How to Grow Dahlias: Grow Bags, Pots or in Ground?

Thanks to squash borers that took out a bed of zucchini plants many years ago, I needed to fill the space with an alternative plant. I ordered a handful of dahlias on a whim, and never looked back.

If you’re a southern gardener wondering whether or not dahlias will work for you.. Check out this post on growing dahlias in the South. 

If you’re ready to grow, and in need of some extra instruction, read on to learn more about growing dahlias at home...

White Perfection Dahlia

Dahlias can be planted in ground or in pots: both have their perks. Here’s a run down of when to plant where, along with growing instructions. 

Planting Dahlias in Pots

Growing dahlias in a pot (or grow bag) is a great choice if you have poor drainage, limited garden space or want to enjoy these stunning blooms up close on a patio or deck. Growing in containers requires more ongoing care - because they're depending on you to supply all their needs, but it's very doable... Here’s how:

Dahlias in Grow Bags

You’ll Need:

• A large pot - I plant 3 tubers per 20-gallon grow bag. If you're planting a single tuber, I wouldn’t go smaller any than an 16” pot - but bigger is better. Dahlias require lots of water so the larger your pot, the easier it will be to stay ahead of this demanding task, as small pots dry out faster.

• Well-draining potting mix and soil conditioner (I use one bag of soil conditioner per 20-gallon grow bag to ensure good drainage and prevent compaction)

• A stake or tomato cage (dahlias get tall!) peony cages work, too

• Dahlia tubers - check out our curated collection of heat-tolerant dahlias for Southern gardens

Steps:

1. Choose a sunny spot

   Dahlias prefer full sun (minimum of 5 hours of good sun), but in warm climates need some protection from punishing afternoon heat - they’ll appreciate a little afternoon shade. If afternoon shade isn’t available, consider a shade cloth.

   
   

2. Prepare the pot:

   Ensure your pot has good drainage - this is essential as tubers are prone to rot. Fill your container until the soil line is ~7 inches from the top of the pot with a mix of high-quality, well-draining potting mix (not garden soil!) and soil conditioner. Mix in a slow-release, balanced fertilizer (like 10-10-10) or organic compost.

   
   

3. Plant the tuber:

   Lay the tuber horizontally with the eye facing up. Cover with a couple inches of soil.

Lay dahlia tuber horizontally with the eyes facing up

1. Water sparingly:

   It’s ok to water lightly at this stage, but wait to water deeply until you see green shoots. Overwatering early on can lead to rot (trust me). It can take up to a month for sprouts to emerge.

   
   

2. Backfill, then mulch: 

   Once a sprout emerges, add another few inches of potting mix/soil conditioner and a layer of mulch on top. In the end, your tubers should be 5-6” deep and the soil/mulch line should fall ~1” beneath the top of the pot (if you fill too high, mulch overflows when watering)

   
   

3. Support them early:

   Insert a stake or cage at planting time to avoid damaging roots later.

   
   

4. Pinch them:

   When the plant is 8-12 inches tall, cut it back 3-4 inches, just above where it branches. This will encourage bushier growth and more blooms.

   
   

   

   Pinching dahlias is an important step to multiply their blooms

5. Monitor and water regularly:

   Watering containers is an art and a science. Many factors impact how quickly containers dry out including temperatures, wind, humidity, container size and the material/color. Check for moisture levels daily and keep soil moist when dahlias are in active growth (once they have leaves / visible growth). In the height of summer, expect to water daily. 

   
   

6. Fertilize:

   Dahlias are heavy feeders and when grown in pots they’ll depend on you for all their nutrients. Some potting mixes incorporate fertilizer (read yours) - these typically feed for about a month, but will need replenishing. You can either apply a granular, slow-release fertilizer to feed for months, or use a liquid fertilizer every couple of weeks.  

   
   

7. Remove lower leaves: 

   This promotes good airflow and helps prevent disease as the plant grows.

   
   

8. Monitor: 

   Regular inspection is the best way to stay ahead of pest and disease.

   
   

9. Harvest: 

   Enjoy your gorgeous cut flowers! (or deadhead them regularly to keep them blooming)

Planting Dahlias in the Ground

If you have a spot with well-drained soil, plenty of sun and the space for them in your garden beds, planting directly in ground is straightforward and yields big, healthy plants. Dahlias planted in zones 8 and up can be overwintered in ground (meaning they don’t need to be dug up). 

Dahlias planted in ground

Steps:

1. Pick a sunny, sheltered spot:

   Dahlias love sun but appreciate protection from strong winds and punishing afternoon heat. They will get tall, so keep in mind they will need staking.

   
   

2. Prep the soil:

   Before applying fertilizer, take a soil test to see what amendments are needed. In-ground soil typically has nutrients available - adding fertilizer without testing can do more harm than good. If your soil is heavy clay or doesn’t drain well, consider planting on a slight mound or raised bed. A healthy dose of organic compost is a great way to improve soil structure and replenish nutrients. Top dress your garden beds with a couple inches once a year. 

   
   

3. Plant the tuber:

   Dig a hole 4-6 inches deep and place the tuber horizontally with the eye (little bump where the sprouts emerge from) facing up. Don't stress if you can't find the eye. The sprout will find its way up.

   
   

   Space dahlias ~12–24 inches apart (larger varieties need more space). Cover with a couple inches of soil. Once a sprout emerges, cover it again with a couple inches of soil. Repeat until the hole is backfilled.

   
   

4. Water lightly until growth appears:

   Upon planting, dahlia tubers are prone to rot. Water lightly after planting, then not again until sprouts appear. Once plants are actively growing they'll need about 1” of water a week. Monitor rainfall, and provide supplemental water as needed. 

   
   

5. Mulch:

   Mulching is important to insulate roots from heat (and cold), to conserve moisture and to suppress weeds - keep the mulch away from direct contact with the stem.

   
   

6. Stake or cage

   Do this early to support heavy flower heads as they grow and avoid damaging the roots.

   
   

7. Remove lower leaves 

   This ensures good airflow and helps prevent disease as the plant grows.

   
   

8. Monitor 

   Keep an eye out daily for pests and disease - it's much easier to nip them in the bud then to deal with a bigger problem later.

   
   

9. Harvest 

   Enjoy those beautiful blooms! (Or deadhead regularly to keep them blooming.)

Dahlias are easy once you get them started, and the payoff is huge—lush, dramatic blooms that last all summer long. Whether you're tucking them into pots or filling your garden beds, they’ll reward your effort with beauty and joy.

Happy planting from my garden to yours!