If your balcony receives relentless sunlight throughout the day, you're right to wonder about its impact on your planter boxes. The direct answer is: it depends significantly on the material.
Plastic planters, particularly cheaper varieties, can become brittle and fade over time when exposed to intense UV rays. The constant heating and cooling cycle can cause them to crack. Wooden planters, especially untreated softwoods, are prone to drying out, warping, and splitting. However, cedar and redwood possess natural rot-resistance and can fare better. Metal planters, like galvanized steel, can heat up excessively, potentially "cooking" plant roots and accelerating corrosion at welded points.
The good news is that many modern planters are designed for these conditions. Fiberglass and resin composites often include UV stabilizers. High-quality polyethylene and thick cedar are also durable choices. To protect your planters, consider applying a UV-protective sealant to wooden ones, using lighter colors that reflect rather than absorb heat, and implementing strategic shading during the hottest part of the day with a sun sail or by positioning them slightly away from the most intense direct rays. Remember, the material's durability directly impacts the health of your plants, as compromised planters can lead to root damage and uneven watering. Choosing the right planter from the start is the best defense against a full-sun balcony.