Much more than just the ‘starch test plant’, geraniums can be used for a wide variety of topics from adaptation to pollen tube growth. Coming from the arid regions of South Africa, geraniums are well adapted to the rigours of life on a lab windowsill.

Practicals

• Photosynthesis: starch as a storage substance. Resource: Can leaf discs make starch in the dark?  (includes teachers' notes, students' sheet and illustrated worksheet) Variegated Pelargonium leaves are best for this activity.

• Photosynthesis: Resource: The response of leaf discs from sun and shade plants to green light

• Photosynthesis: Resource: Investigating the behaviour of leaf discs for photosynthesis

• Cell shape on stomatal peel. Resource: Measuring stomatal density  

• Sexual reproduction growing pollen tubes. Resource: Pollen Tube Growth (includes teachers' notes, students' sheet and illustrated worksheet).

Teaching Topics

• Photosynthesis
• Adaptation
• Cells
• Stomata
• Leaf Variation
• Plant Defences
• Pollen Tubes
• Rooting hormones

Image Resources

• Variegated geranium (Pelargonium) leaf
  

Description

Geranium (Zonal Pelargonium), Pelargonium hortorum hybrid.

Note: although this plant is commonly called a ‘Geranium’, its botanical name is a Pelargonium. The plant family with the Latin name Geranium are the plants commonly known as 'Cranesbills'.

Originates from South Africa, but has been widely bred and hybridised by plant nurseries. There are several types:

• ‘Common Geranium’ with brown-ringed markings: a robust plant very suitable for keeping in the lab, easily propagated by taking stem cuttings.
• ‘Regal Geranium’, with pointed, serrated leaves: this needs a precise watering routine, as it prefers compost that is just moist during the flowering season and even drier during winter. 
• ‘Ivy-Leaved Geranium’, a climber better suited to hanging baskets than to the lab.
• ‘Peppermint Geranium’, again a climber with mint-scented leaves, and not well suited to the lab. This plant needs shading from hot sun.

Variegated and non-variegated types are available: varigated types are preferable for certain activities.

Growing and sourcing

Adapted to: Most of the Pelargoniums we grow have their origins in the arid, sunny landscapes of Southern Africa.

Obtaining: Buy mature plants from garden centres, nurseries and markets. Common Geraniums, as described above, are the most appropriate for the lab.

Propagating: Several new plants can be propagated from one mature plant - the best time to do this is in summer using stem cuttings. When taking cuttings, some growers recommend a rooting hormone, while others say it is unnecessary. See our resource on taking geranium cuttings.

Compost: Use a soil-based compost such as John Innes No. 2

Light: Bright light: keep them on a sunny windowsill.

Water: From spring to autumn, water regularly, but make sure the roots are not sodden. Never let them stand in a saucer of water. In winter, keep the soil just moist to touch with a light watering when the compost dries out. Over-watering kills these plants, as does high humidity. 

Temperature: Normal room temperatures are fine, but if possible, give them a winter 'rest' at 10 degrees C, perhaps in a glass porch. 

Feeding: Feed with a fertiliser such as 'BabyBio' or 'Tomorite' once a fortnight during the summer months.

Eventual size: About 60cm high.

Pruning: Prune your Geraniums in early spring by as much as a third of their total volume.

Notes: Remove dead flowers. These plants can be prone to white-fly.