Plant the tuber(s) with the stem or sprouts facing up and cover with 2” of soil. Planting dahlias early indoors will give you ahead start on summer flowering. (We plant them a little after the tomato plants go in.) Dahlias are beautiful, full-blooming natives of Mexico that can be grown virtually anywhere in the summer. You can do this with tubers you’ve overwintered yourself, or with new tubers purchased from your favorite garden center or mail order source. Even if you have a garden, a container grown dahlia can live on your patio or front porch, bringing those gorgeous blossoms up close and personal. Intro: Dahlia flowers do best in plant containers that are at least 1 by 1 foot, and low-growing and dwarf dahlia varieties are best for container gardening. Amend soil with peat moss or compost. How to Overwinter Dahlias. You will then have a small plant ready at planting time. Dahlias are most often grown from tubers (like bulbs) planted in the spring. Dahlias will not grow in cold soil and their foliage is sensitive to frost. Fill 1 or 2 gallon pots with moist growing mix. In cool climates, dahlias can be planted indoors, 4 to 6 weeks before the last frost date. April 10, ... (Tubers are swollen parts of a root or stem that store food for the next growing season and produce buds for new plants.) Because there are so many varieties, choosing dahlias is much more difficult than growing them. Many nurseries sell the seeds of shorter, bedding-type dahlias. Dahlias. Where to plant. Planting dahlias early indoors will give you ahead start on summer flowering. In hardiness zone 7, dahlias will usually survive the winter outdoors as long if the soil is well drained and the tubers are insulated with a thick layer of mulch.To avoid any risk, bring the tubers indoors, following the instructions for colder zones. Raise the lights as the plants grow. Growing dahlias in containers saves some of this work as you can grow them indoors, or bring the containers in before the weather turns. If you would like to grow larger dahlias in containers the container must be quite large. In cool climates, dahlias can be planted indoors, 4 to 6 weeks before the last frost date. Medium dahlias, also known as border dahlias, are compact 1- to 2-foot-tall plants.These dahlias look great in the front of a perennial bed mixed with other late-blooming perennials, such as phlox and asters. These Mexico natives grow in U.S. Department of Agriculture plant hardiness zones 7 through 10, where the tuberous root systems overwinter well. Growing dahlias in pots.