Lesson Notes By Weeks and Term v4 - SHS 3
HEALTH ISSUES IN CROP PRODUCTION
Download the Lessonotes Mobile Ghana app for faster lesson access on Android and iPhone.
HEALTH ISSUES IN CROP PRODUCTION
Download the Lessonotes Mobile Ghana app for faster lesson access on Android and iPhone.
Subject: Agriculture
Class: SHS 3
Term: 2nd Term
Week: 8
Grade code: 3.4.1.LI.2
Strand code: 4
Sub-strand code: 1
Content standard code: 3.4.1.CS.1
Indicator code: 3.4.1.LI.2
Theme: AGRICULTURE AND HEALTH
Subtheme: HEALTH ISSUES IN CROP PRODUCTION
This page supports the lesson note with a companion video and a short classroom-ready summary.
For class groups and homework, share this lesson page so learners also get the summary, objectives, and full lesson context.
This lesson explores a critical aspect of crop health: the challenge posed by weeds. We often see farmers constantly weeding their fields of maize, cassava, or yam. But have we ever stopped to ask, "How do these unwanted plants get there in the first place, and why do they keep coming back?" Understanding how weeds spread, or disperse, is the first step in effectively managing them, protecting our crops, and ensuring a bountiful harvest. This knowledge is not just for big commercial farmers; it is essential for anyone with a backyard garden or a family farm, helping to save money, time, and increase food production for our communities and for Ghana.
What is a Weed? First, let's be clear on what a weed is. A weed is simply a plant growing where it is not wanted. It is a plant "out of place." Example: A maize plant is a valuable crop in a maize farm. However, if that same maize plant starts growing in the middle of a carefully planted yam mound, it becomes a weed because it will compete with the yam for nutrients, water, and sunlight. Common weeds in Ghana include Speargrass (*Imperata cylindrica*), Guinea grass (*Panicum maximum*), *Tridax procumbens* (often called "Coat buttons"), and the notorious *Chromolaena odorata* ("Acheampong weed").
These weeds are a major health issue for crops because they cause: Competition: For light, water, and soil nutrients. Reduced Yields: Less food for the crop means smaller fruits, tubers, or grains. Allelopathy: Some weeds release chemicals that are toxic to crops. Harbouring Pests and Diseases: Weeds can provide a home for insects and pathogens that later attack our crops. What is Weed Dispersal? Weed dispersal is the process by which weed seeds or other reproductive parts (known as propagules) are moved from a parent plant to a new location where they can germinate and grow.
Think of it like a journey. The weed needs a way to travel from one farm to another, or from the roadside to your vegetable garden. The methods of travel are what we are studying today. Agents and Methods of Weed Dispersal
The primary agents that carry weed propagules are Wind, Water, Animals, and Humans. Dispersal by Wind (Anemochory) This method is effective for weeds that have very small, light seeds or seeds with special structures to help them float in the air. How it Works: The wind blows across the parent plant and picks up the light seeds, carrying them over short or long distances. Characteristics of Wind-Dispersed Weeds: Plumes or Pappus: Feathery or hair-like structures that act like a parachute, catching the wind. Ghanaian Example: *Tridax procumbens* (Coat buttons). You have likely seen its small, black seeds with a white, feathery crown that fly everywhere when you blow on them. Wings: Flat, paper-like extensions on the seed that help it glide. Ghanaian Example: The seeds of the *Tecoma* tree, though not a typical farm weed, demonstrate this principle perfectly. Light and Tiny Seeds: Many grasses have very small seeds that can be easily blown by the wind. Dispersal by Water (Hydrochory) This method is common for weeds growing in or near bodies of water like rivers, streams, lakes, and irrigation channels. How it Works: Seeds fall into the water and are carried downstream. Flooding is a major way water disperses weed seeds over large areas of farmland. Characteristics of Water-Dispersed Weeds: Buoyancy: The seeds can float due to a corky or waxy seed coat or trapped air pockets. Water-Resistant: The seeds can survive in water for long periods without rotting. Ghanaian Example: Water Hyacinth (*Eichhornia crassipes*): A major invasive weed in the Volta River and other water bodies. It spreads rapidly through water currents. Seeds of weeds growing on the banks of rivers like the Tano or Pra can be washed onto farmlands during the rainy season. Dispersal by Animals (Zoochory) Animals, both wild and domestic, are very effective agents of weed dispersal. This happens in two main ways: a) External Dispersal (Epizoochory): On the animal's body. How it Works: Weeds have seeds with hooks, burrs, spines, or sticky surfaces that attach to the fur of animals (like goats, sheep, cattle) or the feathers of birds. Humans also participate when these seeds stick to our clothes. Ghanaian Example: *Bidens pilosa* (Black-jack). After walking through a bushy area, you often find its small, black, needle-like seeds stuck to your trousers or socks. Goats and sheep are major carriers of these seeds. Another example is *Acanthospermum hispidum* (Star burr). b) Internal Dispersal (Endozoochory): Through the animal's digestive system. How it Works: Animals and birds eat fruits containing seeds. The fleshy part is digested, but the hard seeds pass through the digestive tract unharmed and are deposited in droppings far from the parent plant. The droppings even provide some initial fertiliser for the new seedling. Ghanaian Example: Birds eating the fruits of the Turkey berry (*Solanum torvum*) and spreading the seeds everywhere. Guava seeds are also commonly spread this way. Dispersal by Humans (Anthropochory) Humans are arguably the most significant dispersers of weeds, often unintentionally. How it Works: Contaminated Crop Seeds: A farmer might buy maize or cowpea seeds from the market that are accidentally mixed with weed seeds. When these are planted, the weed is introduced to the farm. Farm Machinery: Weed seeds and soil containing seeds can get stuck to tractor tyres, ploughs, or even a farmer's cutlass and boots. When the equipment is moved to another farm, the weeds are transported too. Manure and Compost: If farm animals were fed with plants containing mature weed seeds, their manure will contain these seeds. Applying this manure to the farm spreads the weeds. Deliberate Planting: Sometimes, a plant is introduced for a purpose (e.g., as an ornamental plant) but escapes cultivation and becomes an invasive weed. Characteristics of Weeds that Aid Dispersal Prolific Seed Production: Many weeds produce thousands, or even hundreds of thousands, of seeds from a single plant. This increases the chances of at least some seeds finding a suitable place to grow. Seed Dormancy: Weed seeds can remain alive but inactive (dormant) in the soil for many years. They only germinate when conditions (like light, moisture, temperature) are right, for example, after the soil is ploughed. Specialised Structures: As discussed above, structures like plumes (for wind), hooks (for animals), and buoyant coats (for water) are critical adaptations for dispersal. Vegetative Reproduction: Some weeds, like Speargrass (*Imperata cylindrica*), can reproduce from underground stems (rhizomes). A small piece of the rhizome cut by a hoe or plough can grow into a new, complete plant, making them very difficult to control. Advantages and Disadvantages of Weed Dispersal