A DIY self-watering system for an urban planter box is basically a small sub-irrigated planter. You create a water reservoir at the bottom, keep the soil above it, and use a wick to pull moisture upward. That way you water less often and the plants take what they need.
Here is a simple build. Gather a rectangular planter box, a plastic storage tub or pond liner that fits inside, a sturdy plastic grate or slatted false bottom, landscape fabric, absorbent wick material such as felt, capillary mat, or old cotton towels, a 1-inch PVC fill tube, a drill, and potting mix.
1. Make the reservoir. If your planter box already has drainage holes, line the bottom and sides with pond liner, or set a watertight plastic tub inside it. The reservoir should hold 3 to 5 inches of water. Drill an overflow hole on the side of the reservoir just below the false floor level. This hole sets the maximum water level and keeps soil from getting waterlogged. If you use a tub, drill the overflow through both the tub and the planter box.
2. Build the false floor. Cut a plastic grate or slatted tray to fit above the reservoir. Support it with inverted plastic pots, PVC legs, or bricks so it is stable. The floor should sit 3 to 5 inches above the bottom. Cover the false floor with landscape fabric to stop potting mix from falling into the water.
3. Add the wick. Cut two or three wide strips of felt or capillary mat. Drop one end into the reservoir so it touches the bottom. Lay the other end across the false floor and let it mix into the soil area. For a stronger wick, use a perforated plastic pot filled with potting mix as a wicking column. Set it through the false floor and into the water.
4. Install the fill tube. Place a 1-inch PVC pipe vertically from the reservoir to the top of the planter box. Secure it with zip ties or tape. This is how you refill the reservoir without disturbing the plants. Cut the top at an angle or add a small funnel.
5. Add soil and plants. Fill the planter with lightweight potting mix, not garden soil. Garden soil becomes too heavy and compacts in containers. Plant herbs, lettuce, peppers, or flowers as usual. Water gently from the top once to settle the mix.
6. Fill and maintain. Pour water into the fill tube until it drains from the overflow hole. Wait 20 minutes, then top up again if needed. After that, check the reservoir every few days and refill when it is low. Keep the overflow hole clear. Because reservoir water does not carry many nutrients, feed with a liquid fertilizer every two to four weeks during the growing season.
Tips: In hot weather, a reservoir may last only a few days. In cool weather, it can last a week or more. If plants look yellow or the soil stays soggy, reduce watering and check drainage. If the soil feels bone dry, add more wicks. For a tiny planter, you can also use a plastic bottle with holes and a cotton wick, but a proper reservoir with an overflow hole is more reliable.