How to Grow Dahlias: A Complete Guide
Whether you're a first-time dahlia grower or adding to your collection, this guide covers everything you need to know about growing gorgeous dahlias for all USDA growing zones.
Before You Begin: Know Your Zone
Dahlias are tender perennials native to Mexico. They thrive in warm soil and cannot survive frost. Your USDA hardiness zone determines when to plant, how long your growing season will be, and whether you need to dig and store tubers for winter.
Not sure of your zone? Visit planthardiness.ars.usda.gov and enter your zip code.
When to Plant
Plant dahlia tubers in spring after all danger of frost has passed and soil temperatures have reached at least 60°F. Dahlias will not grow in cold soil — planting too early can cause tubers to rot.
Typical Planting Window - by Zone
Zone 9–10: Mid–Late March
Zone 8: Late March – Early April
Zone 7: Early–Mid April
Zone 6: Late April – Early May
Zone 5: Early–Mid May
Zone 4: Mid–Late May
Zone 3: Late May – Early June
Tip: Think of dahlias like tomatoes — if it's warm enough to plant tomatoes, it's warm enough to plant dahlias.
Getting a head start: In zones 3–5, you can start tubers indoors 4–6 weeks before your last frost date. Pot them up in containers with the eye facing up, place in a warm location with indirect light, and transplant outdoors once the soil has warmed and frost danger has passed.
Choosing a Planting Site
Sun: Dahlias need full sun — at least 6–8 hours of direct sunlight daily. Morning sun with some afternoon shade is acceptable in hot climates (zones 8–10), but more sun generally means more blooms.
Soil: Dahlias prefer fertile, well-draining soil with a slightly acidic pH of 6.5–7.0. They will not tolerate soggy conditions — wet soil causes tuber rot.
Heavy clay soil: Amend with compost and coarse sand or perlite to improve drainage, or plant in raised beds.
Sandy soil: Add compost or aged manure to improve water retention and fertility.
Compacted soil: Loosen to a depth of at least 12 inches before planting.
Shelter: Choose a spot protected from strong winds. Tall dahlia varieties can reach 4–6 feet and are prone to snapping in wind and heavy rain.
Preparing the Soil
Good soil is the foundation for healthy dahlias. Before planting:
Loosen soil to a depth of 12 inches
Mix in 2–4 inches of compost or well-rotted manure
Add a handful of bonemeal or a balanced organic fertilizer (5-10-10 or similar) to the planting hole
Avoid high-nitrogen fertilizers after planting — too much nitrogen encourages leafy growth at the expense of blooms. I use an organic 0-10-10 fish emulsion fertilizer when needed.
Planting Your Tubers
Dig a hole 4–6 inches deep and wide enough to accommodate the tuber horizontally
Tip: Shallow planted tubers will sprout faster, but are more prone to falling over once mature. I plant tubers slightly deeper so the plants need less staking.
Position the tuber horizontally with the eye (the small bud or sprout on the neck) facing up
Cover with 2–4 inches of soil — do not bury too deeply if you live in a very wet climate
Do not water at planting unless conditions are very hot and dry. Watering before sprouts emerge increases the risk of rot. Rain is fine.
Mark the spot with a label so you don't accidentally dig into it before sprouts appear and to track the variety
Install stakes at planting time for tall varieties (see Staking section below)
As sprouts emerge and grow, gradually fill in the hole with soil until level with the surrounding ground.
Spacing:
Full-size dahlias: 18–24 inches apart
Border/compact dahlias: 12–15 inches apart
Watering
Before sprouts emerge: Do not water unless the soil is very dry. Tubers sitting in cold, wet soil will likely rot.
After sprouts emerge: Begin watering regularly. Dahlias need about 1–2 inches of water per week, depending on heat and rainfall.
Watering tips:
Water deeply once or twice per week rather than frequent shallow watering
Water at the base of the plant to keep foliage dry (wet leaves encourage disease)
Allow the top inch of soil to dry out between waterings
Increase watering frequency during hot weather and when plants are in full bloom
Water if plants are dropping during extremely hot and dry days
Container-grown dahlias dry out faster and may need daily watering in summer
Signs of underwatering: Wilting, especially in the afternoon heat
Signs of overwatering: Yellowing lower leaves, soft or rotting tubers
Fertilizing
Dahlias are hungry plants and benefit from regular feeding throughout the growing season.
At planting: Mix a low-nitrogen fertilizer (such as 5-10-10) or bonemeal into the planting hole.
During the growing season:
Once plants are 12 inches tall, begin fertilizing every 3–4 weeks
Use a low-nitrogen, high-phosphorus and potassium fertilizer to encourage blooms (look for NPK ratios like 5-10-10, or 0-10-10)
Organic options include compost tea, fish emulsion, or seaweed fertilizer
Stop fertilizing by early September in colder zones (3–6) to allow plants to harden off and tubers to mature before digging.
Avoid: High-nitrogen fertilizers, which produce lush foliage but fewer flowers.
