Good grief! I had forgotten how much of a pest spotted spurge (Euphorbia maculata) can be in a well-watered area of exposed dirt. What I should have done was apply a pre-emergent, but the schedule didn't permit. Fortunately, spurge is an annual weed so I get another chance next year.
The buffalo grass is sending its runners right over the top of the ground-hugging spurge, so maybe it can choke it out.
The areas where we planted the extra plugs with 9-inch spacing has a dense mesh of runners covering the soil and spurge between the plugs, with some dense satellite clumps from the earliest runners. If your budget can handle the added cost (4 times as many plugs to plant for 9-inch spacing compared to 18-inch spacing) you could have decent coverage by mid-June with a late March or early April planting.