Remember to Leave the Lights on for Your Dahlia Plugs
Gary Legnani and William B. Miller Department of Horticulture, Clemson University

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Introduction

Dahlias offer considerable value to the bedding plant consumer. Dwarf bedding plant types are propagated by seed and grown in cell packs and small pots. Many growers prefer to purchase established plugs from specialized plug growers rather than growing the plants from seed.

Plug production begins during the short days of winter which signal the young seedlings to produce large tuberous roots. Tuberous root formation during plug production is undesireable as it occurs at the expense of shoot growth as a majority of the carbohydrates produced in the leaves are partitioned to the roots. Futhermore the large tuberous roots may outgrow and literally break apart the small plug trays, making transplanting difficult.

the critical photoperiod for tuberous root formation is between 11 and 12 hours. If the days are shorter than this critical daylength then tuberous root formation will be promoted. A 4 hour night interruption between 10 p.m. and 2 a.m. with 60 watt incandescent bulbs (4 feet above the bench and 6 feet apart) is sufficient to stimulate long days. This should result in decreased tuberous root formation and increased shoot growth. The purpose of this study was to investigate the effectiveness of night interruptions to produce a higher quality plug while reducing production time.

Materials and Methods

Dahlia 'Sunny Rose' seeds were sown in size 200 plug trays in a growth chamber with fluorescent light at approximately 65°F. Germination occurred in 4-5 days and trays were transferred to the greenhouse and placed under photoperiod treatments one week following the sowing date. All plants were covered with black cloth from 5 p.m. to 8 a.m. insuring that both long day and short day plugs received the same amount of PAR or photosynthetically active radiation. Long day plugs were seperated by a black cloth partition and were a night interruption between 10 p.m. and 2 a.m. All plugs were given weekly applications of a 33 ppm A-Rest spray starting in the second week of production. Plugs were harvested 2, 4, and 6 weeks following their movement to the greenhouse. The data obtained included dry weights of shoots and roots (fibrous and tuberous), leaf area, and plug height.

Results

Long day plugs showed a 42% increase in shoot dry weight, a 50% increase in fibrous root dry weight, a 35% increase in leaf area and a 31% increase in height by week 6 (Figure 1). Short day plugs showed a 58% increase in tuberous root dry weight and a 41% increase in overall root dry weight by week 6 (Figure 2). While SD plugs did not significantly increase in leaf area between weeks 4 and 6, LD plugs steadily increased their leaf area and by week 6 had 75% more leaf area than SD plugs (Figure 3). Overall plant dry weight differed by only 8% between LD and SD plugs, evidence that a significant change in carbohydrate partitioning is taking place between LD and SD plants.

Discussion

For dahlia plugs grown on a 6-7 week production schedule (sowing to sale) there appears to be significant advantages to giving plugs a 4 hour night interruption. LD plugs given 5 weeks of photoperiod treatment (week 6 of production) showed greater shoot dry weight, fibrous root dry weight, and leaf area than did SD plugs at week 6 (week 7 of production). The only apparent drawback may be increased plug height, but this controlled by increasing growth regulator concentrations or possibly using a fluorescent light source, rather than incadescent. In a related experiment, dahlia plugs were grown under long and short days for 6 weeks, transplanted into 4 inch pots and allowed to grow under natural daylengths (between 12 and 13 hours). After 2 weeks, LD plugs showed a 41% increase in shoot fresh weight, responding much faster to transplanting and increased fetilization than SD plugs. The superior response of LD plugs is likely due to a combination of increased carbohydrate partitioning toward the shoots and increased nutrient uptake as a result of greater fibrous root development.

Conclusion

Using of a 4 hour night interruption during dahlia plug production will help to produce a superior plug in a shorter amount of time.

Last Updated 7/16/98