Persistence of weed

Weeds are highly persistent category of plants. They existed even before the first seed of any crop was planted on earth and they are with us even today, probably in larger number and with greater vigour. This speaks ample of their highly persistent nature. High persistence of weeds results from their multifacet mechanisms. Important among these are:-

(і) Prolific Seed Production:

• Most weeds are prolific seed producers. For instance, the per plant seed production capacity of Cuscuta spp.
• Wasd found approximately 16,000; Conyza 33,992; Chenopodium album, 72,000; and Amaranthus, 196,000. Only in few instances of some perennial weeds, the seed production was weak. Further, the immediate viability of weed seeds has been found to vary from 6 to 78%.
• Perennial sowthistle (Sonchus arvensis) can set viable seeds even when it is cut during its flowering stage and kept in shade.
• Chickweed (Stellaria media) and purslane (Portulaca oleracea) have also been found well adapted to premature flowering and seed setting under adverse environment conditions.
• Chenopodium album may grow as much as 30-50 cm tall before it flowers and sets seeds. But in the events of severe drought it may grow hardly 3 cm high, and still produce some seeds before it withers.

(іі) Dormancy- of Weed Seeds and Other Propagules in Soil

• The majority of weed seeds lying below about 5 cm soil depth remain dormant, and act as source for future flushes of weeds. Thus, the soils, both agricultural and otherwise, acted as “reserve banks” for the weed seeds and their other propagules.
• The phenomenon of dormancy and consequent longevity of weeds seeds in soils is a very important tool with weeds to ensure their everlasting existence against all odds of nature (and man).
• If all weed seeds present in the soil were to germinate at one time, a single tillage operation could wipe these out of the scene. But contrary to it, only a fraction of weed seeds present in any soil germinate at any one time, leaving others to germinate later

Within the soil, weed seeds can remain dormant for three reasons, as follows:-

a) Enforced dormancy:

• Enforced dormancy in weed seeds is due to their placement deeper than 5 cm, resulting usually from tillage of the field.
• Weed seeds under this kind of dormancy germinate readily whenever these are restored to the top 3 to 5 cm layer of soil by tillage, provided adequate soil moisture and congenial temperatures were available in this zone of the soil.
• Enforced dormancy is a non-specific character of the seeds and it is caused by the absence of red light (r) under the ground, which otherwise induces germination in seeds by activating their phytochrome system (P), comprising a responsive chromophore blue pigment attached to the protein molecule in seeds. Far-red light (fr) deactivates the system and thus induces dormancy in seeds.
• Higher soil temperature and NO3content of surface soil may further help in breaking the enforced seed dormancy.

b) Innate dormancy

• Is a genetically controlled character.
• Innate dormancy usually results for reasons of either hard seed coats, like in Setaria, Ipomoea, and Xanthium spp.,or immature embryos, as observed in Polygonum, Juncus, and Eleocharis spp.
• In nature the innate dormancy of weed seeds is overcome with either passage of time, or under the influence of some climatic pressure.

c) Induced Dormancy

• Induced dormancy results from some physiological change in otherwise non-dormant weed seeds under the impact of factors like a marked rise in soil temperature, increased CO2 content of the soil, low O2 pressures, water logging, etc.
• Certain weed seeds, like those of wild oat (Avena fatua), exhibit all the three kinds of dormancies.

(iii) Vegetative Propagation

• Many weeds are extremely persistent because of their ability to propagate by vegetative means.
• When the above ground parts of such weeds are destroyed, their deeply placed vegetative propagules put forth new shoots as soon as the external stresses are removed and favorable conditions revive

(iv) Rapid Dispersal

• It exposes them to different ecosystems so that each weed species can choose its most favourable environment and put up a hard struggle for existence in nature.

 (v) Inherent Hardiness:

• Weeds are highly adaptable and can survive extreme environmental conditions including drought, frost, salinity, poor soil fertility, and pest or disease pressure.
• Some, like Amaranthus and Cyperus, utilize the C₄ photosynthetic pathway for better water and nutrient use. Many weeds also exhibit high transpiration efficiency and low nutrient requirements, allowing them to thrive where crops fail.

(vі) Evasiveness

• Many a weed is capable of evading destruction by animals and man because of their bitter taste, disagreeable odour, spiny nature, and mimicry.

(vіі) Self Regeneration

• Weeds are self-sown plants. They don’t require any artificial, friable seedbeds for their germination.
• Detached from the mother plants, weed seeds and other propagules germinate profusely on undisturbed soils whenever the environment is favourable for the purpose.

(viii) Selective Invasion

• Weed species composition varies with crop type, soil, irrigation, and season. For example, dryland weeds like Tribulus terrestris dominate arid conditions, whereas irrigated fields favor moisture-responsive species like Phalaris minor or Commelina benghalensis.
• Converting fields to paddy introduces aquatic weeds such as Echinochloa and Eclipta. This ability to “select” the most suitable species for prevailing conditions ensures persistent dominance.

(ix) Weed Succession and Resistance

• Continuous cropping and repeated herbicide use promote the evolution of resistant weed ecotypes, called chemotypes.
• For instance, long-term use of isoproturon in wheat fields has encouraged tolerant weeds like Anagallis and Medicago, while repeated butachlor use in paddy favors Cyperus rotundus.
• Over time, weed succession shifts the composition of weed flora, enhancing overall persistence.