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/
Leaf yellowing in geraniums is usually the result of environmental stress, more specifically water stress, caused by either under watering or overwatering. This is most commonly expressed in pot or container grown plants which are more prone to such conditions. Nutrient deficiency will also cause yellowing of the leaves and is the easiest to treat.

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.

Over Watering

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/
Under overly-wet conditions the specialist root hairs (which are responsible for drawing water and nutrients directly from soil) are unable to absorb oxygen (required for plant cell metabolism) from air gaps in the soil as the air has been replaced with water. These specialist root hairs effectively begin to suffocate and will quickly die if conditions do not improve.

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 Watering

Under watered - http://2.bp.blogspot.com/
While geraniums are perfectly suited to dry conditions they are not cacti and neither do not originate from desert regions. So they will require watering, although not as often or as much as most other regular, garden plants. Of course the absence of water will again cause water stress to the plant and while the specialist root hair are still functional a lack of moisture will not cause immediate damage.

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

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'

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