Yellow foliage only where you want it!
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Nanette Londeree
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Want a splash of color in your rich green garden bed? There are trees, shrubs, and perennials that can add a sparkle throughout the growing season, especially plants with brilliant sun-yellow foliage. New plant varieties are constantly being introduced that seem to defy nature with colors from soft-butter yellow to chartreuse. These anomalous leaf colors can add pizazz if they're intentional. But if you have leaves that are supposed to be emerald green but instead look more like shades of lemon, it's time to take a closer look and see what's going on.
Robinia pseudoacacia 'Frisia' is a stunning tree with bright yellow leaves in spring that turn to chartreuse in summer. Photo: Nanette LondereeGreen leaves that fade and turn yellow suffer from chlorosis, a condition caused by the loss of chlorophyll. Chlorophyll is the green pigment that gives plants their color and helps them create their own food through photosynthesis. Any reduction in chlorophyll during the growing season can reduce plant growth and vigor, and plants often produce smaller fruits of poor quality.
Chlorosis is a common problem and can often be remedied if you know what caused it. Overwatering, root damage, nutrient deficiencies, herbicide damage, viruses, and even excess sunlight and high temperatures can produce the yellowing of various types of foliage. As you put on your detective hat, take note of a few things about your plant: where do you see the fading colors – young leaves at the top of the plant or older ones closer to the base? Are the leaves a solid yellow, or are the leaf veins green? Is the soil wet?
The older, lower leaves of this citrus plant are yellow, while the new growth at the top is green, indicating a nitrogen deficiency. Photo: UC ANRFrequent causes of chlorosis in ornamental plants are deficiencies of nitrogen or iron. Soil with a low nitrogen level causes the leaf color to fade to light green and then yellow. The change appears first on older (lower) leaves and gradually affects the entire plant. New foliage will be the typical color of healthy foliage. Iron deficiency is the opposite – it's first seen in young leaves and will gradually extend downward. Leaves will be light green to yellow with contrasting dark, narrow green veins and may eventually show marginal scorching and bleaching.
Most plants get adequate nitrogen from decomposing organic matter, soil, and air. Exceptions include fruit and nut trees, palms, roses, and some other heavily flowering shrubs or plants growing in soil that is very sandy or in containers or planter boxes. When other types of established woody plants show signs of nitrogen deficiency, it's usually related to lack of air in the soil, poor drainage, mechanical injury to roots, soil compaction, root-feeding insects or nematodes, or root decay pathogens that prevent the plant's roots from taking up available nitrogen.
Our soils generally have adequate iron, but it may not be available in a form that plants can use if the soil is alkaline – and has a high pH. Azaleas and rhododendrons, gardenias, and citrus, which do best in acidic soil, are especially prone to iron deficiency when soil pH exceeds 7.5. This malady also occurs when soils are cool, high in calcium, poorly drained, or waterlogged, and like nitrogen deficiency, root health is impaired by root decay pathogens, nematodes, or other biological or physical causes.
Symptoms of iron deficiency include light green to yellow leaves with contrasting narrow dark green veins, marginal scorching, and bleaching.Back to your observations of the plant. If symptoms are due to unhealthy roots, poor soil conditions, or improper care, correct those problems first. If nitrogen is required, choose a slow-release organic fertilizer that is primarily nitrogen. There is no need to add phosphorus or potassium if not needed.
To lower soil pH and make iron more available to plant roots, you can add amendments, such as powdered or pelleted elemental sulfur or compost. This can be a slow process and may take more than a year. Another option is to apply a chelated iron fertilizer, in which the iron is combined with an organic chemical called a chelate that helps keep the iron in a plant-available form.
