That's an excellent and very practical question! The answer isn't a simple yes or no, as it depends entirely on the specific planter box you purchase.
Typically, most standard wooden, metal, or concrete planter boxes sold online or at garden centers do NOT come with a liner pre-installed. You are expected to buy a suitable liner separately. This allows you to choose a liner material (like fabric, plastic, or metal) that matches your needs for drainage, moisture protection for the wood, and root containment.
However, some higher-end or specialty planters may include a built-in, integrated liner or a water-resistant coating. For example, many modern self-watering planters always come with a sealed, integrated reservoir system. The product description is key. Always check the "What's Included" section or product specifications. Look for terms like "includes liner," "integrated basin," or "built-in drainage."
So, when do you need a liner? We recommend using one for wooden planters to prevent rot, for metal planters to avoid soil contact and rust, and to contain soil in any planter with wide gaps. It also makes seasonal cleaning much easier. You can purchase flexible pond liners, landscape fabric, or custom-sized plastic liners from the same stores that sell the planters.
In short, always assume a liner is sold separately unless the product listing explicitly states otherwise. Buying the liner separately gives you the flexibility to ensure perfect drainage and longevity for both your plants and your beautiful new planter box.