This is a common and crucial question for any gardener. Proper drainage is fundamental to plant health. Most quality planter boxes do come with pre-drilled drainage holes. However, the sufficiency of these holes depends on several factors.
Firstly, check the number and size of the holes. A single small hole in a large box is often insufficient. Look for multiple holes spaced evenly across the bottom. The material matters too; plastic and resin planters typically have adequate holes, while some wooden or ceramic pots may have limited or no drainage.
"Sufficient" drainage means water exits freely without leaving the soil waterlogged. To test, water the plant and observe. If water pools on the surface or drains very slowly, the holes are likely inadequate. Stagnant water leads to root rot, a primary cause of plant death.
If your planter lacks holes or they are insufficient, you can easily add more using a drill with an appropriate bit. For decorative pots without holes, use them as a cachepot: place a basic plastic nursery pot (with holes) inside. Adding a layer of gravel or pebbles at the bottom is an outdated method and not recommended, as it can actually raise the water table within the soil.
In summary, while many planters include drainage, always verify. Ensuring excess water can escape is the simplest way to guarantee thriving, healthy plants in your containers.