Increasing Tuber Growth from Cuttings

Can a simple change in timing under grow lights help dahlia cuttings produce more tubers?

In early spring 2026, we began an experiment to explore that question. 

Our first results are encouraging: plants given shorter light exposure during their final two weeks indoors produced more than twice as many usable tubers as plants kept under longer light exposure.

Growers in many climates and with varied soil types report that dahlias grown from cuttings often produce fewer tubers than those grown from tubers. Several explanations have been proposed, but we wanted to investigate one particular possibility: the amount of light cuttings receive early in their development.

Day length helps guide a dahlia’s growth. Longer days encourage leaf and stem growth, while shorter days trigger flowering and tuber development.

That led us to conduct an experiment in 2026 to see if a period of shorter days could trigger young cuttings to produce more tubers during the growing season.

In February, Stonehouse Dahlias supplied us with 150 virus-free cuttings representing 21 varieties. We potted up each cutting in a 4-inch pot filled with quality potting soil, then divided them into two groups of 75 plants. Both groups included all 21 varieties. All the plants went under grow lights with 14 hours of light per day.

For the final two weeks before field planting:

• The long-day group continued to receive 14 hours of light each day.
• The short-day group received only 10 hours of light each day.

At the end of February, we planted all 150 cuttings alongside one another in our field. From that point forward, both groups received the same care: watering, fertilizer, pest and powdery mildew treatments, flower harvesting, and deadheading. Their sun and shade exposure was also the same. 

At the end of September, after seven months in the field, we cut down the plants and dug up their tuber clumps. Each clump was washed and divided on the day it was dug.

Krustine divided all the clumps myself, using the same standard for both groups. To be saved and counted, each tuber needed:

• A firm body.
• An intact neck.
• A visible eye.

This allowed us to compare the number of viable tubers—not simply the number of swollen roots or blind tubers.

The plants receiving shorter light exposure during their final two weeks indoors produced more than twice as many usable tubers as those receiving 14 hours of light.

To illustrate the difference, for every 3 usable tubers produced by the long-day group, the short-day group produced (on average) 6.6. 

We also noticed differences in tuber size and appearance. Tubers from the short-day plants were generally larger and healthier. However, we did not formally measure or score these differences, so we consider these differences observations rather than measured results.

We’ve put together a PDF with photographs and a more detailed explanation of the experiment and results.  We'd be happy to share it. Just send us an email and ask for it.