Marigolds are a common choice for Arizona gardens because of their tolerance of heat and alkaline soil, as well as their low-maintenance reputation. Marigolds are also beneficial to other plants in your garden because they attract helpful insects and repel nematodes, tiny round worms that damage many garden plants. In cooler climates, gardeners need to wait until after the last frost to plant marigolds. However, because Arizona weather is so warm, gardeners there can plant them early in the spring.
Prepare your garden for planting in early spring, when the weather is consistently about 55 or 60 degrees. Marigolds will grow as long as there is no danger of frost, which is almost always the case in Arizona. Barring any unusual weather, planting the seeds in March or early April will be fine.
Choose a location with full sun and good soil drainage to plant the seeds. Prepare the soil by adding a few handfuls of compost or other organic matter and incorporating the compost with the garden soil.
Sprinkle the seeds over the prepared soil. Some gardeners choose to sow marigold seeds in rows, while others simply spread the seeds across a designated area. Most marigold seeds have a fairly low germination rate, so you can drop several seeds in one area and only expect one or two plants to grow. Sow the seeds so there are one or two seeds for every square inch of soil space.
Cover the seeds with soil. Marigold seeds do not need to be planted very deeply, which is why you can just scatter the seeds over top your garden soil. To cover them, brush the topsoil over the seeds with your hands.
Water the ground thoroughly after planting and give the seeds about seven days to germinate. After your seedlings are several inches tall, or about two to three weeks after you plant the seeds, thin the seedlings so they are appropriately spaced. Spacing will depend on what type of marigold you planted, but most types will need about seven inches of space per plant. Check the seed packet for individual spacing instructions.