Mums (Chrysanthemum spp.) grow and flower best in full-sun conditions, though some tolerate or prefer light shade for part of the day. They are perennial plants that can grow in the garden year-round and are also sold as fall-flowering annuals. Whether you grow mums as annuals or perennials, they flower best when given enough sunlight.
Mums require at least six hours of sunlight every day during the summer months. For the best flowers, mums also need fertile, well-drained garden soil or potting mix. These preferred growing conditions are shared by all varieties of garden mums.
Hardy garden mum (Chrysanthemum morifolium, also called Dendranthema morifolium) grows 1 to 3 feet tall and has a wide range of flower colors. It is hardy in U.S. Department of Agriculture plant hardiness zones 5 through 9 and, like other mums, requires full sun. Though they share growing preferences, mum cultivars come in a wide range of sizes and colors. 'Sweet Peg' (Chrysanthemum 'Sweet Peg') is a compact variety which grows 8 to 12 inches tall, has pink flowers and is hardy in USDA zones 5 through 9. The taller variety 'Bronze Elegans' (Chrysanthemum ‘Bronze Elegans’) grows at least 3 feet tall, has bronze-red flowers and is hardy in USDA zones 4 through 9.
Effects of Shade
Though mums can tolerate light shade, they become weak and spindly when they do not get at least six hours of sunlight. Shaded plants also produce fewer flowers. The only situation where shade can be beneficial is when you can supply afternoon shade in warm climates to help keep the plants cool.
To help keep lightly shaded plants compact, pinch 1 inch off the stem tips when the plants reach 6 to 8 inches tall and then again when the new growth is 6 inches tall. You can also pinch plants in full sun to encourage branching. Pinching, even when done in the late spring or early summer, will delay flowering by a few weeks. For more open plants that bloom earlier in the fall, do not pinch.
Day-Length and Flowering
When mums flower is directly influenced by how much light they receive. Mums are "short-day plants," which means they need the longer nights that accompany the fall season to trigger flowering. Mums start to set flower buds once they get at least 11 hours of darkness at night.
This doesn't change the fact that mums need full-sun growing conditions, but it does influence where you plant mums. Do not plant mums near a street lamp or porch light, since artificial light will affect their bloom cycle and may result in fewer flowers.