Sunflower experiments

Run our experiments to learn about how light, water, fertiliser and potting mix conditions affect sunflower growth.

These recommended experiments are a guide only - you can deviate from the suggested trials.

Seed germination

Aim, level and context

Aim: to compare the effect of two seed treatments on successful germination rates of sunflower seeds

Level: basic

Context: seeds need to break “dormancy” before they will germinate. This can sometimes be a problem for agronomists and plant breeders who need to ensure stored seed will germinate.

Materials and methods

You will need at least 12 sunflower seeds. The more seeds you use, the more robust the experiment.

Treatments include:

  • control (no treatment)
  • scarification (cut the tip of the seed open with a sharp knife or nail clippers)
  • freeze-thaw (place the seeds in a freezer for 30 minutes then allow to return to room temperature. You can repeat this several times).

Germinate your seeds on moistened paper towels or on the surface of moist potting mix.

Emergence should occur within a week. Take daily observations.

Observations and data collection

Ask your students to observe the type of germination (epigeal) and draw a diagram. Count the days until the hypocotyl (the plant) and the first root emerges.

Ask your students to write a report on the experiment comparing the three germination rates and time to emergence. Did any treatments cause them to emerge faster? What was the seed germination rate?

Comparison of potting mixes

Aim, level and context

Aim: to compare the effect of three potting mixes on the growth and development of sunflower plants

Level: basic

Context: the soil or substrate in which plants grow is one of the most important determinants in overall health and yield of crops. Soils supply essential macro- and micro-nutrients and have differing capacities to store moisture, influencing the water available for growth.

Materials and methods

You will need three potting mixes (treatments):

  • organic
  • high-quality mix (added fertiliser, trace elements, pH adjusted and perhaps water crystals.) Look for Nursery Association brand – red ticks
  • lower quality mix (look for Nursery Association brand – black ticks)

You will need to source 12 plastic pots (14-litre capacity each).

Fill four pots with each of the potting mixes ensuring you use the same volume of potting mix in each.

Observations and data collection

Ask your students to observe the type of germination (epigeal) and draw a diagram.

Each week, measure the following for each of the treatments (it's best if all measurements are done on the same day each week):

  • plant height (from soil surface to growing apex)
  • stem diameter at the height of the first true leaf
  • number of leaves (exclude the cotyledons).

Ask your students to observe the colour and size of leaves and the differences between treatments. They can also observe biological variability within the same treatment.

Ask your students to write a report on the experiment and draw three graphs (height, stem diameter and leaf number against time). Then discuss how plant nutrition and water availability (or water stress) can influence crop growth.  

Comparison of potting mixes and fertilisers

Aim, level and context

Aim: to compare the effect of three potting mixes and a fertiliser on the growth and development of sunflower plants

Level: intermediate

Context: in most Australian farming systems, native soil fertility levels are decreasing in response to intensive cropping, leading farmers to increasingly rely on fertilisers to provide crops with the nutrients they require for good growth. Maximising the efficiency of fertiliser use is very important as using too little limits crop growth and lowers yield, while using too much reduces profits and has negative consequences for the environment.

Materials and methods

You will need:

  • three potting mixes (treatments):
    • organic
    • high-quality mix (added fertiliser, trace elements, pH adjusted and perhaps water crystals.) Look for Nursery Association brand – red ticks
    • lower quality mix (look for Nursery Association brand – black ticks)
  • 24 plastic pots (14-litre capacity each)
  • fertiliser
    • none
    • a general fertiliser such as Q5 (5.3%N, 5.8%P, 5.0%K, 13.3%S).

Fill eight pots by volume with each of the potting mixes.

Add the recommended rate of Q5 to four pots for each of the potting mixes. The other four pots will be the controls (no added fertiliser). Remember there are nutrients in the potting mix.

You can vary this experiment by adding fertiliser at different times during plant growth, for example, four weeks after planting or at flowering, etc.

Observations and data collection

Ask your students to observe the colour and size of leaves and the differences between treatments. They can measure fresh weight (and dry weight if drying facilities are available: 60ºC for two days or until fully dry) once plants are mature (back of the head turns brown).

Ask your students to write a report on the experiment and to draw three graphs (height, stem diameter and leaf number against time) for each of the six treatments. Are there any interactions between the treatments? What might be causing these interactions?

Students can also weigh a set volume of each soil substrate dry and wet (ie a pot full) and analyse the differences in soil water-holding capacity between the soil types. They should water the soils until water runs freely out the bottom of the pot and then wait 30 minutes before weighing the wet soil. Collate class results for the trial and then discuss the importance of soil water-holding capacities.   

Interaction of potting mixes and fertilisers

Aim, level and context

Aim: to compare the effect of three potting mixes and two fertilisers on the growth and development of sunflower plants

Level: advanced

Context: in addition to increasing dependence on fertilisers to provide crops with the necessary nutrients, the properties of the soil can have a significant influence on the availability of nutrients from fertilisers after they are applied. Chemical and physical properties of soil such as organic matter, pH, texture and water-holding capacity can significantly change the effectiveness of applied fertilisers.

