Weeds – plants out of place• Lesson AimsTo study weeds in the context of competition and how weedkillers can affect

food webs. Learning outcomes By the end of the lesson you should be able to:Explain and evaluate the issues and benefits and drawbacks of weedkillers in the

food chain. Use qualitative and quantitative methods to display the number weeds in

quadrats.

Starter: Quiz

1.2.3.4.5.6.7.8.9.10.

2 minutes

Talk in pairs about what a weed is. Do you know any? Where do you see them?

What is a Weed?A weed is a plant that is growing where it is not wanted, it can have strong and healthy growth, and is able to overgrow valued plants by overcrowding, thus depleting soil nutrients and moisture that would otherwise be available to preferred plants.

Examples of weeds

Dandelion

English Plantain

Nettle

Poppy as a weed in a field of crops

Poppy as a wanted plant in a garden

Prickly Pear Cactus

Area in Queensland, Australia, covered with prickly pear cactus (Opuntia stricta), which had expanded rapidly after being introduced in 1926.

The same area three years later (1929) after the cactus moth (Cactoblastis cactorum) was introduced as a biological control agent for the cactus.

• • We are going to look for weeds on a field

• In pairs you will work what percentage of the quadrated area is covered in different weeds.

• Remember Number of type of weed X 100 = % of type of

Total number of weeds weed

• Add you groups results to the class results • Describe your investigation and what we

found out (Draw a diagram to how you obtained the data)

PAGE 145 Spectrum Biology• Explain why weeds should be removed from

crops • Explain why removing weeds by hand is rarely

used for crop plants • Explain what a weedkiller is • Wild birds often each seeds from weeds. What

will happen if weeds are killed?• Draw and complete this table

Weedkillers in the food chainBenefits Drawbacks

Use of pesticide in crop spraying

Spraying DDT to get rid of mosquitoes