To stop an urban planter box from staining the deck underneath it, focus on keeping moisture, soil runoff, fertilizer, and tannins away from the deck surface. Start by raising the planter on pot feet or deck-safe risers so air can circulate underneath and water does not sit directly on the boards. Place a sturdy drip tray or saucer under the planter, and empty it regularly so it never becomes a shallow pool. If you use a waterproof mat, put it under the tray rather than directly against the planter base, and make sure it does not trap water against the deck.
Check that the planter has working drainage holes, and cover the holes with a small piece of geotextile or mesh to keep soil from washing out. For wooden planters, line the inside with pond liner, but leave a clear drainage path to the tray. Water carefully: give the plants enough to moisten the root ball, then stop before water pours out, and empty any runoff from the tray. Avoid overfertilizing, and rinse away spilled fertilizer or potting mix quickly. Choose resin, glazed ceramic, or metal planters with good drainage, and keep rusty metal from touching the deck directly.
A deck sealer can help, but it is not a substitute for good planter drainage. Clean and dry the deck before placing the planter, lift or move it occasionally to check for trapped moisture, and wipe up stains early with mild soap and water. With raised feet, a well-managed drip tray, and careful watering, your urban planter box can stay healthy without leaving dark rings or watermarks on the deck below.