Pre-Sprouting Dahlia Tubers: A Science-Based Guide for Early Blooms

Copyright © 2025 by Steve K. Lloyd

As I have discussed in other articles and social media posts, for many years I have pre-sprouted my dahlia tubers indoors 4–6 weeks before planting out, a process akin to “forcing” bulbs of other plant species.

When the garden soil has warmed to 60°F and all chances of frost have passed (typically mid to late May in my PNW Zone 8b growing zone), I transplant potted dahlias that are typically 12–18 inches tall and already growing strongly.

This approach gives me dahlia blooms in late June, and virtually all of my hundreds of dahlias are blooming vigorously by late July.

Potted, pre-sprouted dahlia tubers hardening off outdoors before planting.

I was surprised to find a scientific article that suggests some dahlia cultivars actually perform better when tubers are sown directly in the ground, rather than being forced in greenhouse conditions.

The article, titled "Influence of Cultivar and Planting Material on Soluble Dry Matter Content of Dahlia Tuberous Roots," explores the effects of different planting materials (forced vs. unforced tuberous roots) and dahlia cultivars on the accumulation of Soluble Dry Matter (SDM) in dahlia tubers. 

Conducted over two years at the University of Agricultural Sciences and Veterinary Medicine in Cluj-Napoca, Romania, the study specifically examines seven 'cactus' type dahlia cultivars.

Soluble Dry Matter (SDM) represents the portion of a tuber's dry matter that is soluble in water, primarily composed of sugars, starches, and water-soluble carbohydrates. In dahlias, a significant part of SDM is inulin, a carbohydrate valued in the food and medical industries for its properties.

The study measures SDM using a refractometer, which gauges the concentration of soluble solids in a liquid based on refractive index. This method provides an estimate of inulin content and other soluble carbohydrates in the tubers.

SDM serves as an energy reserve that supports initial shoot development and overall plant vigor during early vegetative growth, making it a crucial factor in the plant’s ability to develop robustly.

The study involved planting both forced and unforced tuberous roots of each cultivar in the field using a randomized block design with three replications.

• Forced tubers were started in a greenhouse on March 19th, then transplanted into the field on May 12th.
  
  

• Unforced tubers were directly planted into the field on the same date.

After harvest in the autumn, SDM levels were measured, and statistical analyses, including regression models, were applied to understand the relationship between SDM and plant characteristics like tuber weight and shoot number.

• Depletion of Reserves: Forcing (starting tubers earlier in a greenhouse) can influence SDM content at harvest. It depletes some of the stored energy reserves during the early growth phase, potentially reducing SDM content compared to unforced tubers.
  
  

• Cultivar Variation: The effect varies by cultivar. Some cultivars benefit from this early start with better SDM accumulation, while others retain more SDM when allowed to remain dormant until planting.

• Higher SDM with Forcing:

  • ‘Star Favourite’ (25.47% SDM)

  • ‘Hayley Jane’ (23.97% SDM)
    
    

• Higher SDM without Forcing:

  • ‘TsukiYori No Sisha’ (24.80% SDM)

  • ‘Kennemerland’ (24.27% SDM)

• Tuber Weight: A positive correlation was found between tuber weight and SDM in certain forced cultivars, suggesting that larger tubers might store more reserve substances like inulin.
  
  

• Shoot Number: No significant correlation was found between the number of shoots per plant and SDM accumulation, implying that the number of shoots is not a primary determinant of SDM levels.

Like most experienced dahlia growers, I have long recognized that tubers from which multiple cuttings have been taken seem to get “tired”, making them poor candidates for planting in the garden after their efforts on the cutting bench have finished.

Scientific research backs this. Unforced tubers, which do not use up energy reserves during early growth, have a higher initial SDM content at the time of planting. This provides a significant reserve that supports growth once planted in the field.

Forced tubers can experience transplant shock when moved from a greenhouse to field conditions, potentially affecting growth and SDM levels. Unforced tubers, by avoiding this stress, may develop more steadily.

I have observed that in my own growing, and have learned to transition my pre-sprouted, potted tubers gradually to the conditions they will face in the garden. In the photo below, dozens of my potted dahlias grown from forced tubers are experiencing their first days under natural sunlight.

Dahlia tubers were sprouted indoors before moving outdoors in early spring.

Cultivars that naturally develop strong vegetative growth without needing an early start may perform better unforced. Those that seem to struggle with transplanting should also be left unforced to minimize stress. There is no "data source" where grower's can find this information, so let experience and good record-keeping will guide you here.

Varieties with resilience to changes in conditions, such as those with larger, denser tubers that can sustain early growth, may benefit more from forcing. I have had dahlia growers tell me that in their experience, large tubers take longer to wake up than smaller tubers, which seem to "just want to grow."

Gardeners can experiment with forcing and unforced methods on a small scale, tracking results in terms of bloom timing, plant vigor, and tuber quality to better understand how each cultivar responds. Even without a greenhouse, home growers can start tubers indoors a few weeks before planting time, with no lights or special equipment needed.

The study concludes that the choice between using forced or unforced tuberous roots is highly dependent on the specific dahlia cultivar and the desired outcome, whether for ornamental use or for optimizing inulin production.

The research challenges the assumption that an earlier start always leads to better growth, emphasizing that energy preservation and the efficiency of nutrient storage play crucial roles in determining the suitability of each method for different cultivars.

Based in part on this study, this spring I will pre-sprout most of my tubers as usual, since it has worked so well for me. But I will also plant tubers directly in the garden for a number of the same cultivars, so I can compare the results of the two approaches throughout the growing season.

Cantor, Maria, et al. "Influence of Cultivar and Planting Material on Soluble Dry Matter Content of Dahlia Tuberous Roots." Bulletin of the University of Agricultural Sciences & Veterinary Medicine Cluj-Napoca. Horticulture 73.2 (2016).

This article was written with the help of AI (ChatGPT) to organize research and summarize key findings from scientific sources. While I carefully reviewed the content to ensure that all statements are backed by specific research papers, I rely on AI’s interpretation of the technical material. This is a collaborative effort—AI helps with analysis and structure, while I make sure everything aligns with the intended discussion.