It’s vital to know how to plant dahlia tubers correctly if you want to guarantee a spectacular show of flowers.
When learning how to grow dahlias, you will discover there are varieties available in a huge range of colors and forms – from flawless geometric shapes to flouncy romantic blooms.
‘Few plants can rival the flowering power of a dahlia, and the best part is that growing them is easier than you may think,’ says Gail Pabst from the National Garden Bureau (opens in new tab).
Simply plant dahlia tubers at the right time, and give them the right conditions, and come summer you’ll be rewarded with a glorious display that will last into the fall.
Before the frost sets in and the ground freezes over, make sure you know how to overwinter dahlias, and they’ll return year after year, growing more abundant each time.
Learn how to divide plants and you can propagate more of your favorite dahlia varieties.
How to plant dahlia tubers – expert guide
‘Dahlias are relatively easy to grow and need only a few essentials: good soil, adequate water, and lots of sun,’ says Erin Benzakein, author of Discovering Dahlias (opens in new tab).
Before deciding how to plant dahlia tubers, you must first make sure they are in good condition, and not showing damage or signs of rot. Tubers also need to have all the necessary elements.
‘In order to grow and reproduce, a dahlia tuber must have three key components: the main body, which holds the nutrients and energy needed to produce next year’s plant; an eye or eyes, which will eventually swell and sprout, becoming the stalks of the new plant; and a sturdy, unbroken neck that connects the two other parts. Without any one of these, your tuber won’t grow,’ says Benzakein.
Dahlia tubers are somewhat fragile, so handle them with care. If you need to store your tubers before planting, then keep them somewhere dark, cool and dry.
When to plant dahlias
‘You can plant dahlia tubers directly into the ground in the spring when the ground is warm and there is little chance of frost,’ says Pabst. ‘One good guideline is to plant dahlias at the same time as you would a tomato.’
In many locations, this will be April to May. However, those in warmer climes may start the process a little earlier. If you are experiencing a lot of rain, then it’s best to wait until this has passed.
‘If you want blooms as early as possible, you can start dahlia tubers indoors in good light about a month before outdoor planting time,’ adds Pabst. ‘By starting early you could have a small plant ready to transplant in the spring.’
However, don’t rush to plant your dahlia tubers outside. ‘While it may be tempting to start growing dahlias as soon as possible, ideally wait until the ground has warmed up and all frost hazards have passed,’ says Nikitia, owner of Mit City Farm (opens in new tab).
‘The ground temperature should be at least 60°F (15°C). Check the USDA Plant Hardiness Zone map (opens in new tab) to find out the last average frost date for your growing zone.’
How to plant dahlia tubers in the ground
Before planting dahlia tubers in the ground, it’s important to create the right conditions for them to thrive.
‘Dahlias perform best when grown in full sun and in fertile, well-drained soil,’ says Tammy Sons, horticulturist and owner of Tennessee Nurseries (opens in new tab).
To increase your chances of success, enrich the soil with compost or organic matter, and mix it in before planting.
‘Use an all-purpose granular fertilizer for the biggest blooms and larger plants,’ adds Sons.
When planting dahlia tubers, it’s important to plant them with the ‘eye’ facing upwards. ‘The eye is the point on the shoulder, or crown, of the tuber from which the plant grows,’ says Pabst.
‘When planting dahlia tubers in the ground, the tubers should be 3-4 inches below the surface. Depending on the type of dahlia you have, it is often recommended to plant them 15 inches apart.’
However, larger dahlias may need to be spaced as far apart as 30 inches, so do check the ultimate size of your chosen variety.
Before covering your dahlia tubers with soil, it’s wise to add a stake or other plant support at this stage, to avoid later damaging the tubers. Many dahlias grow to 4-5 foot, or even taller, and their stems can easily be broken in the wind.
Backfill the planting hole with soil, and firm it in, before giving the soil a good watering. Unless the soil dries out, don’t water again until the green shoots appear, or the tubers could rot.
The dahlias should begin to sprout within a few weeks. Once they have grown around 8-12 inches tall and developed three pairs of leaves, pinch out the growing tips to encourage branching. This will ensure a bushier plant with more flowers.
To do this, simply remove 3-4 inches from just above a set of leaves, using sharp, clean pruners.
How to plant dahlia tubers in pots
You can plant dahlia tubers in pots indoors – usually in March – to get a head start on the growing season, before planting them out in the ground in mid to late spring.
Alternatively, if you plan to use them in your container gardening ideas, then you can plant smaller varieties of dahlia tubers into pots outside once the frosts have passed.
The most important aspect of learning how to plant dahlia tubers in pots, is choosing the right container. It should measure at least 1 foot in diameter, and be 15 inches deep. It will also need drainage holes to ensure the soil doesn’t become waterlogged.
‘Dahlias can be planted in pots in two ways: by dividing the tuber into several pieces or planting the entire tuber,’ says Lindsey Hyland, founder of Urban Organic Yield (opens in new tab).
‘If you are dividing the tuber, make sure each piece has at least one healthy bud and at least one node (the point where the stem meets the root).'
Make sure you use a good quality, ideally peat-free, well-draining potting mix.
‘Use a nursery mix or a thick potting soil mixture that's been moistened,’ adds Sons. ‘Fill the container then place the tuber buds – or eyes – facing upwards. Cover the tops with 1-2 inches of soil.’
Water the containers and place them in a bright spot indoors. If you are planting them later in the season, then the pots can go straight into their final positions outside. Water regularly once the shoots have appeared.
As when planting dahlias in the ground, it is advisable to pinch out potted dahlias to encourage flower production.
Do you soak dahlia tubers before planting?
It is not necessary to soak dahlia tubers before planting. However, some gardeners choose to soak them to rehydrate them after storage, and to speed up the growing process.
‘Soaking your dahlia tubers before starting them in pots will stimulate them to develop more quickly, which means they'll be ready to flower sooner,’ says Nikitia. ‘Soak tubers in a bucket of lukewarm water for an hour before planting to properly rehydrate them.
However, after this initial soaking and watering in after planting, she warns of being over-zealous with the hose. ‘Allow the soil to almost dry up before watering again while the plant is forming roots. The tubers will not dry out but may decay if the soil is kept too damp before the roots and top growth get a strong start.’
How deep should I plant dahlia tubers?
When planted in the ground, dahlia tubers should be at a depth of around 3-4 inches below the surface.
When planting dahlias in pots, cover them with 1-2 inches of potting mix.