Biology of weeds is important since it covers all aspects of their establishment, growth and reproduction. All climatic factors have a significant effect on biological processes. A good knowledge of the biology of weeds will ensure that the grower and researcher will be able to apply technology correctly to control any weed problem.
The succession process
Succession is the process through which plants populate an open piece of land. The first plants that establish in an open field are called pioneer plants. Pioneer plants are plants that can survive conditions of low soil fertility, extreme temperatures and low soil moisture content. Most pioneer plants have very short life cycles and produce abundance of seed which increases their probability of survival.
Pioneer plants create an environment with higher humidity, increased soil water content, increased soil humus etc., which is more suitable for perennials. The higher competitive ability of perennials replaces the pioneer plants.
The next type of plant that establishes itself are sub-climax types which are replaced by the climax plants. The climax plants create a stable environment over the long term.
The combination of plants in each phase will be determined by:
- Soil fertility
- Soil moisture content
- Seed availability
- Animal activity
- Biotic factors
- Activity of people
When a piece of land is prepared for agricultural use, a pioneer environment is created. Thus weeds establish themselves primarily on disturbed soils. Quite often this is not bad since some plant growth will maintain moisture in the soil and ensure a healthy bacteria and fungal eco-system. Dying leaves, seeds en stems will plough valuable nutrients back into the soil and keep the valuable C:N ratio in tact. Obviously the farmer must prevent the weeds from going into the reproductive stage of development.
Factors affecting weed development
There are three factors that need to be considered in relation to weed biology, ecology and control include: climatic, physio graphic and biotic aspects.
Climatic factors that affect weeds are:
- Light intensity, quality, duration and photoperiod
- Temperature extremes (high and low), average temperatures, diurnal fluctuations
- Water availability, amounts, percolation, run-off and evaporation
- Wind duration and velocity
- Atmosphere carbon dioxide, oxygen levels, humidity and toxic substances
Physiographic factors include:
- Edaphic soil factors such as soil pH, soil water resistance, fertility and ionic ratio’s, carbon nitrogen ratio, organic content of the soil, soil texture and structure.
- Topographic characteristic of the soil which include the altitude of the land, the slope, the direction of the slope (i.e. north facing or south facing land).
Biotic factors are:
- The variation of plants that have established themselves in the past
- The concentration of allelopathic compounds in the soil
- The concentration of toxins and stimulants in the soil
- The concentration of various diseases and parasitic plants
- The amount and type of animal movement in the area
- Human activity in the area.
Weeds are strong competitors which is the primarily reason why they are so difficult to eradicate and why they have such a significant effect on crop growth. Weeds compete primarily for soil nutrients, light and water. The presence of weeds have a negative competitive effect, resulting in a reduction of commercial yields. Competition results in the decrease of growth potential of two species growing close to each other. Amensalism is another type of competition whereby the growth of one type of specie is negatively affected by another. Allelopathy is a type of amensalism.
Weeds are strong competitors since they have:
- Short growth cycles
- Very high production of seed
- Rapid germination of seed
- Are usually C4 plants which use water more effectively
- Fast growing in the first few weeks
- They have the ability to change into a reproductive phase very early in their growth cycle.
The time when competition takes place in the life cycle of commercial crops will determine its effect on commercial yields. For instance, if the weed population is high early in the growth season, the growth of the commercial crop will significantly be influenced whereas the presence of weeds late in the growth season will only hamper harvesting procedures
Allelopathy is the chemical interaction among plants which includes stimulatory as well as inhibitory influences.
Allelopathic compounds can be released through:
- Root system
- Organic material decomposing on the soil surface
- Through the action of soil bacteria feeding on the dead plant material lying on the soil surface
Reproduction and spread of weeds
Weeds reproduce sexually and asexually (vegetatively). Sexual reproduction involves the pollination of a flower which produces seed.
