Silent killer

With green the buzzword of the 21st century, everyone craves lush
surroundings. But sometimes all green cover does not have a direct
beneficial affect in our lives. Arunayan Sharma explains

BE green, go green is today’s catchword to make the world a better place.
Everyone craves green coverage as much because green is the colour of the 21st
century. We all wish our surroundings have some green cover in the form of
grass, shrubs and trees. But sometimes all green cover does not have a direct
beneficial affect in our lives. At a time when the cry is for “green”, a
silent green killer is spreading and making inroads into various parts of our
surroundings and this phenomenon is common to all parts of our country, be its
cities, towns or villages.
This silent green killer is nothing but a short-lived ephemeral herb that grows
to two metres high in good soil, germinating after rain during any season,
flowering in six to eight weeks and senescing with drought or frost.
With small seeds lasting up to 20 years, this killer is nothing but Parthenium
hysterophorus, one of the worst weeds for agriculture, the environment and for
the animal and human health. It is considered one of the 10 worst weeds in the
world.
The Parthenium weed �" known as white top, whitehead, congress grass or carrot
grass �" is an erect annual herb with alternate, deeply-dissected leaves that
grows to up to two metres high with much-branched inflorescences bearing white
flower heads and numerous obovoid, smooth and black achenes. The seeds do not
germinate immediately after ripening, as the achenes first need to disperse and
release several inhibitors of germination. Parthenium germination reaches its
maximum within one to six months after the achenes ripen. The seeds are not able
to germinate in soil below a depth of five centimetres. Parthenium flowers 30-45
days after germination and the whole plant cycle is completed within about five
months, with a single plant producing an average of 900 flower heads.
Parthenium is derived from the Latin word parthenice �" a reference to the
plant now known as Tanacetum Parthenium Bern or Feverfew �" hysterophorus is
derived from the Greek hystera (womb) and phorus (bearing) referring to the
prolific seeding habit of the plant. Parthenium is also known as Carrot weed or
Gajar ghas (Hindi) �" because the leaves are alternate, sessile, irregularly
dissected and bipinnate, resembling that of carrot (gajar) or chrysanthemum.
Parthenium is also known as White Top, Safed Topi (Hindi) and Chatak chandani
(Hindi �" chatak meaning bright and chandani meaning moonlight) because of its
numerous white flowers.
A notorious weed, it first sneaked into India with wheat grains imported from
USA almost half a century back. It was first observed in Pune in 1955 and has
since become a widespread menace, especially in Bihar, Uttar Pradesh and West
Bengal. The plant is a rapid coloniser and differs from other weeds in its
remarkable adaptability to all kinds of soil and climatic conditions. It is
native to Mexico and the USA and has spread prolifically in central Queensland.
P. hysterophorus is native to the countries bordering the Gulf of Mexico, and
has spread throughout the southern USA, Mexico, the Caribbean and Brazil. A
slightly different race of P. hysterophorus, with yellowish flowers, is native
to central South America (Argentina, Bolivia, Chile, Paraguay, Peru and
Uruguay). The probable geographic centre of origin is the countries around the
Gulf of Mexico.
Parthenium flowers only between February and April in the western parts of the
country and only between August and October in the eastern parts. The blossoms
appear in clusters of tiny yellow-white flowers. However, its excellent adaptive
qualities have ensured that the plant completes its life cycle within a very
short period and flowers three to four times a year. Moreover it can proliferate
vegetatively, if need be.
During a flowering season, Parthenium produces more than 600 million
pollen-bearing units per plant. These can cause airborne contaminated diseases
like asthma, respiratory tract infection, rhinitis, conjunctivitis, hay fever,
acute dramatis and other debilitating allergies. However, it should be noted
that only people allergic to Parthenium suffer its ill effects. It poses some
activity as a depressent to the central nervous system and is reported to be
responsible for the largest incidents of dermatitis in India. Almost every part
of the plant, except the root, is reactive.
Parthenin, an incomplete antigen hepatin in the presence of sunlight
(ultraviolet rays), combines with albumin in the dermal part of the skin to
become a complete antigen and causes photosensitive reaction. This is known as
photophytodermatitis. Consumption of milk from Parthenium-fed livestock could be
hazardous to humans and animals. Since it is an exotic plant, Parthenium does
not have natural enemies such as pests, pathogenic fungi, bacteria and viruses.
Also, herbivores do not eat this weed and this helps its unhindered growth.
There are some incidences where it has been reported that some cattle have died
after consuming Parthenium. After taking root, the plant beats all other
vegetation in the vicinity; it leaches the ground and even inhibits the
germination of other weeds. There is, thus, no room for any plant biodiversity
in a Parthenium-infested area.
Uprooting Parthenium is almost next to impossible, though scientific measures
can check its spread to a great extent. Recent research suggests that the
proliferation of the weed can be checked best through measures at the local
level. The most widespread method used in the country today is manual
extirpation. When this is economical, it exposes a person to health hazards
associated with bodily contact with the weed. Another alternative is the use of
chemicals. This is a broad spectrum controlling procedure. Weedicides such as
atrazine, terbutryn, metribuzin, monuron, neburon, oxadiazon and simazine have
been found effective in controlling Parthenium.
Herbicides such as dicamba, glyphosate and picloram have also yielded good
results. Perhaps the most economical method is by spraying with a brine salt (15
per cent common salt) solution to dry out the plants and then burning them.
However, this method is not always advisable because biomass burning in large
amounts may generate biogenic gases that cause serious pollution. Moreover, this
method is not suitable for densely populated areas. Biological methods have also
been used to curb the spread of Parthenium. Research has shown that wasteland
shrubs, cassia serecia and cassia tora, inhabit germination of Parthenium seeds.
Plant extracts or leachates of marigold have also been found to inhibit
germination of Parthenium seeds.
But, Parthenium is not always a menace, it does have some potential uses. It can
be useful to make green manure, vermin-compost, as an efficient binding agent to
check soil erosion and as a raw material for biogas generation. Parthenium
pollen is also being used these days in the manufacture of pesticides. If you
look around, you will find Parthenium rapidly spreading in and around your
locality. Few are aware of its exact impact on the environment, animal and human
health, but it is spreading with great rapidity over great distances �" along
railway tracks, village roads, near dry ponds, abandoned buildings, etc.
Managing Parthenium calls for proper planning and management and awareness about
this weed should be raised among citizens. Government and non-governmen
organisations should join hands in this endeavour. The administrative
authorities should take measures to control or to stop its spread before it’s
too late.

(The author is director, Centre for Ecological Engineering, Malda.)
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