Yes, you absolutely can and should use mulch on top of the soil in your urban planter boxes to retain moisture. It is one of the most effective and simple practices for container gardening success. A layer of mulch acts as a protective barrier, significantly reducing water evaporation from the soil surface. This means you won't need to water as frequently, a major advantage during hot city summers. Beyond moisture retention, mulch helps regulate soil temperature, keeping plant roots cooler in heat and warmer during cool spells. It also suppresses weed growth, preventing unwanted plants from competing with your vegetables, herbs, or flowers for water and nutrients. As organic mulches like wood chips, straw, or shredded leaves slowly decompose, they add valuable organic matter to the soil, improving its structure and fertility over time.
For best results, apply a layer of mulch about 2-3 inches thick after watering your plants thoroughly. Be careful to keep the mulch a few inches away from the stems of your plants to prevent potential rot. While inorganic mulches like pebbles can also conserve moisture, organic mulches provide the added benefit of improving soil health. By mulching your urban planters, you create a more resilient and low-maintenance garden that makes efficient use of water and supports healthier, more vigorous plant growth.