This is a common and crucial question for any city gardener. The short answer is: most well-designed urban planter boxes do come with drainage holes, and their sufficiency depends on several factors.
Typically, commercially available planter boxes intended for outdoor or balcony use are manufactured with pre-drilled drainage holes in the bottom. This is a standard feature to prevent waterlogging, which is the primary cause of root rot and plant death. The number and size of these holes can vary. For small to medium-sized boxes, one or two holes may be sufficient. Larger planters or troughs often have multiple holes spaced along the base.
Whether the provided drainage is "sufficient" hinges on your specific setup. The key is not just the holes themselves, but ensuring excess water can freely escape. A layer of drainage material, like pebbles or broken pottery shards, placed at the bottom of the planter before adding soil can prevent the holes from becoming clogged. Additionally, using a high-quality, well-draining potting mix—not dense garden soil—is essential. Always place your planter box on pot feet or a stand to allow water to flow out freely from the holes; setting it directly on a flat surface can block drainage.
If you fall in love with a container that lacks holes, you can often drill them yourself using an appropriate drill bit for the material (ceramic, wood, or plastic). For indoor use or sensitive balcony floors, using the planter as an outer decorative pot and placing a smaller, drained plastic pot inside is a perfect solution to manage water. In summary, while drainage holes are standard, their effectiveness is a partnership between the planter's design and your gardening practices.