If you travel a lot, the urban planter box that holds moisture the longest is a large self-watering planter box with a sealed bottom reservoir and a wicking system. This type is often called a sub-irrigated planter, or SIP. Unlike a standard pot, which loses water through the top and drainage holes, a self-watering box stores water below the soil. A wick or capillary mat pulls moisture upward as the soil dries, so the plant drinks slowly instead of depending on you.
Size matters. A rectangular self-watering planter with an 8–10 gallon sub-irrigation reservoir will usually last longer than a small balcony pot. Thick, UV-resistant plastic, glazed ceramic, or composite walls reduce evaporation. Avoid unglazed terracotta for long trips because it breathes and dries out fast. Choose a model with a water-level indicator and a fill tube, so you can top it up before leaving.
For the longest moisture retention, cover the soil with mulch, a plastic lid, or a fabric cover. This cuts evaporation from the surface. Put the planter in partial shade and out of strong wind. Drought-tolerant plants such as succulents, rosemary, sage, or lavender will last longer than tomatoes or ferns. If you are away for more than two or three weeks, even the best self-watering box may need help. Add a drip irrigation timer, an olla, or a smart irrigation system. The most reliable urban setup for frequent travelers is a large EarthBox-style self-watering planter with a covered reservoir, wicking system, and backup irrigation. That combination holds moisture far longer than any ordinary planter box.