Capillary mat subirrigation provides uniform water in the growing medium to optimize seedling growth in plugs. It also offers a closed system that allows the grower to regulate the amount of water available to seedlings and to reduce water runoff. However, root outgrowth into the capillary mat can be a significant problem. Copper hydroxide (Spin Out) was painted on the bottom, outside surface of the plug container to control root outgrowth into the capillary mat. Three square and two octagonal plug sizes were treated with copper. Regardless of the plug size or shape, copper treatment was an effective treatment to control root outgrowth in marigold seedlings. Copper treatment reduced overall root outgrowth by 80% to 92%. Marigold and geranium seedlings in copper-treated square plug containers showed some reduced shoot and root development during plug production, but there were no differences in copper-treated plants compared to nontreated plants following transplanting to cell packs.