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Dahlia Planting & Care

WHEN

Plant after last frost date. See the USDA Plant Hardiness Zone Map here to find your Growing Zone. 

Zone              Last Frost Date

3                           June 1st

4                          May 15th

5                            May 1st

6                         April 15th

7                           April 1st

8                       March 15th

9                         March 1st

10           Feb 15th or earlier

 

WHERE

 Full sun. For hotter climates, afternoon shade is recommended.

 

SOIL

Dahlias like well drained, slightly acidic soils. Add sand or peat moss if you have heavy soil.

 

PLANTING

Plant the tubers 4-6” deep and about 18” apart.

 

FERTILIZER

 A good heaping of compost is always recommended, but no additional fertilizers are necessary at planting time. At 30 and 60 days after planting, apply a low nitrogen fertilizer with a 5-10-10 ratio, or as close as you can find. Too much nitrogen will promote foliage, but few blooms.

 

STAKING

Planting time is also a good time to add a stake for future support, especially for plants that will grow 3’ or higher.

 

WATERING

***DO NOT WATER WHEN PLANTING UNLESS YOUR SOIL IS BONE DRY*** Tubers are prone to rot if presented with too much moisture. Withhold watering until dahlias have begun sprouting. Once sprouted, a good soaking 2-3 times per week is recommended. Soaker hoses are a great option to get the water to the roots and keep foliage dry.

 

PESTS

Snails, slugs and earwigs all love dahlias. Apply snail and slug bait when planting, and reapply throughout the season. Use insecticidal soap to deter earwigs.

 

PINCHING

Once dahlias reach 15-18”, pinch the center stalk right above the 4th or 5th full set of leaves. This will promote branching, more blooms and a less top-heavy plant.

 

CUTTING

The more you cut, the more they bloom! Cut as long of a stem as you can, even if that means sacrificing the side blooms (which are also beautiful in arrangements!). Drop stems in hot water (160°F – 170°F) and allow water to cool. Blooms treated this will survive 4-7 days in an arrangement. Remember to replace your water daily!

 

DISEASES

Dahlias are susceptible to powdery mildew in wet conditions. Keep foliage dry (again, we love soaker hoses). At first sight of fungus, spray with a fungicide every 7-10 days until under control. Neem Oil is a natural fungicide that works well.

 

DIGGING/OVERWINTERING

Dahlias need a good freeze to go dormant and prep tubers for winter. Wait 1-2 weeks after a killing frost before digging them up. In warmer climates where it does not freeze, mid-November is a good time to dig (withhold watering after Nov 1st). Cut stems to a height of 6”. Use a pitchfork to gently loosen soil around the clump and lift it out of the soil. Tap off excess soil and allow clumps to dry in a cool, dry place for 3-5 days. Clumps can be divided now or in the spring.

 

DIVIDING

Wash tubers of remaining dirt. Remove all broken tubers and hairy roots with snips. Using sharp snips or a knife, begin breaking down the tuber into smaller sections by dividing through the remaining stem (you can trim this down to 1” or so to make it more manageable). Locate an eye near the center stalk, and cut each tuber so that it contains one or more eyes. (Search online for some great videos on dividing tubers). Allow fresh cuts to dry overnight, and store in peat moss in card board boxes (no air-tight containers). Store boxes at 40-55 degrees. Check tubers once per month. If you see signs of dehydration (shriveling), mist your peat moss with some water to deter further shriveling. Remove any tubers that show signs of rot to prevent spreading.