Pelargonium (geranium)

Pelargoniums, commonly known as geraniums, are a large, diverse group of mostly evergreen and tender plants used as bedding or houseplants. Although pelargoniums are often called geraniums, this is not correct, as the true geraniums are hardy herbaceous plants.

Pelargonium

Quick facts

Common name Pelargonium, geranium
Botanical name Pelargonium
Group Houseplants, bedding or greenhouse plants 
Flowering time Mainly spring-summer
Height and spread 10-90cm (4in-3ft) by 7-50cm (3-20in)
Aspect Most need full sun, some tolerate partial shade
Hardiness Tender
Difficulty Moderate to Easy

Pelargonium groups

Most pelargonium cultivars are divided to six groups:

  • Regal: Bushy evergreen perennials and shrubs with rounded leaves sometimes lobed or partially toothed, producing single rarely double flowers in shades of mauve, pink, purple or white grown for outdoor or indoor display
  • Angel: Similar to regals, but more compact and bushy. Mostly derived from P. crispum
  • Ivy-leaved: Trailing evergreen perennials with stiff fleshy leaves and single or double flowers used for hanging baskets or containers
  • Zonal: These are mainly derived from P. inquinans and P. zonale. Upright, bushy, succulent-stemmed perennials grown for their single or double flowers. Some have attractive foliage. This type is most commonly used for bedding displays
  • Scented-leaved: Shrubby evergreen perennials and shrubs, which are mainly cultivated for their scented and often distinctly lobed, toothed or incised or variegated leaves
  • Unique: Shrubby evergreen perennials that do not fall into the above categories

Cultivation notes

Pelargoniums can be grown in borders or containers. In borders or beds, plant in fertile, neutral to alkaline soil. Most prefer full sun. Regal cultivars prefer partial shade and zonal cultivars will tolerate some shade.

For indoor or outdoor container cultivation use well drained soil-less multipurpose compost or soil-based compost such as John Innes No 2.

If growing indoors or under glass grow in full light shaded from scorching midday sun.

Watering and feeding

Water moderately during the growing season from spring to summer, avoid the compost becoming too wet and provide good ventilation. Apply a balanced liquid fertiliser according to the application instructions on the packet, every 10-14 days in spring. Once flowers start to form, switch to a high potassium fertiliser such as tomato feed. Water only sparingly in winter. Many will continue flowering if kept at 7-10°C (45-50°F).

Overwintering

Pelargoniums are usually grown as annuals, but with a little care, they can be carried through the winter using one of the methods described below:

Method 1: Overwintering by taking cuttings

This method is useful where there are large numbers of plants to overwinter; it suits soft-stemmed pelargoniums that cannot be kept in a semi-dormant state as per method 3.

  • Take softwood cuttings in late summer and discard the old woody plants
  • Once the cuttings have rooted, they can be overwintered in trays of compost kept on a well-lit indoor windowsill
  • Water the tray only sparingly in winter, allowing the compost to dry out between watering
  • Start feeding in late winter, giving a balanced liquid fertiliser every seven to 10 days according to the application instructions on the packet
  • Pinch out the shoot tips in late winter to encourage bushy growth
  • Pot up individually in mid-spring
  • Harden off and place outside once the risk of frost has passed

Method 2: Overwintering in containers under glass

This method is useful where there are only small numbers of plants to overwinter and space in the glasshouse or conservatory to keep them.

  • Lift and pot those growing in the soil before the first autumn frosts. Cut them back to 10cm (4in) at this time. Use John Innes No 1 potting compost 
  • Keep the plants in their pots over winter, in a light, frost-free position
  • Very little water is needed until growth resumes in spring
  • Re-pot into John Innes No 2 potting compost in mid-spring
  • Harden off and place outside once the risk of frost has passed

Method 3: Overwintering in a semi-dormant state

This method is only suitable for varieties with tough woody stems, but is useful where there are large numbers of plants to overwinter.

  • Lift plants from the ground or pot and shake off excess soil or compost from the roots
  • Allow the foliage and stems to dry off in a frost-free place and then wrap the plants in newspaper or hang them upside down from the shed roof (which must be frost-free)
  • Plants which survive and show signs of growth in early spring should have their roots soaked in water for a few hours before being potted up cut back to about 10cm (4in)

Pruning and training

Many pelargoniums are naturally bushy. They can be pinched back in spring or early summer to encourage further branching. Tall, vigorously growing cultivars can be trained on canes to form a pillar. Young plants of trailing ivy-leaved cultivars are best pruned back to promote branching.

Deadhead plants regularly to promote flowering.

If kept actively growing all year round, the majority of pelargoniums can be renovated by hard pruning in spring. Those that are overwintered in a semi-dormant state (method 3) have already been cut back in autumn and should not need further pruning.

Propagation

Zonal F1 and F2 (bedding type) pelargoniums and species pelargoniums can be propagated by seed. Sow in late winter and grow on in a protected environment such as a heated greenhouse.

Many bedding type pelargoniums are sold as plug plants from mail order suppliers.

Pelargoniums can also be propagated by taking softwood cuttings from spring to autumn.

Problems

Pelargoniums are easy to grow, but there are a few things worth watching out for:

  • Pelargoniums can suffer from viruses transmitted by sap sucking insects such as thrips, or by cross-handling of plants and tools, like knives or secateurs
  • Roots of container grown plants are prone to vine weevil larvae damage
  • Poor air circulation and damp conditions favour diseases such as grey mould and rust
  • They can also suffer from leafhopper, root mealybug and whitefly
  • Dense clusters of distorted leafy shoots, often close to ground level, are leafy gall

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