Growing Dahlias in Zone 6b - How to plant, fertilize, and maintain dahlia tubers throughout the season

Updated November 13, 2023 / Originally written May 21, 2021

Late May is officially dahlia planting season in Zone 6b! We plant approx. 500 tubers every year on our farm (in 2023, we grew over 1,700!) We do all planting by hand on our flower farm and do not employ any machinery in the process. We plant much like a gardener would in their backyard, except our spacing is closer (12”) and we plant in long, straight rows for ease of space, watering and harvesting.

What To Do with Your Dahlia Tubers Upon Arrival

If you’ve ordered dahlia tubers from an online retailer or flower farmer, you’ll want to leave the tubers as-is in the until you are ready to plant. Store your tubers at room temperature (70+ degrees will help wake them up). If eyes begin sprouting or getting leggy, that’s okay, it just means they’re alive and awake. If sprouted eyes break off, that’s okay, too. The tubers will quickly regrow new sprouts right next to where the sprout broke off.

Some tubers take forever to wake up - this is all based on variety and has nothing to do with whether the tuber is alive or not. Some varieties that we grow (such as Penhill Dark Monarch and Nicholas) take weeks longer to sprout than other varieties.

Dahlia Tubers, Explained! What exactly are dahlia tubers?

Every variety of dahlia produces a tuber of a different size and shape - some varieties are long and skinny while some are shorter and stubby. The dahlia tuber is alive! The function of the tuber is to act as a lifeline and food source for the plant while it is in hibernation over winter. People sometimes assume the bigger the dahlia tuber the better, but that is not necessarily the case. Too-big tubers, when planted, cause the plant to rely on the tuber too much as a food source, fail to produce strong roots, and grow weak plants as a result. Sometimes, if a tuber is longer than 6 inches, we will even cut 1/3 of the bottom off with a good pair of dahlia splitting snips.

The general rule is, as long as tubers are the size of a AA battery, they will produce healthy, strong plants if given good growing conditions.

Generally, tubers should be plump and firm; however, if you receive tubers that appear to be a bit dried out (with deep wrinkly lines through them) they will still grow. Dahlia tubers are incredibly tough and retain life inside as long as they possibly can. If your tubers are completely squishy and wet, this means they may have been frozen in transit and are no longer viable.

How to Plant Dahlias in the Garden

Plant your dahlia tubers once all danger of frost has passed and soil temperatures are averaging 60 degrees. Dahlias will not sprout until the soil temperature is in the 60s, so unless you have thousands of dahlias to get in the ground, it doesn’t make sense to plant them before the soil is warm enough. This means the air temperature is at least 70 degrees during the day with consistent sunny days.

If you plant too early (still in the 40s-50s with lots of rain and possible frost), there is a good chance your tubers will rot in the ground. They cannot handle wet, cold soil for a long period of time before establishing roots. It’s best to hold off until your last frost date in your zone has passed (mid-May in Zone 6b).

Dahlias require full sun (8 hours minimum) for successful growth. Dig a hole and plant your dahlia tuber 6 inches deep, laying on its side, with sprouted eye pointing up. If the eye has not sprouted yet, you can plant the dahlia tuber any which way and the sprout will find its way up.

Soil Prep for dahlias and Planting with Soil Amendments

On our flower farm we plant our tubers with Mychorrhizal Fungi Inoculant (2 tsp per plant) and Bone Meal (2 tbsp per plant) straight into the hole with the tuber. We plant 16” apart on our farm, but standard home garden plantings are typically 18” apart.

Prior to planting, we amend our soil with Compost and Alfalfa for adequate nitrogen, organic matter, and trace minerals. We also plant our dahlia rows with Cover Crops (a mix hairy vetch, clover, buckwheat and broccoli raab) and allow the cover crops to grow several feet high until chopping them down and letting them decompose onto the soil before planting into it. Learn how to Cover Crop your garden in our blog article here.

Historically we do not fertilize our dahlia plants throughout the season (due to lack of time and labor) but we plan to design a dahlia fertilization strategy for 2023. To learn more about organic fertilizers for a flower farm or flower garden, visit the “Flower Farming” or “Organic Gardening” sections of our Blog Archive at: heirloomsoul.com/blog-archive

How to organically Maintain Dahlias for Beautiful Blooms

Growing dahlias requires patience! After planting, it can take up to 6 weeks before you see sprouts popping out of the soil (average 3 weeks if planted at the correct time). Refrain from watering your tubers until you see this new growth.

Once your tubers have sprouted: Dahlias are thirsty plants and require daily watering on hot days when it doesn’t rain. It is good practice to add leaf mulch around the base of your dahlia plants to hold moisture in the soil (leaf mulch also provides incredible organic matter to be decomposed by soil microbes and earthworms). FYI, Wood mulch is not recommended for dahlias or other annual plants as it creates an environment where soil microbes tie up nitrogen in the soil. In general, wood/wood chip mulch is preferred for perennial plantings and pathways only.

When your dahlias bloom, cut those flowers! It can be tempting to leave a bloom on the plant until it withers away, but dahlias are “cut & come again” flowers. This means when you cut them, they re-bloom, and they will branch out and produce more stems. Flowers are essentially reproductive organs on full display - their whole purpose is to become pollinated in order to produce a seed, then set their seed on the ground below. When we don’t cut our dahlias, the flower goes into seed-setting mode and will stop producing blooms because it has achieved its life purpose. When we cut the flowers, the plant keeps shooting up more blooms in an attempt to set seeds. So cut, cut away, and bring them inside to enjoy in a vase!

How to Deal with Dahlias at the End of the Season

After a hard frost, the tops of your dahlia plants will turn black. At this point, you have the option to leave the tubers in the ground (the tubers will die) or dig them up to store inside over winter. It is up to you what you choose to do! If you decide to leave your tubers in the ground, it is not for nothing - you’ll be leaving a valuable food source for your soil microbes over winter, and those tubers can stay in the ground to decompose indefinitely.

If you choose to dig your tubers up, do so after two frosts so the plant has been signaled to go into hibernation. Cut the tops of the plants off, gently dig the roots up with a compost fork, gently shake the dirt off, and store in a box or plastic tub full of sawdust in a spot that is consistently between 40-50 degrees. We’ve found that tubers stored in cardboard boxes tend to dry out during storage as opposed to plastic tubs — if you do store in plastic tubs, make sure to crack the lid so moisture can escape.

Some people wash their dahlia tubers down with a hose and let them dry out before packing. We do not wash our tubers simply as we find this an unnecessary, super messy step for our operation. The soil left on the tubers also acts as an insulator, preventing the tubers from drying out faster. You can choose to do what you would like.

For more information on storing your dahlia tubers, visit our blog article: How to Store Dahlia Tubers Over Winter and Get Rid of Mold

Learn how to Store Dahlia Tubers Overwinter Using Sawdust or Wood Pine Shavings!