Dahlias (Dahlia hybrids) are tender perennials native to Mexico and Central America. The tubers are not hardy in most of the US, so they are planted each spring and dug each fall except in the mildest climates. That annual cycle is a small amount of work compared to what they deliver: bold, long-season blooms that range from 2-inch pompons to dinner-plate heads 10 to 12 inches across, in nearly every color. They are also superb cut flowers, and regular harvesting keeps plants producing new stems through frost.
When to plant dahlias
Dahlia tubers go in the ground after your last frost date, once soil has warmed to at least 60 degrees F. Planting into cold, wet soil before it has warmed risks tuber rot before growth begins. Do not rush.
Look up your last frost date using the frost dates tool, then use the planting calendar to identify your planting window. In most of the continental US, this falls between April and late May.
Dahlias can also be started indoors in pots 4 to 6 weeks before your last frost to get a head start. Use a large pot, keep the mix barely moist until shoots appear, then move to strong light.
Site and soil
Dahlias need full sun, at least 6 to 8 hours daily. In hotter climates (zones 8 and warmer), light afternoon shade can protect flowers from bleaching, but in most of the US more sun is better.
Soil should be rich, well-drained, and slightly acidic (pH 6.0 to 7.0). Dahlias are heavy feeders and reward soil preparation: work 2 to 3 inches of compost into the planting area before setting tubers. Avoid fresh manure and high-nitrogen amendments at planting, which can cause rot and encourage excessive leafy growth over flowers. Raised beds with loose, fertile soil are ideal.
Planting dahlia tubers
How to plant dahlia tubers
Set your stake first
For varieties that will exceed 36 inches tall, drive a sturdy stake 12 to 18 inches into the ground at the planting site before you dig the hole. Staking after the plant is established risks spearing the tubers.
Dig the hole
Make it 4 to 6 inches deep and wide enough to lay the tuber horizontally without crowding.
Orient the tuber
Lay the tuber on its side with the "eye" (the growth bud at the neck end) facing up. If there is no visible eye, look for a small knob or node near where the tuber attaches to the crown. No eye, no sprout.
Cover and do not water
Cover with 2 to 3 inches of soil. Do not water until shoots emerge.
Space correctly
Dinner-plate and large decorative types need 3 to 4 feet between plants. Ball and medium types do well at 18 to 24 inches. Smaller pompons and border types can go as close as 16 inches.
Use the spacing calculator to plan how many plants fit your available space.
Pinching for more stems
Pinching is one of the most impactful things you can do with dahlias, and it is easy to forget until the plants are already tall.
How to pinch dahlias
Wait for the right height
When the central shoot reaches 12 to 16 inches tall and has at least 3 sets of leaves, it is ready.
Pinch the growing tip
Nip off the tip just above the top pair of leaves. This removes the dominant apical bud and redirects energy to the lateral shoots below.
Let it branch
Two to four new stems will develop from the nodes below the pinch point. Each of these will produce a flower stem, giving you many more blooms per plant than an unpinched dahlia.
Watering and feeding
Begin watering regularly once shoots emerge. Dahlias need consistent moisture, roughly 1 inch per week including rainfall. Deep, infrequent watering that reaches the root zone is better than light daily sprinkling. A layer of mulch 2 to 3 inches deep helps hold moisture and moderates soil temperature.
Feed dahlias with a low-nitrogen, higher-phosphorus and potassium fertilizer once plants are established and blooming. Too much nitrogen late in the season pushes leafy growth and reduces flower size. Many gardeners use a tomato or bulb fertilizer formula after bud formation begins.
Cutting dahlia flowers
Dahlias are cut-and-come-again flowers. Regular harvesting keeps the plant producing new stems.
How to cut dahlias
Cut in the morning or evening
Avoid midday heat; stems cut during peak heat wilt quickly even in water.
Cut long
Take 12 to 18 inches of stem if the plant has length to spare, cutting just above a leaf node. The node below will push a new shoot.
