You do not need to soak dahlia tubers before planting in most cases. Healthy, firm tubers have plenty of moisture stored inside already. They will sprout just fine when placed in warm soil. Soaking adds moisture that can cause rot, so it does more harm than good for most growers.
I tested this myself with 20 tubers a few years ago. I soaked half of them in lukewarm water for four hours and planted the other half dry. Both groups sprouted within a day of each other. But three of the soaked tubers rotted in the ground within two weeks. None of the dry-planted ones had any problems at all. That test made me stop soaking for good.
Dahlia tubers pull in moisture through their skin. Too much water around a tuber before roots form creates a low-oxygen zone in the soil. Fungal spores love these wet, stale conditions. Without roots to absorb the extra water, the tuber just sits in it and breaks down. Your soil will give the tuber all the moisture it needs once you plant it.
Good dahlia tuber preparation starts with a close look at each tuber. Colorado State Extension says to check for firmness before you plant. Trim any sprouts down to one inch. Look for soft spots, mold, or cracked necks. Set aside any tubers that fail these checks. A firm tuber with a visible eye and solid flesh needs nothing more than warm soil to get going.
When planting dahlia tubers, lay them on their side in a hole 4-6 inches deep with the eye pointing up. Cover with soil and hold off on watering for the first week or two. Let the soil moisture around them do the work. Once you see green shoots break the surface, start watering on a regular schedule. This dry-start method gives roots time to grow before moisture levels rise.
The one time soaking makes sense is when your tubers look dried out and shrunken. These lost too much moisture in storage and need a quick drink. Soak them in lukewarm water for 1-2 hours at most, then plant right away. Do not let them sit in water past that window. The goal is a short sip, not an overnight bath.
Skip the soak for any tuber that feels firm and plump when you squeeze it. Put your dahlia tuber preparation time toward checking for damage and picking a sunny spot instead. Wait for your soil to hit 60°F (15.5°C) before planting dahlia tubers. These steps matter far more than soaking and give your dahlias a strong start for the season ahead.
Read the full article: Dahlia Bulbs: A Grower's Complete Guide
