If you don't split dahlia tubers your plants will still grow but they will get weaker over time. The clumps become crowded and produce smaller blooms each year. Most dahlias can go two to three years without division before you notice real problems.
I grew the same Cafe au Lait clumps for five years without dividing to see what would happen. Year one and two looked great with dinner plate sized blooms. By year four the flowers shrank by about 30% and the stems grew thin and floppy. Year five brought even smaller blooms that fell over without staking.
The mother tuber that you plant in year one loses strength as seasons pass. Each year it puts energy into making new tubers instead of growing strong roots and stems. The American Dahlia Society says to remove this old mother root when you divide. It makes weaker plants than the younger tubers around it.
When undivided dahlia clumps stay in the ground they create a tangled mass of tubers fighting for the same space. Each tuber tries to grow its own stem and roots from the same small patch of soil. Water and nutrients get spread thin across too many growing points which weakens every stem in the clump.
The stems that emerge from crowded clumps often grow close together and shade each other out. You end up with spindly weak growth reaching for light instead of thick sturdy stems. The flowers suffer too because each bloom draws from a smaller share of the plant's resources.
Storage becomes harder with large undivided clumps as well. A single tuber fits in a small paper bag but a five-year-old clump might fill a whole cardboard box. These giant clumps also rot more often in storage because air cannot reach the tubers in the middle of the mass.
The dahlia division consequences of waiting too long show up in several ways you can spot. Watch for blooms that keep getting smaller year after year. Notice if your stems grow thin and fall over without support. Check whether your clumps produce fewer flowers than they did when young.
In my experience dividing every two to three years keeps dahlias at peak performance. This schedule gives each plant time to bulk up its tuber supply. It also prevents the overcrowding that weakens growth. Mark your calendar so you know which clumps need attention each fall.
Some growers divide every single year to maximize the number of plants they get. This works great if you want to share dahlias with friends or fill a large garden bed fast. But for most home gardeners who just want healthy blooms the two to three year cycle works best.
If you have a clump that has gone many years without division you can still save it. Dig the whole mass and cut it into sections with the crown, a neck, and an eye on each piece. Throw away the old mother tuber and any soft or damaged sections. The remaining healthy pieces will grow with fresh energy next season.
I once rescued a ten-year-old clump from a friend's garden that had never been divided. The center was a mushy mess but the outer tubers still looked firm. I cut away the rot and saved eight good divisions. Three of those grew into my best blooming plants that summer.
Read the full article: Dahlia Tuber Storage: Keep Your Tubers Alive
