Peppermint 463
moisture should be selected, and the ground should be well tilled 20–25 cm deep.
The crop cannot tolerate highly acidic or alkaline soils and requires near neutral soil
pH. The crop initially requires lower temperatures and later on, a mean temperature
of 20–40 ∞C is suitable for its vegetative growth. The crop grows well in humid areas
which receive 100–110 cm well distributed rainfall. The plants cannot tolerate frost,
particularly during the sprouting period. They require ample sunshine during most part
of the growing period, and shade is undesirable as it induces higher ester and menthone
content in the oil. Day length is a determining factor contributing towards higher oil
yield and its quality and 15 h day length is essential. Ellis (1960) has established that
for economic production of the oil, the day length must approach 16 h.
Its cultivation has been tried in heavy metal polluted soils in Bulgaria by Zheljazkov
and Nielsen (1996). It was established that heavy metal pollution of soil and air at a
distance of 400 m from the source of pollution decreased the yields of fresh herbage
by 9–16% and the yield of essential oil by up to 14% compared to the control, but did
not negatively affect the essential oil content and its quality. Oils obtained from a
distance of 400 m from the source of pollution have not been contaminated with
heavy metals. Cultivar response to heavy metal pollution was also established. A
positive correlation between Pb concentration in leaves and in essential oil was
found. Heavy metal concentration in the plant parts was found to be, in order, Cd:
roots > leaves > rhizomes > stems; Pb: roots = leaves > rhizomes = stems; Cu: roots
rhizomes = stems = leaves; Mn: roots > leaves > stems = rhizomes; and Zn: leaves
roots > stems = rhizomes. Despite the yield reduction (up to 14%), due to heavy
metal contamination, mint still remained a very profitable crop and it could be used
as a substitute for the other highly contaminated crops.
28.3.2 Planting and varieties
The field for peppermint planting should be prepared by repeated ploughing and
harrowing. The FYM or compost at 10–15 t/ha should be mixed properly at the time
of field preparation. All stubble and weeds should be removed before the land is
levelled and laid out into beds with appropriate irrigation and drainage arrangements.
The new crop is raised through planting stolons although suckers, runners and
transplanting of sprouted plants can also be successful. The stolon is an underground
stem, formed at the end of the creeping rootstock during winter to overcome the
dormancy period. These are white to light cream coloured, smooth, fragile and juicy,
which are dug out fresh for planting. The plants are propagated in the spring, when
the young shoots from the crop of the previous year attain a height of about 10 cm.
January end to early February is the best season for planting stolons. The suckers are
transplanted into new soil, in shallow furrows about 50–60 cm apart, lightly covered
with about 5 cm of soil. The planted field is irrigated immediately. The new sprouts
are produced within 1–2 weeks. They grow vigorously during the first year and throw
out numerous stolons and runners on the surface of the ground. After the crop has
been removed, these are allowed to harden or become woody, and then farmyard
manure is scattered over the field and ploughed in. In this way the stolons are divided
into numerous pieces and covered with soil before the frost sets in, otherwise if the
autumn is wet, they are liable to become sodden and rot, and the next crop fails.
The Central Institute of Medicinal and Aromatic Plants, Lucknow, has developed
improved peppermint cultivars. These are Kukrail, Tushar, Pranjal, CIM Madhuras
and CIM Indus, having herb yield potential between 200–225 q/ha and oil content