Research Aims to Reduce the Time to Flower in Dahlias
February 26, 2026 
For Dahlia ‘Venti Tequila Sunrise’, the time to flower was marginally shorter for those plants exposed to five or 10 short days when evaluated 38 days after treatment. Image: Michigan State University
Greenhouse growers in Michigan and other states report that improperly timed flowering in dahlia (Dahlia × hybrida) is a common problem, either delayed flowering resulting in overgrown plants or early flowering when plants are small.
Dahlias are classified as either day-neutral or facultative short-day (SD) plants. Day-neutral plants do not require a specific day length to initiate flowering, while facultative SD plants flower more quickly when the plant receives short days (less than 13 hours of daylight).
To solve this problem, Roberto Lopez, Ph.D., floriculture specialist at Michigan State University Extension, and his colleagues sought to determine how many SDs are required to induce flowering under non-inductive natural long days (LD) during early to mid-summer. Their objective was to determine the number of SDs required to induce flowering in different dahlia cultivars sold as potted annuals, without reducing flower count or inducing unwanted tubers.
The main question they sought to address: Can we provide enough short days to induce flowering, but few enough to avoid tuber production?
In week 11 (the second week of March), rooted liners of 10 dahlia cultivars were received and placed in a greenhouse with an average daily temperature (ADT) of 70°F, 20-hour photoperiod, and a daily light integral of 15 mol∙m–2∙d–1. Plants were transplanted into 6-inch and trade gallon pots filled with a commercial soilless substrate.
Treatments began during week 14, at which point all plants were moved from their LD conditions to photoinductive 9-hour SDs for five, 10, 15, 20 or 25 days, or remained under continuous SDs (9-hour) or continuous LDs (16-hour) conditions. After their SD treatments, the plants were returned to LDs. SDs were created by opening and closing black cloth at 8 a.m. and 5 p.m., respectively. The time to first open flower (TTF) was assessed for all crops. Plants were grown for an additional four to six weeks after the first open flower. At that point, the total number of spent, open or flower buds showing color was recorded.
Results and Conclusions
For the dahlia cultivars investigated, five to 10 short days was enough to reduce the time to flower by three to 24 days with an increase in flower bud count. Furthermore, providing 15 or more short days promoted tuber formation, reduced flower number and reduced flower quality.
For growers looking to speed up flowering while preventing tuber production in most dahlia cultivars, they should provide five to 10 SDs as an insurance policy.
Learn more here.
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