The key is to eliminate standing water and break the mosquito life cycle before larvae can develop. Start by checking that every planter box has clear drainage holes and that water can flow freely out of the bottom. If the box sits in a saucer or tray, empty it after rain or watering, or fill the saucer with sand so it does not hold a pool of water. Raise the planter slightly on pot feet if water tends to collect underneath.
Keep the soil surface slightly below the rim of the box, and avoid overwatering. Water deeply but less often, letting the top inch or two of soil dry between waterings. Fix leaky drip lines, clogged drainage channels, and uneven soil surfaces where puddles can form. If you use self-watering planters, cover the reservoir opening with fine mesh or a fitted lid, and refresh the water regularly rather than letting it sit for weeks.
For decorative water features, birdbaths, or planter reservoirs that must hold water, refresh them at least once a week and scrub the sides to remove mosquito eggs. You can also use mosquito dunks or granules containing Bti, following the label instructions exactly. Bti is targeted at mosquito larvae and is a good option for many container gardens, but always check the product label and avoid treating water you plan to drink.
Do not rely on mosquito-repellent plants alone; they may help slightly but will not stop breeding in standing water. Instead, inspect hidden spots such as plant saucers, bucket lids, folded tarps, and drainage trays. If you see wriggling larvae, dump the water away from the garden, scrub the container, and let it dry before refilling. A quick weekly walk around your planter boxes is often enough to catch new water pockets before they become a mosquito nursery.