Maximum Yield USA February/March 2020 | Page 44

YELLOW ALERT 7 REASONS FOR YELLOWING LEAVES by Sally Nex Yellow leaves are part of nature’s process but yellowing can mean trouble for your plants. Sally Nex examines seven reasons why leaves go yellow and ways to get your plants back on track. ellow is an alarming shade to find in the greenhouse, especially when you’re not expecting it. From a limey pallor to the startling Y lemon of truly sick foliage, yellow leaves glare out like warning signs against the lush deep greens of healthy foliage. The trouble is, yellow leaves are the plant health equivalent of a headache — a general symptom that could mean anything. Throbbing temples in humans can be caused by a brain tumor or just a good party the night before. To get to the bottom of exactly what’s wrong, a doctor must investigate further, and it’s the same with plants: while yellow leaves are cause for concern, you’ll need to find other symptoms before you can decide on a treatment. Essentially, when a leaf turns yellow, it is dying. We’re all familiar with the process: it happens every fall, as deciduous trees get ready to slip into dormancy for the winter and shed their foliage. Previously green leaves turn spectacular shades of red, orange, or yellow as they die, then detach from the tree and float to the ground. The fall leaf drop is when trees reveal their ‘true’ colors. The green pigment in most leaves is a chemical, chlorophyll, which enables the plant to photosynthesize and turn sunlight into sugar. As the leaf dies, a layer of cells form along the base of the stalk attaching it to the plant, effectively sealing off the pathway of sugar from leaf to plant. The leaf stops photosynthesizing, levels of chlorophyll drop, and the leaf reveals the underlying pigment — usually yellow carotenoids (also responsible for orange carrots and yellow corn). Eventually, this color also fades and becomes the brown of dead tissue (or the necrotic black of rot). So, a yellowing leaf is a natural process. But in a healthy plant in midsummer, when leaves should not be dying, it’s also a signal something is up. There are lots of possible causes, so we’ve put together a symptom checker of the seven most common reasons for yellowing leaves to help you decide what’s wrong, and what to do about it. 44 Maximum Yield