Materials and methods

You will need:

  • three potting mixes (treatments):
    • organic
    • high-quality mix (added fertiliser, trace elements, pH adjusted and perhaps water crystals.) Look for Nursery Association brand – red ticks
    • lower quality mix (look for Nursery Association brand – black ticks)
  • 36 plastic pots (14-litre capacity each)
  • fertiliser
    • none
    • liquid fertiliser (e.g. Thrive or Aquasol)
    • solid fertiliser such as Q5 (5.3%N, 5.8%P, 5.0%K, 13.3%S).

Fill eight pots by volume with each of the potting mixes.

Add the recommended rate of Q5 to four pots for each of the potting mixes. Another four pots will be the controls (no added fertiliser). Remember there are nutrients in the potting mix.

Add the liquid fertiliser weekly using a watering can to the final four pots at the recommended rate, starting two weeks after emergence. Try to add the same volume of water to each plant. All other pots should receive the same volume of water without fertiliser added.

Observations and data collection

Ask your students to note when the star-shaped structure first appears, the colour and size of leaves and the differences between treatments. They can measure fresh weight (and dry weight if drying facilities are available: 60ºC for two days or until fully dry) once plants are mature (back of the head turns brown).

Ask your students to write a report on the experiment and to draw three graphs (height, stem diameter and leaf number against time) for each of the nine treatments. Are there any treatment interactions? Students can also express the results of each treatment as a mean and standard deviation.

Students can weigh a set volume of each soil substrate dry and wet (ie a pot full) and analyse the differences in soil water-holding capacity between substrates. Water the soils until water runs freely out the bottom of the pot and then wait 30 minutes before weighing the wet soil. Collate class results for the trial and discuss the importance of soil water-holding capacities. Also discuss the chemical differences between soils and how this can affect nutrient availability (ie acid soils, clay soils).    

Comparison of light regimes

Aim, level and context

Aim: to compare the effect of two light regimes on the growth and development of sunflower plants

Level: advanced

Context: plants require light for photosynthesis and use it via phytochrome as a signal for physiological changes such as when to flower (photoperiodism), germination of seeds and setting circadian rhythms. Plants will adapt physically when exposed to lower-than-optimal light conditions.  

Materials and methods

You will need

  • a high-quality mix (added fertiliser, trace elements, pH adjusted and perhaps water crystals). Look for Nursery Association – red ticks
  • eight plastic pots (14-litre capacity each).

Fill the pots by volume with potting mix.

Light regimes:

  • natural light
  • 70 per cent shade cloth
  • shade house or structure supporting the shade cloth. You can erect simple frames using PVC pipe and shade cloth.

Observations and data collection

Each week, measure the following for each of the treatments (it's best if you take the measurements on the same day each week):

  • plant height (from soil surface to growing apex)
  • stem diameter
  • number of leaves.

Ask your students to note when the star-shaped structure first appears, the colour and the size of the leaves and the differences between the treatments. Plants that are in the shade will use less water and grow slower. Ask your students to investigate and discuss why this could be the case.

Ask your students to write a report on the experiment and to draw three graphs (height, stem diameter and leaf number against time). On each graph, they can compare the natural and reduced light. Discuss why reducing light would affect growth and photosynthesis.

Interaction of light regimes and fertiliser

Aim, level and context

Aim: to compare the effect of two light regimes on the growth and development of sunflower plants with and without fertiliser

Level: advanced

Context: light and nutrients are essential for plant growth. Light is required for photosynthesis and provides energy for growth, while nutrients provide the building blocks for growth and all biological processes. In addition, plant nutrient and water uptake is closely linked to light exposure with water moving from the soil to the leaves in response to light-driven stomatal opening.  

Materials and methods

You will need:

  • a high-quality mix (added fertiliser, trace elements, pH adjusted and perhaps water crystals). Look for Nursery Association – red ticks
  • 16 plastic pots (14-litre capacity each).

Fill the pots by volume with potting mix.

Light regimes:

  • natural light
  • 70 per cent shade cloth
  • shade house or structure supporting the shade cloth, simple frames can be erected using PVC pipe and shade cloth).

Fertiliser:

  • none
  • a general fertiliser such as Q5 (5.3%N, 5.8%P, 5.0%K, 13.3%S).

Add the Q5 at the recommended rate to eight pots (place four pots in natural light and four under shade). Equally distribute pots without fertiliser between the shade house and natural light.

Observations and data collection

Ask your students to note when the star-shaped structure first appears, the colour and size of leaves and the differences between treatments. Under shade, plants will use less water and grow slower.

Ask your students to write a report on the experiment and to draw three graphs (height, stem diameter and leaf number against time). On each graph, they should compare natural and reduced light with and without fertiliser. Are there any interactions between treatments? Discuss how and why photosynthesis and plant nutrition are linked.