When Is the Right Time to Dig Dahlias? Beyond the 120-Day Rule

For decades, dahlia growers and collectors have followed an old rule: “Wait for the first frost, then wait two more weeks before digging up your dahlias.”
Another common saying is the so-called “120-day rule” — if a dahlia has been in the ground for more than 120 days, its tubers are mature and ready to be lifted.

Like much traditional advice, this is only partly true. Dahlias don’t follow the calendar — they follow natural signals that tell them the season is coming to an end.

Beyond the 120-Day Rule

The “120-day rule” assumes that dahlia tubers mature on a fixed timeline, but research and grower experience show that environmental conditions play a much greater role.

If your dahlias experience short days and cool nights, tuber formation happens faster.
If days remain long and nights are warm, the process can be delayed or even inhibited — regardless of how long the plants have been growing.

So instead of relying on the calendar, watch for nature’s cues:

• Days become shorter
• Nights turn cooler
• Leaves begin to yellow or brown — a sign the plant is entering dormancy

These signals tell you that the tubers are maturing and getting ready for lifting and storage.
If your soil freezes in winter, be sure to dig your dahlias before deep frost reaches them.

When Tuber Formation Actually Begins

Dahlias begin forming tubers late in the season, and the trigger is day length, not cold.
When the day shortens to around 12 hours, the plant’s hormones shift: flowering slows, and the plant starts directing carbohydrates from its leaves into its roots.
That’s when true tuber development begins.

From this point, dahlias need about six weeks of active growth in the soil to fully mature — to accumulate starch, strengthen cell walls, and develop their inner structure.

Mild nights around +10 °C (50 °F) allow the plant to continue channeling sugars from leaves into the tubers, even while the flowers are still blooming.
As night temperatures drop toward +7 °C (45 °F), this movement slows — the plant senses the season’s end and begins preparing for rest.
When temperatures fall below roughly +7 °C, growth stops, and a two-week curing period begins. During this time, moisture levels stabilize and the outer skin of the tuber toughens for winter storage.

Why Those Two Weeks Matter

The traditional advice to “wait two weeks after frost” isn’t about finishing tuber growth — it’s about stabilizing moisture.
When frost kills the top growth, the plant stops drawing water upward. Moisture begins to redistribute within the tubers and slowly evaporates from the outer layers.

Over about 10–14 days, cells in the outer tissue become firmer and drier, forming what growers call the “winter skin” — a thin, corky layer that protects the tubers from drying out or rotting in storage.

If you dig tubers immediately after frost, their skin is still soft and full of water — easily damaged, and they store poorly.
Give them those extra two weeks in the ground, and they will become firm, leathery, and resilient.

If Frost Never Comes

In milder or coastal regions, frost may never arrive at all. You can mimic the natural process by creating an artificial pause:

• Cut off all blooms and buds to stop flowering.
• Stop watering for about two weeks.
• Let the plants remain in the ground another 10–14 days before digging.

This replicates what nature does with frost — halting growth and allowing moisture to balance inside the tubers.

The True Tuber Cycle of a Dahlia

• Photoperiod signal: Day length drops below 12 hours → tuber formation begins
• Growth phase: About 6 weeks of underground starch accumulation and structural strengthening
• Curing phase: Another 2 weeks after growth stops → moisture balancing and skin hardening

So, the old “frost + two weeks” rule is really just a simplified version of a deeper truth:

Dahlia tubers need six weeks of growth and two weeks of rest.

This full cycle takes around four to five months — the time it takes for a dahlia to grow from seedling to storage-ready maturity.
That’s the natural rhythm of a dahlia’s life: first creation, then rest.

In Conclusion

By learning to read nature’s rhythm, you’ll know exactly when your dahlias are ready.
No chart or rule can tell you more accurately than the plants themselves.
Each season, each garden, and each variety behaves a little differently — so let your dahlias show you when it’s time for rest.