Optimizing Glycerin-to-water Ratios for Preserving Eucalyptus Foliage

Abstract

Fresh eucalyptus stems can be preserved via systemic absorption of glycerin, but the optimal rate of glycerin and its effect on foliage color and stem pliability during holding have not been well-described. We harvested stems of Eucalyptus pulverulenta ‘Baby Blue’ and treated them in solutions of 0% (control), 25%, 50%, and 75% glycerin:reverse-osmosis water for 2 weeks. Leaf greenness was measured nine times during this period; the change in stem weights and solution uptake were measured as well. Stem pliability was evaluated after 2 weeks in solution as well as 1 week after holding the stems dry. The 25% and 50% glycerin treatments increased stem weight, whereas the control and 75% treatments decreased stem weight. The glycerin treatments absorbed less than 50 mL solution per three stems during 2 weeks, whereas those in the control absorbed >270 mL water. The color of foliage was similar during the first 5 days in treatment solutions, but stems in 25% and 50% glycerin treatments were less green than those in the control and 75% treatments by day 6, and this trend continued. No difference occurred in stem pliability among treatments after 2 weeks in glycerin solutions; however, after 1 week of holding, the top 9 cm of stems in glycerin treatments were more pliable than those in the control. Based on these results, we recommend a rate of 25% glycerin to preserve eucalyptus foliage for use in botanical artistry.

Abstract

Fresh eucalyptus stems can be preserved via systemic absorption of glycerin, but the optimal rate of glycerin and its effect on foliage color and stem pliability during holding have not been well-described. We harvested stems of Eucalyptus pulverulenta ‘Baby Blue’ and treated them in solutions of 0% (control), 25%, 50%, and 75% glycerin:reverse-osmosis water for 2 weeks. Leaf greenness was measured nine times during this period; the change in stem weights and solution uptake were measured as well. Stem pliability was evaluated after 2 weeks in solution as well as 1 week after holding the stems dry. The 25% and 50% glycerin treatments increased stem weight, whereas the control and 75% treatments decreased stem weight. The glycerin treatments absorbed less than 50 mL solution per three stems during 2 weeks, whereas those in the control absorbed >270 mL water. The color of foliage was similar during the first 5 days in treatment solutions, but stems in 25% and 50% glycerin treatments were less green than those in the control and 75% treatments by day 6, and this trend continued. No difference occurred in stem pliability among treatments after 2 weeks in glycerin solutions; however, after 1 week of holding, the top 9 cm of stems in glycerin treatments were more pliable than those in the control. Based on these results, we recommend a rate of 25% glycerin to preserve eucalyptus foliage for use in botanical artistry.

The preservation of foliage extends its ornamental value by preventing decay. Early plant preservation included the replacement of the internal moisture of fresh flowers with glycerin (Hartshorne 1865), a sugar alcohol used in food and skin products. Today, using glycerin to preserve foliage via systemic absorption is common (Bale 2006; Condon 1982), but the limited available information about preservation methods does not define rates of use, or the methods are difficult for florists and growers to implement. For example, Stevens (1998) recommends determining the amount of glycerin to add based on the weight of the plant material to be treated. In addition, glycerin-treated plant material does not retain its natural color (Stevens 2000). The objective of this study was to determine an optimal glycerin:water ratio for systemic absorption to preserve eucalyptus foliage by characterizing the effects of glycerin on foliage color and stem pliability.

Materials and methods

Plant material

Fresh-cut Eucalyptus pulverulenta ‘Baby Blue’ was started from seed (Johnnys Selected Seeds, Albion, ME, USA) and grown in a glass greenhouse range in Manhattan, KS, USA, for 17 months in a cut flower bed of soilless substrate (Berger BM6; Berger, Saint-Modeste, QC, Canada). Stems with a length of 46 cm were harvested from woody eucalyptus stock plants on the day of setting in treatment solutions.

Treatments and experimental design

The experiment was a completely random design with four preservation solution treatments × five replicates of each. Treatments were water only (control), 1 part (25%), 2 parts (50%), or 3 parts (75%) glycerin solution to 3 parts, 2 parts, or 1 parts reverse-osmosis water, respectively. The glycerin source was 99.7% vegetable food-grade glycerin (CLI Syntec, St. Charles, MO, USA). We prepared 400 mL of each glycerin solution and added 2.6 g/400 mL citric acid (Mrs. Wages, Kent Precision Foods, Muscatine, IA, USA) to modify the solution pH (Stevens 1998). Three 46-cm stems were placed in each of 20 glass vases (1.5 L) after measuring fresh weight. Vases were kept in an interior classroom with 24-h lighting at a temperature of 72 °F (22 °C). Cling wrap was placed over the top of each vase to reduce evaporative water loss.

