The appearance of a plant gives lots of clues as to its health and well-being. Yellow leaves may be an indication of a problem and are best investigated to find out the cause, ideally at an early stage before any problem becomes too advanced. The horticultural term for yellowing leaves is chlorosis.
Why do leaves turn yellow?
Yellowing leaves develop for a wide variety of reasons. Often yellow leaves are a sign that something is wrong, and action is needed. For example, your plant may be telling you it has been given too much water or needs feeding. Leaves turning yellow can be a sign of pest or disease problems, an indication that the plant is growing in the wrong place, or due to extremes of rain. Though in some cases, such as with evergreen shrubs and trees, some yellow leaves are simply a natural part of the plants’ life cycle.
Causes and remedies of yellowing leaves on plants
Overwatering – older leaves turn yellow first. Overwatering is easy to do on house plants that sit in decorative potholders, and patio plants are susceptible when pot drainage holes become blocked or if water has built up during periods of heavy rain. Overwatering may be obvious on first inspection if the plant is sitting in water. If this isn’t the case, check the compost moisture by the simple means of sticking a finger in the compost to feel it. Remedy by standing the plant in a light, airy spot and clear the drainage holes so the root ball drains completely and wait until the compost is on the dry side before watering again. Cut off leaves that have become completely yellow.
Lack of nutrients – depending on the type of nutrient deficiency, leaves discolour in different ways. Yellowing young leaves and general weak growth is likely to be nitrogen deficiency while leaves that turn yellow around the edges are likely to be due to potassium deficiency. Both are major nutrients that are present in general fertilizers. Leaves that are yellowing between the veins are suffering from iron deficiency – this can occur on acid-loving plants that are growing in unsuitable (limy) soil. While this can be corrected in the short term with a suitable plant tonic, moving or repotting the plant into lime-free or ericaceous compost is a long-term answer to the problem.
Under-watering – yellowing of young leaves may result from lack of water. Checking the compost by inserting a finger is a quick and easy way to test. Or, if the plant is lift-able, test the weight – if very light, it’s likely to be dry. Stand the pot in a sink or bucket of water for an hour or so to completely re-wet the compost.
Low light – house plants that need good light, but which are placed in dark spots are likely to produce growth overall that is yellow green, becoming more yellow, instead of green. Affected leaves are likely to turn brown and then die. Check the plant’s growing requirements and move to a suitable site, though don’t move a plant from deep shade to bright sun in one go as the foliage could be scorched by strong sunlight.
Pot bound or root bound plants – these are likely to exhibit pale or yellowing leaves because the roots are congested. Remedy by potting on, into a larger container.
Natural leaf-drop – this occurs on evergreen trees and shrubs such as bay, olive, holly, laurel, and many more, particularly in summer, when a small proportion of the oldest leaves are shed by the plant.
Plants that may suffer from yellow leaves
House plants
House plants with large leaves are particularly noticeable when the foliage changes colour, so yellow leaves on Swiss cheese plant, rubber plant or calathea, really catch the eye. The most common cause of yellow leaves on house plants is overwatering, and this should be the first point to check.
Fleshy-leaved plants such as Chinese money plant and types of cacti that develop yellow leaves are highly likely to be caused by an excess of water, because these plants naturally store water in their stems and leaves and are susceptible to overwatering. Orchids may also develop yellow leaves from overwatering as they need to be grown in a very open and free draining compost.
Growing plants in conditions that are too dark may also be a cause of yellow leaves – even shade tolerant house plants like peace lily may develop leaf yellowing if placed in a dark spot.
Pot-bound plants (where the plant has grown too big for its pot) is also common with house plants which are often sold in small pots, particularly trailing ones like devil’s ivy.
Ornamental garden plants
The often-repeated mantra of ‘right plant, right place’, avoids many problems in the first place, including that of yellowing leaves. Select plants that suit your garden’s soil type and the aspect (the amount of sun or shade a site receives). Check the level of acidity or alkalinity (the soil pH) before buying ericaceous or lime-hating plants such as camellia, Pieris, and rhododendron.
Soil fertility is important for ‘hungry’ plants such as roses, sweet peas, and dahlias, that need a rich, fertile soil and which may develop yellow leaves if short of nutrients.
Yellow leaves may also be a sign of pests or diseases, and inspection is likely to reveal clues as to the problem. On roses, yellowing is often seen in conjunction with black spot disease, for example.
Vegetable plants
Yellowing leaves can be a sign of disease, which requires prompt action to minimise the risk of the disease spreading. Common problems include yellow leaves on mature tomatoes and potatoes are likely to be the fungal disease, late blight, while cucumbers and courgettes may suffer from cucumber mosaic virus.
Remove and bin yellow leaves that are affected by fungal or bacterial disease. With virus diseases, dispose of the whole plant, as there’s no cure. In future, choose plant varieties that are naturally resistant to diseases.
from BBC Gardeners World Magazine https://ift.tt/JAZomsY
