The following points highlight the top six factors affecting the persistence of weeds. The factors are: 1. Selective Invasion 2. Prolific Seed Production 3. Longevity of Weed Seeds in Soil 4. Vegetative Propagation 5. Rapid Dispersal 6. C4 Plant Physiology.
Factor # 1. Selective Invasion:
Weed species differ widely in their soil and climatic requirements. Every soil seems to be a seed bank of several weed species; their composition depending upon the chance a particular weed had to reach a particular site. The nature makes a critical selection out of the lot and allows only those weed seeds to germinate at a time which were most adapted to the environment prevailing then.
The seeds of rest of the weed species wait in soil till the environment outside changes. At this stage suddenly another set of weed species takes over, the ground. The major environment elements which determine the weed species composition on the ground comprise the available soil moisture, soil pH, temperature, photoperiod, and solar energy.
To cite some examples, in a typical dry farming situation the weed flora often comprises moistures hardy species like Tribulus terrestris, Argemone mexicana, Eragrostis cilianesis, Euphorbia hirta, Celosia argentea, and Heliotropium eichwaldii. When such fields are brought under irrigation, soon these weed species are replaced by better moisture responsive weeds like Trianthema monogyna, Phallaris minor, Asphodellus tenuifolius, Malva parviflora, Commelina benghalensis, and Brachiaria ramosa, depending upon the season.
Further, if such fields are irrigated more intensively and turned into paddy fields by ponding water, there shall be another shift in weed species. In this situation weeds like Echinochloa, Caesulina, Butomus and Eclipta spp. will dominate the scene. Likewise, weeds in sugarbeei grown on the salt affected soils are different from those found on normal soils.
Some salt tolerant weeds which tend to dominate the salt affected soils are- Chenopodium murale, Salsola kali, Taraxacum officinale, Polygonum spp. and Distichalis stricta. Weed species vary with soil texture also. For instance, while Celosia argentea and Tribulus terrestris are adapted most to sandy and light texture soils, weeds like Trianthema monogyna and Amaranthus viridis prefer heavy or fine texture soils for their luxuriant growth.
Weed species also differ with the micro-climate offered by a particular crop. For instance, weeds like Coronopus didymus and Cichorium intybus prefer to associate with closely sown and frequently irrigated crops like lucerne and berseem (Egyptian clover) while they avoid open row crops like chickpea and rape & mustard.
Nutrient status of the soil may also sometime determine the dominant weed flora at the site. Weeds like Crotolaria, Ipomoea, Carolina and Cassia spp. become prominent on phosphorus deficient soils while Plantago and Rumex spp. are more adapted to potash deficient soils. The time of sowing of crops is also important in dictating the weed flora.
Under similar soil and climatic conditions, an early sown crop is very likely to be infested with weed flora which is quite different from the one sown timely or late, in the same area. For example, when lucerne and berseem are sown in the beginning of October (in North India), Amaranthns and Trianthema spp. are found to dominate. But when these crops are sown in November, Chenopodium, Anagallis and Vicia spp. become the prominent invaders.
Climatic and seasonal changes also force rapid changes in the weed flora. That is why we have typical summer and winter weeds. In fact, the presence of particular weed species on a given land at a particular time is an interesting decision of nature, involving the interplay of several known and unknown factors, including the agricultural activities of man. The mechanism of such an armour available in weed seeds seems to be a perfect remote sensing system.
Factor # 2. Prolific Seed Production:
Most weeds are prolific seed producers. Cusuta arvensis often produces as high as 16,000 seeds per plant. Conyza sp. is known to produce up to 33992 seeds, Chenopodium album 72000 seeds, and Amaranthus viridis 196000 seeds per plant. Only certain weeds produce few seeds. The viability of weed seeds has been found to vary from 6 to 78%, depending upon the species.
Thus, by and large even if a few weedy plants escaped the control measures, these are sufficient to rein-fest large fields and continue their progenies year after year. Besides, many a time the weeds may mature their seeds on the uprooted plants. Chakrawarti (1966) compiled some good information on this aspect of weed biology.
