Understanding weeds in grasslands and grassy woodlands

Introduction

This activity would suit primary and early high school age children. It aims to encourage children

·        to understand the principles of competition

·        to understand the difference between annuals, biennials and perennials

·        to apply knowledge to field observations

·        to do independent research

This is an activity that can take all day but should be limited to about 30 minutes for young children and teenagers (maximum 10 at a time).

Materials

Tent pegs, field magnifying glass, copies of Our Patch: Field Guide to the Flora of the Australian Capital Region as photographed in the Aranda Bushland and students’ notepads.

Suggested Introductory Text

Do you like weeds? Most people dislike weeds because they conjure up images of our vegie patch and flower garden growing all sorts of plants which don’t belong there. Have you heard the song “I’m a lonely little petunia in the onion patch and all I do is cry all day”? In this case the petunia is a weed because it belongs in the flower garden and not the onion patch!

In the bush there are lots of weeds that come from people’s gardens. Honeysuckle and periwinkle are two problem weeds arround Canberra.

What makes a problem weed? Problem weeds are those with the potential to kill out the native plants we wish to conserve. There are various ways they can do this, which we will explore.

European grasses, such as fescue and phalaris are adapted to living in a much colder climate than Canberra. They start to grow away in the spring several weeks earlier than our native grasses, like kangaroo grass and wallaby grass. As a consequence they use up the nutrients and moisture and space before the native grasses begin to grow. When they do they are at a big disadvantage in the competition for nutrients, water and space.
                                                                  

Oh dear!
                                                                   Where can I grow?
Other European plants are what we call biennials, which means they begin to grow in one year and flower the next. Many of our common vegetables, such as carrots and onions, are biennials.

A number of our problem weeds in the bush are also biennials. The most common of these are Paterson’s curse and thistles.

In the first year when they germinate they grow a rosette, which made up of rings of leaves, around the central growing point and very close to the ground. The rosettes can grow quite large, up to 50cm or more.

In the next spring or summer they send up a flowering stalk which if it grows to maturity can have many hundreds of seeds.

The problem with them is they too take up the space. Most native plants in grassland are annuals, i.e. the germinate, grow and flower in the same year. If there are lots of biennial weeds there is no space for them to grow.
                                                            

Hey! I need to be able to see the sun
                                                                    if I am to survive and grow.
Another group of plants is called perennials, i.e. they grow for a number of years.

A problem weed, that is a perennial is St John’s wort. You can find St. John’s wort in the grassland too. It has beautiful yellow flowers and can produce hundreds of seeds which can stay in the soil for many years.

If you look carefully at a plant you can see last year’s flower stalk and where this year’s growth began at the base of the old stem.

When we try to get rid of St. John’s wort, we encounter problems:

·        if we pull the plant out of the ground, leaving even a tiny piece of root, it will grow again

·        the seeds in the ground can live for many years.

                                                              
Food for thought!

Paterson’s curse and St John’s wort are not problem plants in the countries where they normally grow. There they help to feed a number of insects and other organisms. Can you find out about the bugs that are being introduced into Australia in order to control these problem weeds?

Clue: When back at school, search the web for information about the CSIRO’s Biological Control Program.

On-site activity

This activity will demonstrate to participants the ‘step point’ method of assessing the concentration of particular species in a selected area. It involves several simple steps:

• Insert a tent peg in lace-up shoes so that the pointy end of the tent peg faces forward and the curved end is hooked under the laces.
• Select an area between permanent markers (e.g. two trees or firmly grounded stakes) that is suitable for counting species in, and identify what you are looking for. It may be a particular kind of weed, or particular annuals or perennials.
• Step forward slowly and record how many of what you are looking for can be seen as you look straight down at the pointy end of the tent peg.

This activity can (and should) be repeated at regular intervals to monitor change in plant diversity and to evaluate the effectiveness of weed control methods.

Return to Activity Home