Data collected

Fresh weights of eucalyptus stems were measured just before and 14 d after treatment (DAT). Foliage greenness was measured with a soil plant analysis development (SPAD) meter (Konica Minolta, Ramsey, NJ, USA) on 1, 3, 4, 6, 8, 10, 12, and 14 DAT by averaging one reading for each of the three stems in an experimental unit; measurements of a leaf that was approximately seven leaves below the stem tips were collected. The glycerin solution volume in each vase was 400 mL at 0 DAT, and it was measured again at 14 DAT to determine solution uptake. Solutions were added to the vases as needed and the amount added was tracked.

Stem pliability, which is necessary for florists to manipulate stems to create floral designs, was evaluated 18 cm from the tip at 14 DAT by bending each treated stem at a 90° angle on a table top. This was repeated at 9 cm and 20 cm from the tip after holding stems dry in vases without solution for 7 d. Pliability ratings were recorded as follows: 0, no break; 1, a break through the epidermis only; and 2, a break through the entire stem.

Data were analyzed with one-way analysis of variance using JMP Pro 16 (SAS Institute, Cary, NC, USA), and means were separated using Tukey’s honestly significant difference mean separation procedure.

Results and discussion

Glycerin solution uptake

After treatment, the stem weight in 25% and 50% solutions increased similarly, but it decreased in 0% and 75% solutions (Table 1). Similar amounts of solution were absorbed by stems in 25%, 50%, and 75% treatments. Although not statistically different, the data trend suggested that as glycerin increased, the solution uptake decreased. Glycerin treatments took up less than 50 mL solution during the 2-week period, whereas the control absorbed >270 mL water, which was more than five-times the amount of solution as that absorbed by the stems in 25% glycerin solution (Table 1).

Leaf greenness

By day 6 in glycerin solutions, the 25% and 50% glycerin treatments were less green compared with the other treatments. This trend continued and the color difference between treatments increased during 7 to 14 DAT in treatment solutions (Fig. 1). The data suggested that glycerin absorption is correlated with a decline in green color; therefore, dyes are often necessary to produce a desirable preserved product. If excessive glycerin is absorbed, then the product may form droplets of glycerin on the foliage surface or from the cut end (bleed) during storage or use (Stevens 1998).

Stem pliability

After 14 DAT, stem pliability was similar across treatments (Table 1). However, after dry holding for 1 week, pliability was similar for all glycerin treatments and better than that of the control at 9 cm from stem tips. At 20 cm from the stem tips, increased glycerin in solution resulted in increased stem pliability, with 50% and 75% glycerin resulting in the least stem breakage (Table 1). DelPrince (2020) reported that the optimal rate was 50% glycerin and 50% water; however, a 30% glycerin solution often provided adequate results.

Based on these results, we recommend a rate of 25% glycerin to preserve eucalyptus foliage for use in botanical artistry. Results of both 25% and 50% glycerin solutions were similar across most parameters measured, but 25% can be used to reduce the cost of glycerin solutions while gaining the preservation benefits of pliability. The 75% glycerin solution created oily, slick stems caused by glycerin bleeding. All glycerin treatments resulted in foliage color loss that could be covered by opaque spray paints or systemic dyes.

References

Bale S. 2006. Preserving flowers and foliage. Univ Kentucky Cooperative Extension Bull HO-70. https://publications.ca.uky.edu/sites/publications.ca.uky.edu/files/ho70.pdf.

Condon G. 1982. The complete book of flower preservation. Pruett, Boulder, CO, USA.

DelPrince JM. 2020. Glycerin-preserved foliage. Miss State Agric Exp Stn Res Bull P3039. https://extension.msstate.edu/sites/default/files/publications/publications/P3039%20web%20rev2023.pdf.

Hartshorne H. 1865. A monograph on glycerin. JB Lippincott, Philadelphia, PA, USA.

Stevens A. 1998. Field grown cut flowers: a practical guide & sourcebook. Avatar’s World, Edgerton, WI, USA.

Stevens A. 2000. Preserving flowers and decorative foliages with glycerin and dye. Kansas State Univ Ag Experiment Station and Cooperative Extension Service MF-2446. https://bookstore.ksre.ksu.edu/pubs/preserving-flowers-and-decorative-foliages-with-glycerin-and-dye_MF2446.pdf.