He reported that when perennial sow thistle (Sonchus aroensis) was harvested at its full flowering stage and kept in shade, it still produced completely viable seeds. Chickweed (Stellaria media) and purslane (Portnlaca oleracea) were also found to mature their seeds on the uprooted plants. No crop plant exhibits such phenomenon.
Further, weeds ensure some seed production even under most stress conditions. For example- under normal soil moisture condition a weed like Chenopodium album often grows up to 50 cm or more in height and may produce over 70,000 seeds per plant.
But in moisture stress condition, as found on dry walks and bunds of the fields, it grows as a small plant of only 10-15 cm height, though still it produces some seeds before withering away. Crop’ plants have no such mechanism to adjust to the vagaries of nature.
Both prolific and ensured seed production are important factors in the persistence of weeds. Added to this are mechanisms like seed dormancy and preservation of seed viability in compost pits, soils, and animal digestion tracts.
Factor # 3. Longevity of Weed Seeds in Soil:
Most weed seeds present in the soil are distributed in the top about 30 cm of soil depth, although some may be found much deeper, reaching there through the soil cracks. Of all these weed seeds only the ones present in the top 3-5 cm soil are induced to germinate, almost simultaneously, while the deeper ones continue in a state of dormancy, waiting for their turn to get into the top soil layer. Some large seeded weeds like Xanthium strumarium may, however, germinate from up to 10 cm soil depth.
Long term experiments have been laid out for decades in the USA to study the viability of certain weed and crop seeds burried deep in soil. Perhaps the oldest seed burial experiment was started in 1879 by late Dr. W. J. Beal in the USA. The viability tests reported from this experiment by Kiviban and Bandurski (1973) showed that certain weed species retained their viability even after 100 years of storage in soil. Similar seed burial experiments have been laid out by some other scientists, time and again, in the USA and some other western countries.
The phenomenon of extended longevity of weed 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 different depths of the soil were to germinate at one time, a single tillage operation could wipe these out from the land. But it is not so.
Only a fraction of weed seeds present in a soil germinate at any one time, leaving others to germinate later. Also, a few weedy plants which escaped attention in a field are often sufficient to recoup the seed bank in the soil.
Under the circumstances, while on one hand tillage of the field tends to bring the deeply placed weed seeds up in the top layer of the soil for their germination, on the other hand it burries deep and preserves the fresh seeds to act as source of future weed populations. In this context the old gardener’s saying, one year seeding is seven years’ weeding, still holds good.
Factor # 4. Vegetative Propagation:
Many weeds are persistent because of their ability to propagate by vegetative means. They possess modified organs like rhizomes, rootstocks, tubers, bulbs, and runners for the purpose. The runners are located on the soil surface, while others grow below the soil surface; their depth depending upon the weed species.
Such weeds are very persistent, for instance, bermudagrass (Cynodon dactylon), Johsongrass (Sorghum halepense), nutsedge (Cyperus rotundus), and mile-a-minute (Mikania micrantha). Even if environment remained unfavourable for such weeds for considerable period, they go dormant underground and regrow in favourable times with full vigour. Tillage helps such weeds to increase in numbers by fragmenting their vegetative propagules. Each such fragment then puts forth new mother plants.
Factor # 5. Rapid Dispersal:
Weed species explore new venues for their establishment by adopting one or more of the dispersal mechanisms. Rapid dispersal is one key factor in developing persistent weed populations for every possible niche in the environment.
The overall level of persistence of a weed depends upon its capability to adopt one or more of the above cited features. Weed species that combine majority of these features are surely horrible weeds, for instance, Sorghum halepense, Saccharum spontaneum, Solanum elaegnifolium, and Mimosa spp.
Factor # 6. C4 Plant Physiology:
Many of our persistent weeds are C4 plants. The C4 physiology confers many characters that are advantageous to them under varied field situations. Their light and CO2 compensation points are low whereby they continue to be productive under vary low light and CO2 levels as found under a crop canopy.
At the same time the C4 weeds can withstand high temperatures and high light intensities, besides being free of photorespiration. Some common C4 weeds are- Cyperus, Cynodon, Echinochloa, Eleusine, Sorghum, Imperata, Eichhornia, Portulaca, Chenopodium, Digitaria, Convolvlus, Avena, Amaranthus, Paspalum and Rotboellia spp.