For urban planter boxes, using bagged topsoil alone is generally not recommended. While topsoil is great for in-ground gardens, container gardening has different needs. Planter boxes require a special potting mix, often labeled as "potting soil" or "container mix.
The key difference is structure and drainage. Topsoil can become dense and compact in a confined planter, suffocating roots and leading to poor water drainage and root rot. A quality potting mix, however, is specifically engineered for containers. It's typically lighter, fluffier, and contains ingredients like peat moss, coir, perlite, or vermiculite. These components ensure excellent aeration, moisture retention without sogginess, and proper drainage—all critical for healthy root development in a limited space.
Potting mixes are also usually sterile, free from weed seeds and soil-borne diseases common in bagged topsoil. You can use a small amount of topsoil to amend a large raised bed, but for most urban planter boxes and pots, a dedicated potting mix is the safer and more effective choice for thriving plants. For the best results, look for mixes formulated for the specific types of plants you are growing, such as vegetables, herbs, or flowers.