|Nutrient deficiency - http://www.gardeningknowhow.com/|
Bedding geraniums (Pelargonium hybrids) are among the most popular of all flowering garden plants and while they seem to thrive on neglect this doesn't meant that they are indestructible. In the majority of hybrids the parent plants originate from southern Africa. They are naturally drought and heat tolerant, and can even tolerate minor frosts. As selected hybrids, they express an even more vigorous and hardy characteristics although they are still fairly intolerant to freezing conditions.
|Bacterial wilt - http://www.cals.ncsu.edu/|
Fungal diseases can also be a contributing factor to yellow leaves but this kind if infection usually only takes hold on plants already weakened by over watering. If you are unlucky enough to get an infection of bacterial wilt (characterised by collapsed leaves which turn chlorotic and eventually necrotic) then there is no treatment other than to burn infected plants. Verticillium Wilt will give similar results. There is no chemical control for verticillium wilt and so the best course of action is to destroy the plant.
For plants which receive any kind of cultivation the most common reason for yellowing leaves is overwatering. Pelargonium species have evolved in regions of low rainfall and as such have roots which are intolerant to prolonged conditions of damp or waterlogging.
|Verticillium wilt - http://pnwhandbooks.org/|
If the root hairs have died then they are unable to draw in any water or nutrients into the body of the plant and so the plant begins to dry out. Luckily, geraniums have an effective method of coping and recovering from this kind of damage. Nutrients and moisture are drawn out of the older, lower leaves and are diverted to produce new foliage and root growth. New leaves are the most efficient regarding photosynthesis and help to produce the much needed energy to powers the growth of new root hairs.
This is the important point. The green chlorophyll pigments are broken down in the older, lower leaves to produce the building blocks required for the new leaf growth. As the chlorophyll breakdown and are removed the yellow carotenoid pigments responsible for protecting the chlorophyll against damage from ultraviolet radiation become visible. This is why geranium leaves go yellow when subjected to water stress.
|Under watered - http://2.bp.blogspot.com/|
Extended periods of dough will case the same symptoms as waterlogging which is why the characteristic yellowing of the lower leaves occurs with both over watered and under watered plants.
Nutrient deficiency usually occurs on mature pot or container grown plants. A lack of nitrogen, magnesium or potassium will cause all plants to struggle to produce the green chloroplasts necessary for photosynthesis. Without the green chlorophyll present in the leaves the yellow carotenoid pigments will begin to show through.
For related articles click onto the following links:
GERANIUM pratense 'Johnson's Blue'
HOW TO GROW GERANIUMS FROM SEED
WHY ARE MY GERANIUM LEAVES TURNING YELLOW?