During the second year of growth, sideoats grama will reach maturity. It's root system will grow to a depth of five feet or more. Smooth brome only sends roots to a depth of one foot or less.
Several years are required for a stand of native grasses to reach maturity. During the first tow years native grass stands may appear unkept and weedy.
However, when they are fully established, there are very few weeds which will be able to compete with native grasses for nutrients and water in the soil.
Native grasses will eventually starve out, and hold out, undesirable weeds.