Yes—but the planter box itself is not the problem; standing water is. Mosquitoes need shallow, stagnant water to lay eggs, and a planter box can easily become a breeding site if water collects in a saucer, a clogged drainage hole, an uneven corner, a cracked liner, or a leaky irrigation line. Even a small amount of water that sits for about a week can support mosquito larvae, especially in warm weather.
To reduce the risk, choose boxes with clear drainage holes, raise them slightly off the ground, and empty saucers every few days. Check after rain and after watering, fix leaks, and refresh any decorative water promptly. If a planter must hold water—for example, a bog garden or water feature—use mosquito dunks or bits containing Bti, which target mosquito larvae without harming most beneficial insects. Keeping water moving with a small pump can also help.
Stagnant water is not just a mosquito issue; it can also make soil soggy and attract fungus gnats or other moisture-loving pests. Good drainage, thoughtful watering, and regular inspection are the simplest ways to keep urban planter boxes healthy and comfortable to use. So the short answer is yes, they can attract mosquitoes if they hold water—but with a few routine habits, they do not have to.