On a 100°F day, an urban planter box can turn into a miniature heat island. The soil warms much faster than garden soil because it is exposed on the sides and top, and dark plastic or metal containers can push root-zone temperatures even higher than the surrounding air. Small boxes are especially vulnerable because they hold less soil and less water, so they heat up and dry out quickly.
The first thing you may notice is that the soil dries unevenly. The surface can become crusty and hydrophobic, meaning water runs off instead of soaking in. Plants may wilt at midday even when the soil still feels slightly damp, because high heat forces them to lose water through their leaves faster than their roots can replace it. Leaves can scorch, flowers can drop, and vegetables such as lettuce may bolt or turn bitter. Roots near the container walls can be damaged by heat, and beneficial soil microbes become less active when the soil is hot and dry. Evaporation also concentrates fertilizer salts near the surface, which can stress plants further.
A planter box in extreme heat needs more attention, not just more water. Check soil moisture daily, preferably in the morning, and water deeply until it drains from the bottom. If the soil has become hard and repellent, water slowly in several passes so it can soak in. Early-morning watering is usually best because it gives plants a reserve before the hottest hours. Add a layer of mulch to shade the soil, move the box into afternoon shade if possible, or use shade cloth during a heat wave. Light-colored containers, larger soil volumes, self-watering systems, and grouped pots can help moderate temperature and moisture. Avoid heavy fertilizing during extreme heat, and choose heat-tolerant plants for exposed urban boxes. With a little protection, an urban planter box can survive 100-degree days, but it will need closer monitoring and a more careful watering routine.