Condition immediately
Place stems in deep cool water for a few hours before arranging. Some growers sear the freshly cut stem end in boiling water for 30 seconds to extend vase life.
Deadhead what you do not cut
Any spent flower you leave on the plant will signal it to slow down. Remove them all, either by cutting for the vase or snapping off faded blooms at the stem.
Staking and support
Dahlias taller than about 36 inches need support. The most reliable approach is a single sturdy stake driven in at planting time (before the tuber is placed), standing 4 to 5 feet above ground for large types. Tie stems loosely to the stake as they grow using soft garden twine or stretchy ties. For very large dinner-plate types, a cage or multiple stakes around the plant provides better support against wind.
Overwintering: when and how to dig
In zones 8 to 10, tubers can be left in the ground year-round in well-drained soil. In zones 7 and colder, dahlias must be dug after the first killing frost damages the foliage.
How to dig and store dahlia tubers
Time the dig
Wait until frost has blackened the foliage, but do not delay more than a week after the first hard frost or the tubers may rot in cold, wet ground.
Cut back the stems
Cut stems to 3 to 4 inches above the crown.
Loosen the soil first
Use a garden fork to loosen the soil in a wide circle, at least 12 inches out from the stem base, then lift the clump. Avoid cutting through tubers.
Wash and cure
Gently rinse off soil with a hose. Allow tubers to cure in a dry, shaded, well-ventilated spot at 60 to 70 degrees F for 1 to 3 days.
Divide if desired
This is a good time to divide clumps into individual tubers, each with an eye attached to the crown neck. Label everything with a permanent marker.
Store for winter
Pack in dry peat moss, vermiculite, or coarse sawdust in a cardboard box or mesh bag. Store at around 40 degrees F in a cool, dry location such as a basement or garage. Check monthly and remove any tubers that show rot.
Common problems
- Powdery mildew: common on dahlia foliage late in the season. Improve air circulation by spacing adequately and removing the lowest leaves to lift the canopy. Water at the base only.
- Aphids: cluster at stem tips and under leaves. A strong water spray knocks most off; heavy infestations may need an insecticidal soap.
- Spider mites: hot, dry conditions favor them. Stippled, bronzed leaves are the sign.
- Thrips: distorted or silvery-streaked petals, most visible on light-colored blooms.
- Gray mold (Botrytis): appears on fading flowers in wet, cool weather. Remove faded blooms promptly.
- Tuber rot at planting: almost always caused by planting into cold, wet soil or watering before shoots emerge. Ensure soil is above 60 degrees F and hold off watering.
When should I plant dahlia tubers?
After your last frost date and once soil has warmed to at least 60 degrees F. Use the frost dates tool for your specific last frost, then check the planting calendar. Planting into cold or wet soil before it warms risks rotting the tubers before they sprout.
Do I have to dig dahlias every year?
In zones 8 to 10, no: tubers can overwinter in well-drained soil. In zones 7 and colder, yes. Dahlia tubers are killed by hard freezes. Dig them after the first killing frost, cure briefly, and store in a cool dry space at around 40 degrees F through winter.
Why are my dahlias not blooming?
Three common causes: planting depth too deep (more than 6 inches can slow emergence), insufficient sun (they need at least 6 full hours), or too much nitrogen in the soil or fertilizer (promotes foliage over flowers). Also check that you pinched the main stem to push branching: unpinched plants put most energy into one central stem and bloom later with fewer total flowers.
How do I know if a dahlia tuber has a viable eye?
Look for a small bud, knob, or bump near where the tuber attaches to the crown. Eyes are sometimes hard to see on freshly dug tubers but become more obvious after a few days of curing. A tuber with no eye will not sprout. When dividing a clump, make sure each section has at least one visible eye attached to a piece of the crown.
How do I keep dahlias blooming all season?
Cut or deadhead every flower as it fades, before seeds form. Regular harvesting signals the plant to keep producing. Dahlias are cut-and-come-again flowers: the more stems you remove just above a leaf node, the more branching and flowering you get. A plant that is never cut will slow down and put energy into seed production.