How to Grow Dahlia Tubers

How to take care of your dahlia tubers BEFORE planting

Keep tubers in a place that is between 40-50 degrees with about 80-90% humidity before you plant them. Otherwise, they can freeze, rot, dry out, or all of the above. 

If you are waiting to plant cuttings, keep them watered and warmer than the tubers–near a sunny spot. You have to wait until ALL danger of frost has passed before these babies go into the ground.

When to plant dahlias

For tubers and cutting, choosing the right time to plant is the key to a successful dahlia crop. Dahlias prefer warm soil. Plant after all danger of frost has passed and when soils have warmed to at least 55-60 degrees. If the forecast is predicting a prolonged stretch of wet weather, it is best to hold off on planting because tubers are prone to rot in cold/wet soils. In our zone 5, early May is typically the earliest we can plant outside. If you want earlier flowers, you can start your tubers indoors in pots about a month before planting outdoors. 

Site preparation for planting dahlias

Dahlias thrive in full sun and should be planted in a location that receives a minimum of 8 hours of sunlight a day. Dahlias planted in an area with less sunlight will grow taller and won’t yield as many blooms.

Dahlias are heavy feeders and like soil with lots of organic matter. Because tubers are prone to rot, it’s important to make sure that your soil isn’t too heavy and drains freely and easily; if not, amend it with peat moss or sand.

Dig a hole or row that is at least 6 inches deep. Layer the bottom with compost and Sluggo (slugs love dahlias and Sluggo is an organic way to avoid them).

Planting dahlia tubers

After amending your soil with compost and Sluggo, I also add mycorrhiza. The major role of mycorrhiza is to enhance the nutrient and water uptake by the plant by exploiting a larger volume of soil than the roots can do alone. It’s like a symbiotic relationship between plants and fungi.

Plant tubers horizontally, with the eyes facing up, about 4 to 6 inches deep. Place them about 12 inches apart from each other. 

If you are planting cuttings, you should still add compost, the mycorrhiza and Sluggo, but you don’t have to dig as deep of a hole. Just cover the bottom quarter of the stem with soil and nestle in. 

Place a stake or flower support next to the tuber while planting, so you avoid spearing the tuber later on. Dahlias, in addition to having the most beautiful flowers ever, are also HEAVY. They will often fall over without support.

If you want to remember the variety you planted, put a label marker in the ground when you plant, because trust me, you will not remember later. 

Watering dahlia sprouts 

DO NOT water it until the tuber has sprouted. I know this goes against every ounce of your flower mama instinct, but just don’t do it! Watering before sprouting often causes the tuber to rot. The plant is getting all of its nutrients from the mother tuber and is essentially eating itself in order to send up sprouts. You can start watering after it sprouts

You can water cuttings right away since they are already established.

For disease prevention, it’s always better to water at the bottom of the plant or with drip irrigation rather than overhead watering. 

Fertilizing dahlia plants

Once the dahlia plants are established and about 1-2 feet tall, I do a once every 2-3 week liquid kelp spray. You can also add organic fertilizer to the soil every few weeks, but on a larger scale, I find spraying is less time consuming. 

If you plan to dig up your tubers to store and plant the following year, you need to also wrap a plant tag around the stem with the variety name written on it once it is tall enough to do so. The reason being–after the first frost, everything turns black and there is no way of knowing what it looked like.  

Pinching back dahlia plants

A regular practice for dahlia growers is to pinch your dahlia plants to promote lateral growth and a more balanced plant with more stems. When your plant is about 12” tall, snip out the growing tip, leaving 3-4 pairs of leaves (counting from the bottom). Again, this is HARD TO DO when your plants are looking so healthy and are FINALLY taking off, but I promise you will be rewarded later for doing this. This will encourage the plants to send up growth that will not only give you more flowers to cut, but will also distribute their weight better so they are less likely to topple in strong winds and rains.

A lot of varieties of dahlias have very heavy flowers, so you need to give them support. Try using netting, tying them to the stake you put in at planting time, or use a tomato cage to bolster them. Do something, because the last thing you want are your giant beautiful blooms laying on the ground. 

Harvesting dahlia flowers 

Once your dahlias bloom, keep cutting them and they’ll keep producing more stems! Cut just above a leaf set and the plant will continue branching from right below where you cut. Cut at a 45 degree angle. Make sure you cut when you’re happy with how the flower looks, because it won’t open further once cut, and buds won’t open afterwards either.

Always cut when it’s the coolest part of the day – either morning or sunset – to avoid wilting. Strip the foliage off of the leaves as you cut to prepare them for conditioning.

Harvest and deadhead dahlias regularly to promote additional flowering. If you let the flowers go to seed, it will dramatically slow down bloom production.

To extend their vase life, dip the stems in boiling water for a few seconds immediately after cutting them. Let them cool for 10-ish minutes and then place in cool or room temp water. 

Large dinnerplate dahlia varieties never last as long in a vase as smaller more compact varieties do. Sadly, it just is what it is.  

Digging dahlia tubers in the fall for winter storage

After 2 hard freezes, dig up the tuber clump, wash it, divide the tubers, and store them wrapped in plastic wrap between 40-50 degrees with 80-90% humidity throughout the winter. Congratulations, because by doing this you just multiplied your supply of tubers by anywhere from 4-15! 

 

YouTube resource for digging and dividing tubers:

https://www.youtube.com/watch?v=V_GzIEfXXBw

 

YouTube resource for storing dahlias the way I do, with the plastic wrap method:

https://www.youtube.com/watch?v=kcc0jHpeZWc

 

Next spring, after there is no longer a threat of frost, unwrap them and repeat this process!

(All of the above steps and tips all came from A LOT of trial and error on my part. Everything that could go wrong has gone wrong at one time or another. Here’s to hoping that the above tips save you some trouble and keep you coming back for more! Dahlias are ADDICTING!