466 Handbook of herbs and spices
were associated with high rates of nitrogen fertilizer (100, 200 and 300 kg nitrogen/
ha), and high levels of irrigation (50 mm per week) throughout the last half of the
growing season. The composition of oil extracted from herb at the commercial harvest
date was not significantly affected by either nitrogen or irrigation treatments. The oil
yield from regrowth within the same growing season was significantly affected by
irrigation and nitrogen treatments applied prior to the first harvest. When 300 kg
nitrogen/ha and 50 mm of irrigation weekly (during the last half of the growing
season) were applied, the oil yields from regrowth approached the commercial yield
obtained from one harvest. Oil from regrowth contained high levels of menthol,
menthyl acetate, menthofuran and limonene, and low levels of menthone and cineole.
28.3.4 Pest management
The crop requires frequent tillage as successful mint-growing implies clean culture at
all stages of progress. The presence of weeds among the peppermint, especially other
species of Mentha, is an important cause of deterioration on quality of the oil. Use of
herbicides significantly reduces the cost of weed management. Due to slow sprouting
and growth rate of the crop in the initial stages, weeds may dominate the crop if
proper weed management is not followed. The critical periods for weed interference
in peppermint has been found to be between 30–50 days after planting and 15–30
days after the first harvest. Usually 2–3 manual hand weedings are required to keep
the weed growth under check. Pre-emergence spray of herbicides like Oxyflourofen,
Pendimethalin and Diuron has been found quite effective. However, considering the
use of herb and oil for edible purposes, the use of herbicides should be avoided.
Experiments at different locations in Poland were carried out in 1971–1973 on the
usefulness of a herbicide Sinbar, which can be successfully applied for the control of
dicotyledonous weeds. Successful results were obtained when the preparation was
applied at 1–1.5 kg/ha dose before or after the shooting growth of mint (Golcz et al.,
1975). Sinbar does not control monocotyledonous weeds but it inhibits their
development. Sprayings with Sinbar in an appropriate dose and at fixed dates do not
have any negative effect on the development of mint and the concentration of essential
oil in the crude drug. The residual terbacil (an active ingredient of Sinbar) in Herba
Menthae piperitae is 0.008 ppm during harvesting, when Sinbar is applied before and
after drying of mint. Later sprayings increased the concentration of terbacil in the
crude drug (0.21–0.27 ppm).
Briggs (1973) applied three polybutene preparations to healthy peppermint plants
and assessed the effects of these products on the yield and quality of oil derived from
the plants. The treatments caused premature ageing of the leaves and the quantitative
composition of the oils had altered relative to controls, resulting in low quality.
Preparations containing polybutenes are unsuitable for application to peppermint and
should not be applied to crops that are to be extracted by steam distillation. The effect
of ten different herbicides on the yield of herb, oil yield and oil composition of
peppermint was studied by Skrubis (1971). The herbicides increased the yield of
fresh herb, but the yield of peppermint oil was not affected. A gas chromatographic
examination of the oil showed that the composition of the oils varied with different
herbicides.
In another study, 40 different chemical substances with weed killing properties
were studied by Pank et al. (1986) in cultures of peppermint over thirteen years
duration. It was summarized that Chlorbromuron, Simazin and Terbacil can be