Marigolds bring bright color to containers and window planter boxes. The orange and yellow flowers bloom all summer long and are prone to very few disease or insect problems. There are varieties available with both small, dainty flowers and large flowers. Marigolds require very little care once established, though there are a few issues that must be attended to when they are in planters as opposed to in the ground. Choose smaller varieties for containers and save the large varieties for bed plantings.
Choose planter boxes with plenty of drainage holes. Use boxes with attached drip trays or drill holes in existing boxes and place them on separate drip trays.
Place the planter boxes where they receive full sun. Set them on a south-facing porch, balcony or window for the best sun exposure, or choose an east-facing location, as morning sun is better than afternoon sun.
Space plants 6 to 8 inches apart in the boxes. Stagger rows in wider boxes so that the plants can quickly fill in the spaces and provide a fuller look.
Water your planters regularly, and do not allow the soil to dry out. Apply a 1-inch layer of mulch around the plants to help preserve soil moisture. Empty the drip trays once the excess water is done draining.
Fertilize your marigold plants once a month with a timed-release fertilizer that is high in phosphorous. Water well after fertilizing, and avoid touching the plants directly with fertilizer.
Pinch off dead blooms beneath the spent flower head and above the nearest leaf or bud before they go to seed and to encourage further blooming.