The plants commonly known by gardeners as geraniums are in fact pelargoniums. There are thousands of varieties grown for their attractive, colourful flowers or aromatic foliage. There are both annual and perennial types and varieties that creep along the ground or are erect bushes. Pelargoniums grow fast and flower profusely and are a staple of the low-maintenance beginner's garden as well as a popular container and bedding plant.
Pelargonium leaves turning red is a sign that the plant is stressed in some way. This may be because the plant is receiving too much water or too much sunshine, or has been planted outdoors too early or in mineral-deficient soil. Pelargoniums that are planted too close together also tend to develop red leaves. Diagnosing the problem is a case of looking at the exact conditions your geranium plant is growing in.
Red leaves on a pelargonium plant are often a sign that it is receiving too much water. These plants need very little water, and should be planted in well-draining soil that is kept barely moist. Potted pelargoniums with a saucer under the pot often suffer from waterlogging and will develop red leaves. If overwatering or waterlogging is not corrected, pelargoniums will drop leaves and die.
Pelargoniums planted in direct sunshine will turn red as a defence, starting with the older leaves. Moving potted plants into semi shade or providing some shade for plants out in the garden will remedy the situation.
Cold weather will turn pelargonium leaves red, especially if they are planted out too early in the year. Annual plants that have been grown from seed or as cuttings in the house or under glass should be exposed to colder outdoor conditions gradually. If your pelargonium leaves turn red just after being planted or moved outdoors, it is likely because it is still too cold for them. In the fall, red leaves are a sign that it is time to move pelargoniums back indoors or to take cuttings from annual cultivars.
Pelargoniums are fast growing with small root systems and are therefore vulnerable to phosphorus and trace mineral deficiencies. Treat any plants with red leaves or slow growth with a high-phosphorus liquid fertiliser applied every two weeks during the growing season. Also apply a trace mineral-rich fertiliser or foliar spray containing boron and magnesium. Cold temperatures inhibit the uptake of trace minerals in pelargoniums. Cover the soil around your plants with black barrier cloth to increase soil temperature, or move plants indoors during cold weather.