Many flower species grown as cuts would be more marketable if their stems were longer.  What manipulations could we use to lengthen stems, and not sacrifice convenience of harvesting or yield?  We know from previous work of many, that shading will enhance stem elongation.  Stems also grow longer if plants are crowded, because they can sense the light reflected from neighboring plants.  This reflected light is relatively rich in far-red wavelengths, and triggers a ‘shade avoidance’ response.  The recent development of  red shade netting that alters light quality may allow us to mimic that response.  We also know that plants that are grown in a windstill environment in which plant movement is minimized will be taller than those buffeted by winds.
In the summers of 2004 and 2005,  we grew lisianthus ‘Echo Champagne’, trachelium and rudbeckia ‘Prairie Sun’ in shade canopies, to measure the effect of lower light, altered light quality and reduced plant movement on stem length and productivity. Seedlings were sown in a greenhouse in both years, and transplanted to field plots on a sandy loam soil in the third week of May.  In 2004,  we compared an unshaded control with shade canopies of black saran blocking 30% of the light, and a red netting reducing light by either 30 or 50% (Chromatinet, donated by Signature Supply Co.,  The canopy structures measured 4 feet wide, 15 feet long and 5 feet high, were constructed of metal posts tied together by wire at the top (Fig. 1).  The netting extended over the top, and 3 sides, leaving the north side free for harvesting.  Rows ran east-west.  Lisianthus and trachelium were spaced 9 x 9 inches apart in four-row beds while rudbeckia were planted at a 12 x 12 inch spacing.  All three species were planted under each shade structure, and there were 3 replications of all treatments.
In the 2005 trial we compared a 50% black with the 50% red netting material as covers for the shade canopies, and also tested side curtains of the same materials around 3 sides of each plot, in comparison to an exposed control plot in each of the three replications.  In both years, flowers were harvested when they reached marketable stage, and stem length was measured.  The harvest season lasted from late June to mid-September in 2004, and until late November in 2005.
The results of the 2004 trial indicated that shading could increase stem length by as much as 17% (Table 1).  Heavier shade was more effective than the 30% shade level, but also reduced the number of stems per plant.  All three species reacted in the same way to the shade treatments.  Rudbeckia ‘Prairie Sun’ produced the longest stems and the most stems per plant, with lisianthus ‘Echo Champagne’ the least productive.  The indications that the red shade fabric was more effective than black netting in increasing stem length prompted a direct comparison of those two fabrics in the 2005 experiment.
Table 1.  Effect of shading on stem length and number of stems per plant, for three cut flower species grown in an outdoor experiment in summer, 2004. 

 Data are averaged across Species  
 Shade level Stem lenght, cm Stems per plant
 No shade 51 a 13 b
 30% black 54 ab 10 a
 30% red 56 bc 12 ab
 50% red 59 c 10 a
 Stat. sign. *** *
 Data are averaged across treatments.  
 lisianthus 55 a 3a
 rudbeckia 60 b 19 c
 trachelium 50 a 12 b
 Stat. Sign. *** ***

Means in a column followed by the same letter are not significantly different at the 5% level, using Duncan’s Multiple Range Test.

Again, shading stimulated stem growth, but there was no difference in the effects of red vs. black materials (Table 2, Fig. 2).  Shading the plants under a canopy significantly decreased yield.  The side curtains that provided wind protection produced a more modest stem length increase, but did not decrease yield significantly.  Comparing the three species, rudbeckia was again the most productive, but this time lisianthus grew somewhat longer stems than the other species.  In both years, stem thickness was proportional to stem length, and the longer stems were not more spindly (not shown).

So to answer the question posed in the title: “Yes, but there are tradeoffs to be considered”.  Although shading was effective in increasing stem length, the 32% reduction of yield in 2005 due to the shade canopy, may discourage adoption.  On the other hand, side curtains increased stem length by 14%, but did not reduce yield significantly.  This indicates that avoiding plant movement through installation of windbreaks in cut flower fields would pay dividends.  Alternating rows of tall and short cut flower species might have a similar sheltering effect, and is often seen on commercial cut flower farms.  High tunnels also tend to provide a more windstill environment, and do often foster increased stem length compared to production outside.  More work is needed to determine if red net covering on tunnels could benefit stem length, for our results showed positive effects in one season but not in the next.  Crowding plants together on beds may provide similar benefits, but it remains to be seen if a red covering over closely-spaced plants would stimulate stem growth even more.

Table 2.  Stem length and stem number per plant of three cut flower species when shaded with 50% shade fabric, or provided with side curtains, in an outdoor experiment in 2005.

 Data are averaged across species  
 Treatments Stem length, cmStem number/plant
 Control 37c 11a
 Canopy, black 46a 7c
 Canopy, red 47a 8bc
 Side curtain, black 41b 10ab
 Side curtain, red 43b 10ab
 Data are averaged across treatments.  
 Treats. stat. signif. *** ***
 lisianthus 46a 6c
 rudbeckia 41b 13a
 trachelium 42b 9b
 Species stat. signif. *** ***