How to Grow Dahlias

by Jan Cashman

No flower is showier than a dahlia.  The colors of these showy flowers range from white to rich yellow through pinks and red to purples, and sometimes are bicolored.  They bloom in August here and continue to bloom until the first frost.   Native to Mexico, they were first imported to Europe by Andreas Dahl (hence the name) as a vegetable in the 1700’s.  Interest in growing them as an ornamental flower began soon after, and, in Belgium, large, double flowers were bred as early as 1815.  Today there are a range of sizes and colors unmatched in other garden flowers.

Dahlias are classified according to the size and form of their flowerheads.  Flowers range in size from one inch to over 10 inches.  The large-flowered cactus and dinnerplate dahlias have become popular today.  Some of the dwarf bedding dahlias are grown from seed, but most dahlias are grown from tubers.  Plants range in height from 12” to over 5 feet!

Growing Tips:

Dahlias won’t survive frost.  So the plant has time to produce flowers in our short summer season, start the tubers indoors in April and plant outside after the soil has warmed up and the danger of frost has passed.  Or grow them in a cold frame to protect them.  Choose a sunny site out of the wind when you are ready to plant them outside.  Dahlias are not fussy about soils as long as the soil is warm and not water-logged, in fact, they prefer slightly alkaline soils.   If you have heavy clay soil that retains lots of water, work in organic matter to help lighten it.  Dahlias are heavy feeders, so dig in a slow release fertilizer before planting.

Space the large flowering varieties at least 18” apart so their roots don’t intertwine.  If they are crowded too close together, the plants will reach for the light and become tall and spindly.  Because of their shallow root systems, most dahlias will need to be staked or they will blow over.  Pound a tall stake next to each plant and tie them to the stake when they get to be a foot tall.  Continue to tie them to the stake every 18”.  Or you can use tomato cages or other commercial plant supports.

Expert dahlia growers pinch back the central shoot to encourage the development of side shoots.  Later, extra side shoots can also be pinched off so you get fewer, but larger, flowers.  Slugs love dahlias, so it’s a good idea to spread slug bait around the plants before they emerge.

In the fall, after the first frost and when the foliage is withered, trim back the stems of your dahlias to about 6”.  Lift the tubers. Do not let the tubers freeze, but dry them and store them in perlite, moss, or soil pep in a cool place.

Dahlias will grow well in a container, using a soilless growing mix.  When planting them, place the tubers horizontally (sprouts up), 5 to 6 inches deep in the container, as you would in the ground.   Barely cover the tuber with soil and backfill as it grows.  The pot does not have to be deep because dahlias’ roots stay near the surface, but make sure the pot has plenty of drainage holes.  Keep your potted dahlia inside in a sunny spot until the danger of frost is past.  You’ll need to stake your dahlia whether it is left in the container or transplanted into the ground.

There are a number of gardeners in Bozeman growing dahlias, so we know it can be done here.  One local gardener says she uses chicken compost on her dahlias and they are tall and beautiful.  Missoula and Kalispell both have dahlia societies that have shows in August and September. Try growing dahlias yourself, either in a container or in the ground.  You might grow some nice enough to enter!