Sick plants:-
If a plant is sick, its leaves may turn yellow. Yellow leaves could be due to a number of problems:
If a plant is sick, its leaves may turn yellow. Yellow leaves could be due to a number of problems:
1. Summer heat is too great for the plant.
Although Bauhinia can survive in the sun during the summer, it will display heat stress. Note that the tips of the leaves are scorched and the yellowing (or in this case whitening) is worst around the edge of the leaf. This is because water is being transpired so fast from the leaves that it cannot reach the edges. Since Bauhinia is a tree, you cannot easily move it in to the shade during the summer and you will just have to accept that it may lose much of its leaves (but new ones will appear in the autumn). In smaller plants that are heat stressed during the summer, it is best to cut back the plant to a smaller size, so that it has less leaves to lose water and the stems are shorter (so water can reach the leaves at the tip). I would always do this for Madagascan periwinkles, as an example. |
2. Too shaded (needs sun).
Some plants (in this case, Allamanda) must be grown in the sun or their leaves turn yellow. Note that the leaves are not damaged (unlike heat stress above) and are a uniform yellow, with green veins. Also note that in the background, you can see Asystasia (top left) and Impatiens (left), both with dark green leaves, because they like living in the shade. 3. Lack of nutrients, like nitrogen, will produce the same yellowing of the leaves. You should give fertiliser e.g. N/P/K every month. |
4. Overwatering causing root rot.
Leaves look similar to (1), but without scorching at the tips. If the soil is waterlogged, there will be no air present and the roots will die from lack of oxygen. In this situation, the plant is unlikely to recover. If you pull it up, you will find it has no roots. Madagascan periwinkles (this photo) are especially prone to this sudden death. 5. Underwatering will result in leaves as in (1), since there is not enough water to reach the leaf tips. |
Other leaf problems that you need to watch out for include:
6. Twisted leaves. Could be due to a virus, but most likely cause is a plant-sucking bug, such as a mealybug. These inject chemicals in to the plant causing deformed growth of the leaf. You will need to inspect closely because: a) they hide among the leaf folds that they have created; b) baby mealybugs are extremely small and difficult to see.
Mealybugs are Homopteran bugs, but closely related are Heteropteran bugs. These are even worse at injecting leaf-deforming chemicals and also destroy the individual cells of a leaf to produce pale areas. In addition, Heteropterans destroy the plant cells one by one (note the pale areas on the leaves). This photo of Thunbergia erecta is thus more likely due to Heteropterans. Hibiscus are often attacked by tiny black Heteroptera. |
7. Leaf miners. These are due to the larvae of tiny Agromyzid flies. Each larva spends its life inside 1 leaf (so the tunnel gets wider as it gets older). By eating the chlorophyll-filled cells and damaging the veins, the larva reduces the efficiency of the leaf. However, plants (unless very young) are little affected by considerable leaf damage and so in most cases, leaf miners can be ignored. But, if the infestation is severe, the plant can be killed.
Because the larva is inside the leaf, they can only be killed with systemic insecticides. Alternatively, you can cut off infected leaves and destroy them (but you may have a rather bald plant afterwards). |