How To Grow Dahlias

Basic Needs

  • Sun: Minimum 6 hours/day
  • Water: 1 inch/week
  • Staking: if 30 inches tall or more

Starting Indoors

For late blooming varieties, earlier blooms on others and delayed spring weather.

  • in greenhouse, sun room, sunny window or under lights
  • flower pots or seedling trays
  • slightly damp potting soil
  • water lightly, tubers rot easily
  • heat (70F pref ) to promote growth

Outdoor Preparations

  • Prefer rich, sandy, well drained soil. Will tolerate other soils that are not too wet.
  • Use raised beds in wet or poorly drained areas
  • Remove all weeds and roots
  • Add manure, compost, decomposed leaves, etc.
  • If using fertilizer, use low nitrogen for vegetables or flowers
  • Soil ph, prefer between 6.5 and 7

Outdoor Planting

  • Plant when the lilacs bloom and the corn is planted
  • 2 feet apart unless a dwarf variety, 2.5 feet for very tall varieties, 1.5 for dwarfs
  • In holes 1 spade depth deep
  • Put a stake, of suitable height for the grown plant, in the hole
  • 1 tsp 6-8-6 in the bottom of the hole, cover with soil
  • Tuber on its side. Eye/shoot close to the stake about 6 inches below the surface
  • If it has a shoot, leave the top exposed and fill the hole in as it grows
  • Very little water after planting, until growing well. Tubers rot easily.
  • Protect against cut worms and slugs by placing a toilet paper roll, partially buried in the ground, around the shoot

Growing Your Dahlias

  • 1 inch of water a week from rain or by deep root soaking with irrigation or soaker hose.
  • Tie to stake every 12 inches or so
  • Prefer cool roots, mulch late in June after earwigs have nested
  • If using fertilizer, 1 heaping tsp of super triple phosphate 0-46-0 per plant every three weeks from when the plants are 12 inches tall until late August. Water in well. This gives strong roots and stems and lots of flower
  • Late July add 1 Tablespoon of Muriate of Potash 0-0-50 every 3 weeks until the end of September. Potash builds big, strong tubers.


  • Stops the upward growth of the main stem of the plant
  • Creates a shorter, bushier plant
  • Allows for more flower development
  • Done when the plant has 4 sets/pairs of leaves
  • Part leaves and snap off the centre growing stalk just above the 4th pair of leaves. You could use a knife or scissors if you wish


  • Creates longer stems on flowers, better for cut flowers
  • More energy per bloom so bigger blooms especially in the large and giant varieties
  • Dahlias grow buds in groups of two or three
  • When the buds are small but can be individually seen, pinch out the smaller side bud(s)
  • For long stems you may need to take off one or two sets of leaf side shoots. Depends on the variety.

Pests and Diseases

  • Strong, healthy plants can thrive in spite of some insect damage
  • Soap and water sprays control most problem insects
  • Clean cut broken stems to reduce the chance of bacterial infections
  • Cut off rotten stems well below the infection so it doesn=t spread to the tubers

Dahlia Virus

  • Stunted, twisted plants, pale curled leaves- probably dahlia virus
  • Dig and destroy, preferably burn. Do NOT compost
  • Sterilize tools used.
  • Spread by nematodes and sucking insects
  • Some plants may be carriers

Dig up

  • After killing frost or 1st of October
  • Tubers rot in cold, damp soil, even if not frosted on top
  • Cut off top, 6 inches above ground
  • Leave for a few days up to 1 week to allow eyes to set
  • Tie label on stem
  • Dig with a long-bladed spade 9 - 12 inches from the stem. Must be a complete circle with overlapping cuts to sever long roots
  • Using the spade, lift the tuber clump
  • Clean the worst of the earth off
  • Wash clump and leave on its side to drain and dry for 24 hours, protect from frost.
  • Split clump removing broken, rotten, diseased, old and eyeless tubers
  • Leave to dry and scab over cuts for 24 hours
  • Place in storage medium (vermiculite, shavings, peat, etc) and containers.
  • Store cool (about 5C, 35 - 45F) and dark