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.
| — | Water-holding capacity | ||
|---|---|---|---|
| Humectant | 30% RH | 60% RH | 90% RH |
| Propylene glycol | 10 | 40 | 270 |
| Ethylene glycol | 13 | 37 | 180 |
| 1,3 butylene glycerol | 10 | 30 | 205 |
| Dipropylene glycol | 6 | 18 | 96 |
| Diethylene glycol | 9 | 23 | 115 |
| Triethylene glycol | 7 | 22 | 100 |
| Glycerol (glycerine) | 12 | 41 | 215 |
| Sorbitol | 2 | 30 | 135 |
| Polyethylene glycol 400 | 6 | 26 | 127 |
| Polyethylene glycol 600 | 5 | 20 | 90 |
| Sodium chloride | 15 | 15 | 950 |
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.
| — | Relative humidity (%) | — | — | — | — | — | — | — | — | — |
|---|---|---|---|---|---|---|---|---|---|---|
| Temperature | 10 | 20 | 30 | 40 | 50 | 60 | 70 | 80 | 90 | 100 |
| 10 | 1 | 2 | 3 | 5 | 6 | 8 | 9 | 10 | 11 | 12 |
| 15 | 2 | 3 | 5 | 7 | 9 | 10 | 12 | 13 | 16 | 17 |
| 20 | 2 | 4 | 7 | 8 | 11 | 13 | 16 | 19 | 21 | 23 |
| 25 | 3 | 7 | 10 | 13 | 16 | 19 | 22 | 26 | 29 | 32 |
| 30 | 4 | 8 | 12 | 17 | 21 | 25 | 29 | 34 | 38 | 42 |
| 35 | 5 | 11 | 17 | 22 | 28 | 34 | 39 | 45 | 50 | 56 |
| 40 | 8 | 15 | 22 | 30 | 37 | 44 | 52 | 59 | 67 | 74 |
| 45 | 10 | 19 | 29 | 38 | 48 | 57 | 67 | 77 | 86 | 96 |
| 50 | 12 | 25 | 37 | 49 | 62 | 74 | 86 | 99 | 111 | 123 |
| 55 | 16 | 32 | 47 | 63 | 79 | 95 | 110 | 126 | 142 | 158 |
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.