Several years ago, I visited a dahlia farm in Holland and was able to see these festive flowers blooming on a massive scale. The fields of cheery, bright colored dahlias were breathtaking. You can imagine my surprise when I saw that the growers were actually lopping off the blooms, leaving behind stems and foliage. It turns out that this practice makes for better dahlias for our gardens because removing the blooms shifts the plant's energy inward producing big, healthy tubers.
Although the farm I visited was in Holland, dahlias actually hail from Mexico. They love warm weather and abundant sunshine typical of their native land, but they can be grown just about anywhere for summer color. After all Holland and Mexico are diverse climates.
There are so many varieties of dahlias available I don't know where to begin in describing all the choices. They can range in height from dwarfs, only 15 inches tall, to the giants that reach 6 feet or more. Bloom shapes include anemone, ball, cactus, orchid, decorative, collerete, single, waterlily, pompom and fimbrated, which has delicately forked petals. There is almost every color under the sun save blue and flower sizes go from giant dinner plates to miniatures. Whew! That's a lot to choose from!
Plant dahlia tubers after the last frost date in your area and soil has had a chance to warm up. I plant them about the same time that I plant tomatoes. Select a place with full sun. Dahlias aren't fussy about soil, but they don't like "wet feet" so it's important that the area drains well. Amend heavy clay soil with sand or grow your dahlias in containers. Dwarf dahlias are perfect for pots and you can get the soil just right.
Plant the tubers about 6-inches deep and about 24-inches apart. Add some compost and a spoonful of bone meal or soft rock phosphate to the planting hole and place the tuber horizontally with the eye pointing upward. Backfill the hole with soil and water lightly. Once the green shoots emerge from the ground, give the area a good soak.
If you selected a variety that grows over 3 feet tall, set up supports at planting time.
Caring for Dahlias
Give you dahlias a deep soak once a week, more if this weather is especially hot and dry.
When dahlias are about 12 inches tall, feed with a fertilizer that is low in nitrogen. Too much nitrogen will cause plants to produce weak stems and fewer blooms. Apply as directed on the package. Pinching back the stems at this time will make the plant full and compact.
About a month before blooms appear start a routine of feeding with a water-soluble fertilizer high in phosphorous every 7 to 10 days. Continue this until plants die back in autumn.
Dahlias will bloom from late summer until the first frost. Removing faded flowers helps keep the blooms coming.
What to Do With Dahlias in Winter
Dahlias are perennial in regions where winters are mild. Simply cut them back in fall and cover with mulch.
If you live in a part of the country where cold winters and hard frosts are the rule, it's important to lift dahlia tubers from the ground and store them. I suggest digging them with a one-foot diameter root ball, lift soil and all and put them in a cool, dark, dry place. Cover them with dry sand or sawdust until you are ready to replant them in the spring. The way I see it, these flowers are worth any extra trouble you may have to go through to keep them from one year to the next.