In most cases, yes. A wooden urban planter box will perform better and last longer if it has a liner, especially when it is exposed to rain, irrigation, soil moisture, and frequent planting changes. Wood is an organic material, so constant contact with wet soil can lead to rot, warping, staining, and structural weakness. A liner creates a barrier between the soil and the timber, helping protect the wood while also making the planter easier to maintain.
A good liner also helps control moisture and keeps the planter cleaner. In city gardens, balconies, rooftops, and courtyards, planter boxes often face limited drainage, reflected heat, and heavy watering. A waterproof liner with proper drainage holes can direct excess water away from the wood and reduce waterlogging around the roots. It can also stop fine soil from washing out through gaps in the timber. For deeper boxes, a geotextile layer over the drainage area can help prevent soil from clogging the drainage holes while still allowing water to pass.
However, a wooden planter box does not always require a full liner if it is made from a naturally durable wood such as cedar, larch, or teak, and if it is well built with adequate drainage, raised feet, and a breathable finish. In that situation, some gardeners use only a drainage layer and a permeable geotextile fabric rather than a sealed waterproof liner. This approach can work well in dry climates or for short-term seasonal planting, but it usually shortens the planter’s lifespan compared with a lined box.
If you choose to add a liner, avoid completely sealing the inside without drainage. Standing water is one of the fastest ways to damage both the wood and the plants. Use a food-safe or garden-safe waterproof membrane, such as EPDM, heavy-duty pond liner, or sturdy plastic, and puncture or fit drainage outlets at the lowest points. Leave a small gap or drainage path so water can escape, and avoid trapping moisture behind the liner. A slatted false bottom or a layer of gravel can also help water move freely toward the drainage holes.
For urban planter boxes, the practical answer is this: use a liner unless the wood is naturally rot-resistant, the drainage is excellent, and the box is designed to dry out between waterings. A liner is not just about protecting the wood; it also improves soil retention, simplifies cleaning, and makes the planter more reliable over time. If you want the longest service life and a tidier urban garden, line the wooden planter box properly and make sure drainage comes first.