Staking and Support
Most dahlia varieties benefit from staking. Tall varieties (3–6 feet) absolutely require support — heavy blooms and hollow stems make them prone to snapping in wind and rain.
When to stake: Install stakes at planting time to avoid damaging tubers and roots later.
Staking methods:
Single stake: Drive a sturdy 5–6 foot wooden or metal stake 8–10 inches into the ground next to the tuber. Tie stems loosely to the stake as they grow, adding ties every 12 inches.
Tomato cage: Place over the planting site at planting time. Works well for bushier, medium-height varieties.
Corral method: For rows of dahlias, place T-posts every 3–4 feet and run twine horizontally between them at 12-inch intervals as plants grow.
Tip: After years of staked dahlia plants falling over, I now use bailing twine, T-posts, and the corral method in raised beds and in-ground patches.
Pinching for More Blooms
Pinching encourages bushier plants with more flowering stems. When your dahlia plant reaches about 12–16 inches tall and has 3 sets of leaves:
Locate the center growing tip (the topmost growth)
Pinch or snip it out just above the top set of leaves
The plant will respond by sending out multiple side shoots, each of which will produce flowers
Tip: When pinching for cut flowers, I will also pinch the two side blooms that form at the end of each stalk to encourage the one remaining bud to bloom faster.
Note: Pinching delays the first blooms by a couple of weeks but results in a bushier plant with significantly more flowers throughout the season overall.
Deadheading
To keep dahlias blooming continuously from midsummer through frost, remove spent flowers regularly.
How to deadhead:
Identify spent blooms (petals fading, center exposed or going to seed)
Follow the stem down to just above the first set of full-sized leaves
Cut at an angle with clean, sharp snips
Tip: Learn to distinguish buds from spent blooms. Buds are round and firm. Spent flowers are more pointed and feel soft or hollow.
Deadheading redirects the plant's energy from seed production back into making more flowers. I stop deadheading my flowers six weeks before the end of the season (mid-September for Colorado) so the plants start producing seeds.
Cutting Dahlias for Bouquets
Dahlias make exceptional cut flowers. Regular cutting actually encourages more blooms — the more you cut, the more you get.
When to cut:
Harvest in the cool of early morning or evening
Cut when blooms are about 95% open — they will not continue to open much once cut
How to cut:
Use clean, sharp snips or a knife
Disinfect tools between cuts to reduce the spread of disease between plants
Cut stems deep into the plant (at least the length of your forearm), just above a leaf node to encourage deep branching growth
Immediately place stems in cool, clean water
Some growers blanch the ends of dahlia stems in boiling water to lock moisture into the stem
Extending vase life:
Strip leaves that will be below the waterline
Change water every 1–2 days
Keep arrangements out of direct sun and away from heat sources
Dahlia vase life is typically 4–7 days
Common Pests and Problems
** Some of these recommendations are based on general guidance and not my personal experience growing dahlias since Colorado thankfully does not have many of these common pest issues. For regionally specific issues, I recommend contacting your local dahlia society chapter for guidance. **
Slugs and snails: Love young dahlia foliage. Handpick in early morning, use beer traps, or apply an organic slug deterrent around plants. Reapply after rain.
Earwigs: Chew on petals and buds, causing ragged edges and holes. Remove garden debris where they hide. Trap them in rolled-up newspaper.
Aphids: Cluster on new growth and buds, causing distorted leaves. Spray off with water or release beneficial insects like ladybugs.
Spider mites: Cause stippled, yellowing leaves and fine webbing. More common in hot, dry conditions. Spray foliage with water to increase humidity and wash off mites.
Japanese beetles: Skeletonize leaves and damage flowers. Handpick into a bucket of soapy water in the morning when they're sluggish. Protect blooms with organza bags when physical removal isn’t working.
Powdery mildew: White, powdery coating on leaves, usually late in the season. Improve air circulation, avoid overhead watering, and strip each plant’s leaves from the bottom 16 inches of each stalk. Treat with diluted milk and water spray if severe.
Dahlia mosaic virus and other viruses: Cause stunted growth, mottled or yellowing leaves, and distorted blooms. There is no cure — remove and destroy infected plants to prevent spread. Do not compost. Sanitize tools with diluted bleach water between plants.
Crown and leafy gall: Causes lumpy, tumor-like growths or excessive stalks on the tuber base. Destroy affected plants and do not replant dahlias in the same spot the following season. I rotate where I plant my dahlias each season to avoid spreading gall. Some forms of soil solarization or cover cropping will also treat gall-infected soil between seasons, but I am not an expert on this practice.
Disclaimer: I don't use insecticidal soap, neem oil, or diatomaceous earth in my garden, as these products can harm pollinators and other beneficial insects. Instead, I rely on physical barriers like organza bags, Japanese beetle traps, hand-picking pests into soapy water, and companion planting to manage pest pressure while protecting the broader ecosystem.
End of Season Care
When frost arrives: The first light frost will blacken dahlia foliage. This is called the “killing frost” and signals the end of the growing season and that tubers are ready to be lifted (in zones where digging is required).
