some of these tubes are cut. This reduces food transfer to the roots and stunts tree growth. If bark is cut or damaged all the way around the trunk, trees will die.
Weed and grass free circles around trees can be maintained in several ways in addition to pulling or cultivation. Edging barriers such as bender board, metal or plastic can be placed at the outer edge of circles. Edging prevents grass from growing into the circle. Edging can be moved outward as trees grow larger. Concrete edging can also be used, but it is difficult to enlarge. So grass free areas should be made larger in anticipation of tree growth.
The best way to prevent weed and grass growth within the circle is to apply black landscape weed barrier fabric. Landscape fabric is woven so it contains holes for air and water to pass through. The fabric is not only a physical barrier, but prevents light from reaching the soil beneath. Plants cannot grow without light. Landscape fabric is usually covered with bark or other mulch to improve appearance. After a year or two weed seeds will blow into the mulch, but they are easily controlled because of shallow roots. Do not use ordinary black plastic as a weed barrier. It does not have holes so air and water can reach the roots.
Weeds can also be controlled by spraying weed killers such as Roundup (glyphosate). Granular weed preventers such as Casoron or Trifluralin (Preen) can also be used. Corn gluten, an organic weed preventer, is usually available only from full service nurseries and garden stores.
Flowers planted around trees will also be competitive for water and nutrients, but not to the same extent as grass and weeds. I like to keep flower plantings at least 2 feet away from tree trunks.