Essential Hemp Oil / 4. Discussion

4. Discussion
Essential hemp oil is thought to be synthesized at the same location as the cannabinoids, that is in the epidermal glands or glandular hairs (Malingré et al., 1975). The highest density of glandular hairs is found on bracts of the female flower and surrounding leaves (Hemphill et al., 1980, Pate 1994). Thus, maximizing the essential oil yield can be done by maximizing the amount of female flowers per area and increasing the essential oil (or glandular hair) content per flower.

Strain
Using monoecious varieties, it was possible to increase the bud yields (Fig. 5). However, the essential oil yield of dioecious varieties was higher or equal to monoecious varieties (Fig. 2). Differences between scent scores of essential oil of different culticars were reported (Mediavilla and Steinemann 1997). The big variability of Cannabis sativa L. may be used to discover new odours which might be of interest for the perfume industry.

Plant density
Maximizing the bud yield can be done by optimizing the plant density. In our experiment the bud yield was highest at the sowing rate of 5 kg/ha or about 15 plants per square metre (Fig. 5). At this plant density weeds were not sufficiently suppressed by the crop. We assume that the ideal plant density for highest essential oil yield to be the same density as for highest bud yield.

Harvest time
It is clear that the abundance of glandular hairs depends on the developmental stage of the individual female flowers, the bud, the plant and the whole population. The optimal harvest time for highest yield may be when the abundance of intact functional glandular hairs is biggest. This is not a calendar date but an ontogenetic time depending on the strain.
Highest yield of essential oils was gained when about 50% of the seeds had reached maturity.
The ideal harvest time for best quality (scent scores) was somewhen between female flowering and seed maturity (Fig. 4). Depending on the strain this may be quite a long period. For the strains Kompolti and Futura 77 best quality was found about one week before seed maturity (75% mature seeds), for the strain Félina 34 about three weeks before. It seems that the harvest time for best quality has to be determined for every strain. The increase of quality during senescence is astonishing and cannot be explained. Compositions and amounts of the detected substances by chemical analyses did neither correspond to the quality marks of the scent tests nor to harvest dates. Thus, smell quality can still not defined by chemical composition.
Unfortunately the yield and the quality never have been highest at the same time. Therefore the optimal harvest time depends on whether the farmer or his customer is more interested in, yield or quality. The time interval when both yield and quality are high is rather small (Fig. 4).

Prevention of pollination
The female hemp flowers are thought to be fertile only during a rather short time. Hemp is known to build large amounts of new flowers as long as it does not become pollinated. Prevention of pollination leads to greater amounts of flowers and also to significant higher essential oil yields (Tab. 3). This strategy is known by people growing hemp for the production of drugs and considered as the "sinsemilla" technique. Prevention of pollination in fields with a size of economical relevance is nearly impossible yet but easy to carry out in a greenhouse. This difference in the yield is expected to be dependent also on the harvest time.
The quality of oils was rated as „very good“ or even „excellent“, regardless if pollination took place or not.

Weather
As mentionened above the weather during the period between female flowering and seed maturity in 1997 was dry. There is evidence for increasing resin production, glandular trichome density and cannabinoid content for plants grown in areas with a lower humidity (Pate 1994). The same should be true for the production of essential oil. In addition it is imaginable that hard rainfall could destroy glandular trichomes and thus decrease the yield of essential oil. The relatively low yields of the two last harvesting dates (Fig. 2) could be due to rainfall (Fig. 1). In seasons with more rain input the yields are expected to be smaller.

Other constraints
The daily time of the harvest is said to strongly influence the yield of essential oil. For example clary sage (Salvia sclarea) is recommended to harvest at daybreak early in the morning. Lavender (Lavandula spec.) and french basil (Ocimum basilicum) should be harvested on bright sunny days (Husain 1994). In the case of hemp it is not yet clear whether the daytime of the harvest influences the yield, this still has to be tested. We harvested early in the afternoon on bright sunny days.
The quality of the water and the material of the distillation equipment may have a strong impact on the quality of the oil (Mächler 1997). The same is true for the harvest technique. Best quality is gained when only flowers harvested by hand are used for distillation. Harvest by machine whereby the whole plants (flowers and stems) are chopped lead to lower quality (unpuplished data). Although hemp stems have a typical smell it was not possible to gain essential oil from them.

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