Geranium

Light and exposure

The pelargonium wants full sun. The more it gets, the more it flowers: aim for at least six hours of direct sun a day, a balcony that catches the midday and afternoon sun, a window box against a pale wall that reflects the heat. In shade it grows leggy, turns green and flowers poorly.

It is a plant for pots and window boxes above all, which is convenient anywhere it will have to come indoors before the frosts. In open ground it works too, but then give it the warmest and sunniest bed, and still take it in before the frosts if you want to keep it. Under a scorching summer, light shade during the hottest hours keeps the flowers from burning and costs nothing in flowering.

Watering

Water regularly in summer, but let the surface of the compost dry out a little between waterings. The pelargonium copes better with a slight shortage than with an excess: a permanently waterlogged substrate rots the base. In hot weather a window box may need water every day; in overcast weather, space it out.

Water at the base, not on the flowers or foliage, and empty the saucers after rain. Water standing at the bottom of the pot is enemy number one, especially in late season when the nights turn cool and the compost dries slowly.

Soil and repotting

It needs a free-draining compost, never heavy or waterlogged. A planting compost for geraniums, lightened with a handful of sand or perlite, does the job very well. At the bottom of the pot, a layer of clay pebbles or gravel keeps the roots from sitting in water.

Repot or set out the window boxes as spring arrives, in a container with drainage holes. Do not crowd the plants: a pelargonium given room spreads and flowers more. In a window box, allow one plant every twenty to twenty-five centimetres.

Temperature and humidity

This is the critical point. The pelargonium is frost-tender: it already suffers around 3 or 4 degrees and a frost kills it outright. It is genuinely perennial outdoors only in USDA zones 10 and 11. It thrives between 15 and 25 degrees, in the open air, right through the growing season. Never put the pots out while night frost is still possible, a single night scorches the foliage: wait until nights settle above 8 to 10 degrees.

Above 30 degrees it suspends its flowering without being ill for all that, and takes it up again as soon as the heat drops. As for humidity, it prefers dry, well-ventilated air to a stuffy atmosphere. A window box crammed against a wall, poorly ventilated in damp weather, encourages disease. Let the air circulate between the plants and avoid wetting the foliage in the evening.

Pruning and upkeep

Remove faded flowers very regularly, snapping or cutting the flower stem at its base. This simple gesture keeps the flowering going continuously: a pelargonium that is kept clean flowers from late spring right up to the first frosts, a neglected pelargonium exhausts itself making seed and slows down.

Pinch the young stems at the start of the season to get bushy plants rather than lanky ones. Feed throughout the flowering period: a special fertiliser for flowering plants every eight to fifteen days in full season. Abundant flowering means high demand, a poorly fed pelargonium yellows and flowers less.

Pests and diseases

In a damp climate, two diseases keep coming back. Grey mould, a grey felting on flowers and leaves in mild, stuffy weather, settles on the faded parts left lying around. Pelargonium rust marks the underside of the leaves with small brown powdery rings, especially in a damp late summer. In both cases: ventilate, remove the affected parts, do not wet the foliage.

Excess water remains the real danger. Waterlogged compost rots the collar and the base, and the plant collapses all at once. As for pests, watch out for aphids as growth restarts in spring and the pelargonium caterpillar, the leaf roller, which bores into the buds and hollows out the stems.

Propagation

Stem cuttings in late summer are astonishingly easy, and it is the best way to renew your plants for free. Take a non-flowering stem ten centimetres long, remove the lower leaves, let the cut dry for an hour or two, then insert it into a light, barely moist compost. Rooting takes two to three weeks.

You can also sow under warmth at the end of winter, but it is slow and the young plants only flower late. For most gardeners, the late summer cutting remains the royal road: it gives plants ready to take off again the following spring.

Toxicity

Barely toxic to humans, with no real danger on contact. It is however toxic to dogs and cats: eating leaves or stems causes vomiting, loss of appetite and lethargy. Keep the pots out of reach of a pet that nibbles.

Month-by-month care calendar

Green Hub tips

• Never put pelargoniums out while a night frost is still possible. A single night is enough to scorch all the foliage: wait until nights hold above 8 to 10 degrees.
• Bring the pots into a frost-free place in autumn, in a cool, bright room between 5 and 10 degrees, cut back by half and barely watered: they will start again in spring.
• Remove faded flowers without delay, it is the gesture that keeps the flowering going and makes it last from late spring right up to the first frosts.
• Take cuttings at the end of summer to renew your plants for free: a ten-centimetre stem roots in two to three weeks in a light compost.