Rhododendron leaves turn yellow because of sun exposure, according to the Victoria Rhododendron Society. Sunlight tends to bleach exposed leaves. When this occurs, adding nitrogen helps turn the leaves green again.Know More
The Victoria Rhododendron Society adds that older leaves naturally turn yellow, and they are replaced every one to three years. When the leaves are yellow but the region along the veins remains dark green, the plant likely suffers from chlorosis, which is caused by a magnesium, nitrogen or iron deficiency. In this case, the soil’s pH must be checked. A very high pH hinders the plant's ability to absorb necessary trace metals. If the pH is close to 5.5, adding Epsom salts is a recommended solution. If this does not work, it helps to add a nitrogen source and chelated iron.
Chlorotic leaves are usually a sign of iron deficiency and can be dealt with by using iron sequestrene, according to the Millais Nurseries. Chlorosis is also caused by poor conditions, such as drought, soil pH or drainage problems. Over-fertilizing must be avoided, as the edge of the leaf will likely turn dry and brown. Chicken and farmyard manures are not advisable as fertilizers, because they contain too much nitrogen for rhododendrons. Similarly, bonemeal must be avoided, as it has too much calcium.Learn more about Botany
Azaleas and rhododendron shrubs, both members of genus Rhododendron, should only be fertilized sparingly in late autumn or early spring, if at all. The American Rhododendron Society suggests that these shrubs should only be fertilized if they obviously display some sort of deficiency.Full Answer >
Unless forced to turn red through carefully controlled exposure to light, poinsettia leaves are naturally green. The poinsettia plant needs 14 hours of darkness per day for eight weeks before it acquires the bright-red leaves traditionally associated with it.Full Answer >
Fruits turn brown due to exposure to oxygen. This causes the oxidation of phenolic compounds that are naturally present in fruit tissue. This reaction takes place in the chloroplasts by means of an enzyme called polyphenol oxidase.Full Answer >
Trees lose their leaves in the winter as they enter their dormant period in order to protect cells from freezing temperatures. In this stage, they no longer need the food that the leaves produce. Trees respond to changes in available light and begin to form a layer of dead cells in the stem of their leaves, so each leaf breaks from the limb and falls to the ground.Full Answer >