Position the bathtub in your yard where you would like to locate it for your marigolds. Select a sunny location, as shaded areas will delay flowering for marigolds.
Pull the plug out of the bathtub drain. Fill the bottom 6 inches of the bathtub with gravel. This will provide good drainage for the bathtub planter.
Fill the remaining depth of the bathtub with a quality potting mix. Smooth the top surface with your hands.
Dig holes for the marigold seedlings with a trowel. The hole should be large enough to accommodate the root system of the seedlings. Since there are many varieties of marigolds, it is best to refer to the plant spacing instructions that came with your specific flowers.
Water each hole thoroughly using a watering can. Place one seedling in each hole and fill in around it with soil. Gently pack the soil around each seedling with your hands. Water the bathtub planter thoroughly using a watering can. Keep the soil moist throughout the growing season.