Remember to Leave the Lights on for Your Dahlia Plugs

Sunny roseGary Legnani and William B. Miller
Department of Horticulture, Clemson University

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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.

Sunny rose(image: Dahlia 'Sunny Rose' grown under LD (left) and SD (right)) Plug production begins during the short days of winter which signal the young seedlings to produce large tuberous roots. Tuberous root formation during plug productionComparing plants 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.

Plugs after 2 weeks(image: Dahlia 'Sunny Rose' plugs used in this study approximately 2 weeks following sowing)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.

Sunny rose plugsMaterials and Methods

 (image: Dahlia 'Sunny Rose' plugs grown under SD (left) and LD (right), 6 weeks into production)) 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 separated 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.Dry weight of rootsDry weight of dahlia roots


(Figure1: Dry weight of dahlia 'Sunny Rose' roots and shoots under long and short days. Photoperiod treatments begin.) 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 Plant compared (Figure 2). While SD plugs didLeaf areas 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.

Figure 2: Dry weight of fibrous and tuberous roots under long and short days

Figure 3: Leaf areas under long and short days

DiscussionDahlia plugs

(image: Dahlia 'Sunny Rose' plugs grown under LD (left) and SD (right), 2 weeks following transplanting to 4" pots, growingunder natural daylengths.) 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). Comparing plantsThe 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 incandescent. 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 day lengths (between 12 and 13 hours). Dahlia plugsAfter 2 weeks, LD plugs showed a 41% increase in shoot fresh weight, responding much faster to transplanting and increased fetilization than SD plugs. Dahlia rootsThe 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.

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

Images: Dahlia 'Sunny Rose' plugs grown under LD (left) and SD (right). Note increase in tuberous root development under SD and an increase in fiberous root development and shoot development under long days (bottom).

Last Updated 7/16/98