While waiting for your seedlings to develop into plants, root some basil cuttings from bundles purchased at your local market. Place a few cuttings in a bottle of fresh water. In a few weeks you'll have rooted basil plants ready to be grown in your garden or in containers. For more unusual varieties purchase a few plants from a garden centre. Once established in your garden, take cuttings and root them as well.
Cuttings rooted from already rooted and thriving plants have a higher success rate.
This will fill up the empty spaces until the late harvest plants have filled out more, providing less places for weeds to grow. It will also keep the soil around the plant shaded and moist which means less watering.