Geranium leaves turn red when they are exposed to low temperature. They also turn red when the soil pH is too low, when they are under-watered or over-watered, and when they are deficient of phosphorus or magnesium. When this plant is under stress, its leaves may turn red.
Geraniums can survive a light frost but may develop red leaves as a sign of displeasure. Moreover, the availability of nutrients is also affected by the pH of the soil. To prevent geranium leaves from turning red, make sure the soil has a pH of 6.5.
Watering geraniums too much or too little can cause stress to the plants and lead to reddening of leaves. Lastly, to prevent nutrient deficiency, using foliar feeds on the plant is recommended. Foliar feeds can also be applied to the geranium leaves so they absorb the nutrients directly.