Preserving plant foliage with glycerine

Glycerine (glycerol) is the most widely used substance to help preserve plant material although many chemicals possess hygroscopic properties.

Cellulose, the major constituent of the plant 'skeleton', is brittle when dry. Adding a hygroscopic chemical (humectant) prevents cellulose from drying out completely, which keeps the plant material supple.

About half of most plant fresh weight is water but brittleness is usually only a problem if the water content falls below 10%.

Suitable chemicals

Hygroscopic chemicals suitable for use as humectants for ornamental foliage can be grouped into four classes:

• polyols (glycerol, glycols)
• sugars (sorbitol, sucrose)
• salts (calcium chloride CaCl₂, sodium chloride NaCl)
• quarternary ammonium compounds.

Glycerol is the most efficient humectant because it has the greatest water-attracting capacity (Table 1). Diethylene glycol and the polyethylene glycols have been used as humectants for ornamental foliage. However, their lower water-holding capacity means that a greater quantity is needed for a given degree of suppleness.

Knowing the humidity (easily measured by wet bulb temperature depression) and temperature, a quick calculation will reveal whether glycerining under such conditions is possible (that is, the VPD is positive). If not, the humectant solution can be heated to increase the VPD.

Experiments have shown that an acceptable rate of glycerining occurs if the VPD is greater than 10 millibars (see working example below).

Working example

A preserving solution will only be taken up by plant foliage if the correct environmental conditions are provided. The vapour pressure deficit (VPD) must be positive and should be as high as possible (preferably greater than 10 millibars) for rapid preserving. VPD can be established from Table 2 if ambient temperature and relative humidity are measured. Relative humidity may be measured directly with an inexpensive humidity probe or calculated, using a wet bulb or dry bulb thermometer and the appropriate tables.

Taking a hypothetical example, assume that atmospheric conditions are 20°C and 80% RH. The VPD is calculated by subtracting the value of the vapour pressure at 20°C and 80% RH obtained from Table 2 (19 millibars) from the vapour pressure at 20°C and 100% RH (23 millibars).

That is VPD = vapour pressure at 100% RH minus vapour pressure at atmospheric RH = 23 millibars -19 millibars = 4 millibars.

In the example, the calculated VPD is small (4 millibars) and glycerining will be slow. It can be speeded up by heating. Solution temperature should not be increased beyond about 40°C as there is a risk of killing the foliage.

Other factors

Water quality has been found to have no deleterious effect on the uptake of humectants (except possibly where calcium levels are extremely high).

In fact, salts such as sodium chloride (NaCl) and potassium chloride (KCl) enhance uptake when added to the humectant solution at between 5 and 10g/L. Similarly, a solution pH in the range 2 to 8 does not affect uptake.

Sweating

The amount of water that a humectant will hold depends on the atmospheric humidity. At very high humidities (over 80%), small variation in humidity will greatly affect the water-holding capacity of the humectant.

'Sweating' occurs when a sudden drop in humidity from a high level results in a drastic reduction in the water-holding capacity of the humectant, which then releases free water. As this water cannot evaporate quickly enough, it collects as droplets on the surface of the plant material where it provides an ideal environment for bacterial or fungal growth.

Sweating foliage should be hung in an area with plenty of air circulation to allow rapid evaporation of this surface water.

Sweating can be minimised by:

• keeping the level of humectant in the plant tissue as low as practicable
• storing glycerined foliage at low humidity
• avoiding exposure of glycerined foliage to conditions where rapid decrease in humidity will occur (that is, do not open storage/drying sheds on a fine day immediately after a week of rain).

Glycerined foliage destined for humid climates can be sealed in plastic before shipping, to prevent excessive mois­ture absorption in transit.

Dyes

Dyes may be added to the humectant solution. Acid dyes (that is food dyes) generally produce good results when added to the uptake solution. Basic dyes can be used in the immersion method. Most humectants, especially glycerol, cause foliage to brown. The darker dyes, green, red, blue and black, will easily mask this discolouration. It is often beneficial to sun bleach uptake-dyed foliage for about a week after glycerining to bring out the full intensity of the dye.

Pastel colours require the foliage to be bleached before dyeing and glycerining, to remove all background colour. It is critical that all traces of bleach be removed from the foliage before attempting to dye. Either sulphuring or dipping the foliage in sodium metabisulfite, prior to immersion glycerining and dyeing will ensure that residual bleach is removed.

Hygiene

Glycerining solutions provide an excellent environment for the rapid growth of bacteria and fungi. These micro-­organisms may build up to such levels that they block stems and prevent solution uptake.

Buckets used for glycerining should be kept clean. Residue should be scrubbed out and the buckets preferably sterilized with 1% bleach (sodium hypochlorite) between uses. The high cost of glycerol means that the solution must be re-used several times. Thus, a biocide should be included in the solution to suppress pathogens.

Benzalkonium chloride (for example Alginox®) at a concentration of 0.2g/L is an effective biocide. Copper sulphate (0.2g/L) may also be used unless it is planned to bleach the foliage at a later date. Copper catalyses the decomposition of bleach and should be excluded from bleaching solutions.

Chlorine-releasing compounds such as sodium dichloro-isocyanurate (DICA) at 0.2g/L are excellent biocides but must be replenished regularly because they break down within a few weeks. Organic fungicides should not be added to the glycerining solution because they are not water soluble and may clog stems.

Acid dyes are incompatible with most of the above biocides. Hydroxy quinoline sulfate (HQS) is one of the few biocides which can be included with acid dyes. HQS is available as a fresh flower biocide (for example Chinosol®) and should be used at 0.2g/L of active ingredient.

Economics

Sorbitol is the cheapest of the readily available liquid humectants. Glycerol is the next most expensive, then the glycols.

Prices vary widely depending on purity and quantity. Some quaternary ammonium salts show promise as humectants. However, data on their water-holding capacity is not available.

Storage and transport

Cardboard cartons are used for storing glycerined foliage during transit. Wax-coated or plastic-lined cartons can be used for shipment to humid climates to prevent contamination by atmospheric moisture.

Humectant quantity

The quantity of humectant needed for a supple product is dependent upon both the nature of the plant foliage (is it naturally brittle?) and the humidity of the intended market. As it is the water that the humectant 'binds' which provides much of the plasticising effect on the foliage, rela­tively less humectant can be used for a market where the humidity is high.

It is convenient to consider final market humidities as either dry (about 30% RH), intermediate (about 60% RH) or humid (about 90% RH). Humidity ranges for major export markets are listed in Table 3.

Glycerol application rates for some commonly preserved foliage lines have been determined experimentally. Table 4 refers to the number of grams of glycerol required to retain suppleness in 1 kilogram of fresh foliage.

Note that the lower value of the range suggested is applicable to a humid market and the higher value would be required for suppleness in a dry environment.

Some broadleaf eucalypts and native bamboo have been found to require high levels of glycerol to prevent leaf curl. Export of such products to humid markets requires special care because the product is especially prone to 'sweating'.

The data presented in Table 4 should be considered as a starting guide only. It may be necessary to vary the quantity of humectant in order to arrive at the optimum preserved product. It is prudent to err on the side of less glycerol (where leaf distortion is not a factor), because the danger of 'sweating' will be reduced.