If you don't get frost: In zones 9–10, dahlias may not experience frost. You can cut plants back to the soil level once they naturally decline in late fall, or leave them to go dormant on their own. If you want to inspect your dahlia tubers for disease or plan on selling or sharing your tubers, you may still want to lift, divide, and store your tubers over the winter even in warmer climates.
Digging and Storing Tubers
Whether you need to dig your tubers depends on your zone:
Zone 3–7: Required — Dig and store tubers indoors for winter
Zone 8: Optional — tubers may survive in ground with heavy mulch, but digging improves survival rates
Zone 9–10: Tubers can generally overwinter in ground; digging still recommended for dividing and inspecting
When to dig:
After the first frost blackens foliage, or
After tubers have been in the ground at least 120 days (in frost-free zones)
How to dig:
Label plants before frost while you can still identify varieties
I use a wax pencil and flagging tape (a permanent marker - even a garden grade one - will wash off)
Cut back stalks to 4–6 inches above ground
Loosen soil in a wide circle (10–12 inches) around the plant with a garden pitchfork
Lift the clump carefully — tubers are fragile and attached by thin necks that break easily
Shake off loose soil and gently rinse the remaining soil off with a hose
Inspect tubers — discard any that are soft, rotten, partially severed, or show signs of disease
Dividing tubers:
Tubers can be divided in fall after curing or in spring before planting. Each division must have:
At least one tuber (the food source)
A portion of the crown/neck
At least one visible eye (the growth point)
Use a clean, sharp knife that is disinfected in diluted bleach water between tubers. Let tubers cure for a few hours in a cool, dry, well-ventilated area out of direct sun (curing time is dependent on climate conditions). I let mine cure for about an hour until dry to the touch.
Storing tubers:
Pack tubers in a breathable material: vermiculite or wood shavings work best
Place in cardboard boxes, paper bags, or plastic crates with ventilation
Store in a humid, cool, dark location: 40–50°F is ideal (garage, basement, or unheated room)
Do not let tubers freeze
Check monthly — remove any tubers showing rot or mold; lightly mist any that appear overly shriveled
In late winter/early spring, move tubers to a warmer spot (over 70°F) to encourage eyes to sprout before planting
Note: Some growers in zones 5-6 successfully overwinter their dahlia tubers in the ground by covering them with a thick layer of mulch and a vapor barrier. I have never tried this method myself, but it works for some gardeners willing to take the risk.
Overwintering in the Ground (Zones 8–10)
If you choose to leave tubers in the ground:
After frost or natural dieback, cut stems to 4–6 inches
Cover the area with 4–6 inches of mulch (straw, leaves, or wood chips) to insulate against cold snaps
Ensure the area has good drainage — tubers will rot in waterlogged soil over winter (you will need to use a vapor barrier over the area for high moisture areas)
In spring, pull back mulch as temperatures warm and watch for new growth
Note: Even in mild climates, lifting tubers every 2–3 years allows you to divide, inspect for disease, and refresh the soil.
Troubleshooting Common Issues
Tubers didn't sprout:
Tuber may have lacked a viable eye
Soil was too cold or too wet, causing rot
Tuber dried out in storage
Lots of foliage but few flowers:
Too much nitrogen fertilizer
Not enough sun
Plants need pinching or deadheading
Plants are tall and floppy:
Need staking
Not enough sun (plants stretching toward light)
Too much nitrogen
Flowers are small:
Plants are stressed (too dry, too hot, or nutrient-deficient)
Overcrowded — thin plants for better air circulation
Consider disbudding for larger blooms
Leaves have holes:
Slugs, earwigs, or Japanese beetles (see Pests section)
White powder on leaves:
Powdery mildew—improve air circulation and treat if needed
Note: Dahlia plants that are used for production cutting will always look more sparse.
Quick Reference: Dahlia Growing Calendar
Early Spring (Zones 8–10):
Plant tubers after last frost
Start fertilizing once plants are 12 inches tall
Mid-Spring (Zones 5–7):
Start tubers indoors (zones 3–5) or plant outdoors once soil reaches 60°F
Install stakes at planting
Late Spring (Zones 3–5):
Plant tubers outdoors after last frost
Pinch plants when 12–16 inches tall
Summer (All Zones):
Water deeply 1–2 times per week
Fertilize every 3–4 weeks with low-nitrogen fertilizer
Deadhead spent blooms regularly
Monitor for pests
Late Summer–Early Fall:
Continue cutting flowers and either deadheading or leaving spent blooms to go to seed
Stop fertilizing by early September (zones 3–6)
Label varieties while still blooming
After First Frost (or Late Fall):
Zones 3–7: Dig, cure, divide, and store tubers
Zone 8: Dig or mulch heavily
Zones 9–10: Cut back and mulch, or dig to divide
Winter:
Check stored tubers monthly
Order new varieties for next season
Questions? Contact littlelapinfarm@gmail.com
