I have a couple of plants where the leaves are starting to turn yellow, and I wondered why. It turns out there are no straight answers for this, and you really need to understand plants to find out the reasons - and a gardening guru I am not! But a little detective work gave me some clues.
Dehydration is the number one problem, especially after a sudden hot spell. The plant will shed leaves in order to try to prevent the plant dying. Sometimes the way the plant is being watered can be the problem – to encourage root growth, water deep, wait until the soil begins to dry, and then water again.
Overwatering is another cause, too much is just as harmful as too little! If there’s a green crusty appearance to the soil surface, this is algae, and it too is an additional symptom of overwatering. If the plant is in a pot, check if the pot has enough drainage, and if roots have turned black, this will indicate decomposition and almost certainly a death sentence. Try repotting it, cutting the black roots out and leaving only the healthy white ones. Sit back, and hope for the best!
Pests can be a cause of yellowing leaves or at least yellow spottiness, and infestations can be caused by one of the following: mites, aphids, mealybugs, thrips, scale, or whiteflies. Repeatedly washing the plants or applying an insecticidal or horticultural soap are treatments that are often effective as well as environmentally safe. Check the underside of leaves, which is where the little critters will hide - get your magnifying glass out, and try to identify any you find.
Nutrient deficiencies can be a cause of yellowing leaves too, and if the top leaves of your plant are yellowing, or there is an unusual pattern of yellowing on the leaves, such as the veins remaining dark while the rest of the leaf is turning yellow - it’s most likely a nutrient deficiency. The type of deficiency is where you have to bring your detective skills in, as each symptom needs different treatment. Deficiencies in potassium, nitrogen, calcium, magnesium and iron can all cause yellowing, some with or without brown spots or veins – and on top of this, you are supposed to test your soil to maintain a pH below 7!
Temperature - a significant temperature change can leave the tips of your plants looking burned. Shifts in temperature–hot or cold–affects the health and colour of leaves. When prolonged cool temperatures or late spring cold drafts occur, your plants–especially tender vegetable plants such as tomatoes and cucumbers may lose their colour. Keep an eye on evening temperatures as well.
Too little sunlight can cause yellow leaves to appear, and with partial shade varieties, too much sun can be the cause. Leaves are the solar panels of the plant; the place where their energy is produced. When a plant becomes thick and bushy, it sometimes blocks light from reaching inner and lower leaves. These leaves cannot photosynthesize and chlorophyll production comes to a halt. Since the leaves are non-productive, the plant no longer needs them.
Old Age– Sometimes a plant has just outlived its natural plant life, and is dying off!
Mentioned above is testing your soil for pH. Fortunately, you can test soil pH without a soil test kit for next to nothing, and you probably have all you need in the kitchen – white vinegar, baking soda and distilled water. If you are really keen to try, collect 1 cup of soil from different parts of your garden and put 2 spoon fulls into separate containers. Add 1/2 cup of white vinegar to the soil in the first pot. If it fizzes, you have alkaline soil, with a pH between 7 and 8.
If it doesn’t fizz after doing the vinegar test, then add distilled water to the second container until the soil is muddy. Add 1/2 cup baking soda. If it fizzes you have acidic soil, most likely with a pH between 5 and 6.
If your soil doesn’t react at all, it is neutral with a pH of 7 and you are very lucky!
Lastly, please remember that whatever the cause of your plant’s illness, it may take weeks or even months for a plant to recover and return to normal growth. Good luck with your detective work, Sherlock!