Asexual reproduction involves the development of a new plant from a vegetative organ such as roots, stems leaves or modification of these organs. Vegetative reproduction takes place through underground stems (rhizomes), aboveground stems (stolons), tubers, corns, bulbs and bulblets. Environmental factors such as day length or photoperiod can have a significant affect on the development of these organs. Thus the longitudinal location of the weed can determine if the weed will spread or not.
The process through which weeds spread their seed is called seed dissemination. There are six methods through which weeds scatter their seed, these are:
- Seeds are scattered through crop seed, grain feed during harvesting, the production of hay and straw.
- Many seeds are light enough to be scattered through wind action
Some weeds require water, such as in rivers or heavy storms so that the seed can travel with the water to another location. This method also ensures that there is enough soil moisture when the seed germinates.
- Many seeds are able to resist digestion by animals and even humans in order to be scattered. Some seed however will only germinate after they have been digested. The digestion process removes the hard outer layer of the seed so that water can penetrate the seed surface and allow it to germinate. Many seeds germinate still in the droppings of animals. This also ensures that there is enough moisture and nutrients for the young seedling to develop and survive the first few weeks, which is usually critical.
- Some seeds have very hard surfaces or “outer skin” and some type of scarification is needed in order for water to penetrate the outer coat and germinate
- Weed seed can be nutritious and is often used in feed mixes. However the process through which these weeds are ground will destroy its viability thus reducing the risk of scattering. Heating seeds during pelleting also destroys their viability.
Characteristics of weeds
The fact that weeds are a problem can be ascribed to their ability to survive and spread their seed. Each type of weed has its own set of characteristics which makes it unique.
Commercial crops vs. weeds
Comparing the survivability of between commercial crops and weeds, one would quickly see that weeds have the advantage which has evolved over time. Commercial crops have been bred to have very specific characteristics, of which survivability in harsh conditions is not one of them. In the case of weeds, only the toughest has survived over the years. They have adapted to their surroundings for instance, many weeds have a C4 photosynthetic pathway which provides the weed an advantage during high temperatures and high light intensities. Other weeds are adapted to low temperatures. Many weeds will thus survive temperatures extremes between -10 and 45 °C whereas commercial crops are designed to grow between 12 and 32 °C.
Weeds have the ability to lose more water through dehydration and restore to normal growth than commercial crops. Some weeds can lose up to 75 % of its water in cells and still have the ability to grow after cell water content have reached normal levels.
Amount of seed produced
Weeds tend to produce much more seed per plant than any commercial crop. For instance Striga asiatica (witchweed) has the ability to produce up to 500,000 seeds per plant per season in comparison with wheat which produces only 200.
Commercial seed has an average germination percentage in the first year of 95 % after which the value decreases sharply in the second year if the seed is not kept in cold storage. Weed seed can maintain its germination viability for many years even if it is exposed to extreme climatic conditions. Some seeds have survived more than 10,000 years in ice sheets in Canada after which scientists have germinated them.
Forced seed dormancy
Some weed seed have the ability to go into a phase of forced seed dormancy. The seed will stay dormant even if environmental conditions are favourable for germination. It has been estimated that the amount of dormant seed in 1 ha can exceed 1 billion.
Forced seed dormancy may also occur when one factor is not optimal for germination. Factors such as soil moisture, oxygen levels, light levels, CO2 levels and temperatures.
Primary seed dormancy
Primary or inherent seed dormancy prevents the germination of seed before some action has taken place. The seeds own characteristics prevent germination. Germination is prevented through various factors such as:
- Hardiness of the outer layer of the seed which requires some sort of scarification or digestion by animals or fire
- Some seed epidermis is impermeable to oxygen in the first year which prevents it from germinating.
- Specific temperature requirements such as freezing or very high temperatures found in fires
- Some seed have some chemical substance present that prevents it from germinating. Once the seed comes in contact with the soil, the chemicals are leached out of the seed and germination takes place.
Induced seed dormancy
Many seeds are ready for germination, however some external factor prevents the germination process to initiate. External factors such as
- Seed lying on the surface of the soil
- Seed that is too old and have depleted their internal energy resources
- The build-up of toxic substances inside the seed