The Right Place
While there are lots of mature, blooming chrysanthemum plants available for sale in the fall, spring is the best time to plant mums in the ground to assure survival of their roots over their first winter outdoors. Spring nursery stock will be small cuttings or potted seedlings that will mature over the season in your yard and bloom in the fall. Plant new, young garden mum seedlings in a sunny location in spring after the soil has warmed up. In the Northeast or Midwest that means well into May. If you intend to keep them in the garden for several years, be sure to purchase types listed specifically as “hardy”.
Fall nursery stock will be full-grown plants. Choose plants in pots 6 to 8 inches wide with buds just ready to open. Imminently blooming plants with swollen buds that show just a hint of color are preferable to those in full flower because you will get to enjoy them longer and the earlier planting gives their roots more time to establish for winter. Plant the mature plants for fall display in containers and in masses in sunny, bright beds as you would annuals during the summer. If they do not survive the winter, pull them up in the spring and replace them with summer annuals.
Planting Nursery Stock
In addition to lots of sun (at least 5 hours daily), garden mums like soil that drains well and is mildly acidic (pH 6.0 to 6.5). They do poorly in clay soils that hold too much moisture. If you have clay soil add organic matter such as peat moss to the soil or build planting beds higher than the surrounding soil surface and then add lots of organic matter (such as chopped leaves, peat moss) to the soil to improve drainage. Or plant them in containers in soilless planting medium that is light and drains well.
Clear the planting area of weeds and debris. Prepare the soil by loosening it with a trowel or shovel, digging down about 6 or 8 inches and turning it over. Break up large clumps of soil and remove any stones, then smooth and level the planting area. Dig a hole for each plant that is roughly the depth of its pot and somewhat wider. Remove each mum seedling or plant from its container by gently tapping its bottom as you tip it into your hand. Loosen any roots that are tangled or matted, then set each plant in its hole. Be sure that it is at the same level in the soil that it was in its pot, then fill the space in the hole with soil. Firm the soil around the plant stem and over the rootball with your hand to remove air pockets in the soil and water well. Space young plants about 18 inches apart to allow for bushiness as they mature over the summer. Plant mature plants as close as their size permits, since they are already full-sized.