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by Nanette Londeree, Consulting Rosarian

Chlorosis is a fancy name for yellowing of green plant foliage due to a lack of chlorophyll development. When this happens, leaves donít have their normal green color; they may be pale green, yellow, or yellow-white. The affected plant has little or no ability to manufacture carbohydrates and may die unless the cause of its chlorophyll in-sufficiency is treated. Yellowing leaves are generally a good indicator that your rose is suffering. While this condition can affect just about any green plant, roses, especially in their rapid growth stage, can be prime candidates.

Symptoms of chlorosis can vary depending on several factors. Mild chlorosis usually starts as a paling (lighter green to lime-green color) of interveinal (between veins) tissue, whereas a yellow color indicates a more serious condition. In some cases, only part of the plant is chlorotic. Affected areas (or the entire plant) may be stunted or fail to produce flowers. In addition, chlorotic leaves are more prone to scorching and leaf diseases. In general, the longer the plant has been chlorotic, the more severe the condition.

Many different situations can cause chlorosis, with one of the most common being low iron. Plants need iron to make chlorophyll, the green color in plant cells necessary for photosynthesis. Soils with pH above 6.5 bind up the iron and make it unavailable to the roots. High concentrations of limestone (calcium carbonate) make the soil more alkaline (higher pH). Roots may be damaged by a lack of oxygen in over watered or poorly drained soils, typical after a rainy spring. Chlorosis is aggravated by extreme soil temperatures and con-ditions that restrict air movement into soil like plastic sheet mulching, compaction, along with soils that contain high levels of zinc, manganese, phosphorus or copper.

Before you can try to eliminate the chlorotic condition, you need to look closely at the affected leaves to determine the cause. If a plant is iron-deficient, its newest leaves are more yellow than old ones, and the interveinal areas show chlorosis while the veins remain green. If the chlorosis is a result of oxygen deficiency (as a result of over watering or inadequate drainage), the veins of the leave will be yellow, while the remainder of the leaf is green. You can distinguish chlorosis from nitrogen-deficiency where the old leaves are yellow and the new ones green and the veins are the same color as the rest of the leave.

Once youíve identified the cause, you need to remediate it. If the cause is oxygen deficiency, then evaluate your watering, drainage or soil compaction, and correct the condition. The fix may be simple like adjusting your watering schedule or more involved if you need to improve drainage and soil aeration. Drainage and soil compaction may require that you remove your plant, amend the soil with sand and organics. You might choose to wait for the winter when the plants are dormant to undertake drainage improvement; in the meantime, try to create some pathways for air to get to the roots - using a manual aerator (like the type used on lawns) may be an option.

If the cause of chlorosis is related to a soil pH, you can adjust that accordingly. If the pH of your soil is high, then reducing soil to a neutral or slightly acidic pH, 7 - 6.5) is the single best way to remedy iron chlorosis; it also improves air and water movement through soil. Apply powdered soil sulfur (that may require a lot of sulfur and a year or more to produce results) or soluble forms of sulfur with iron and manganese. To raise soil pH you can add limestone.

Chlorosis due to a true absence of iron from the soil can be resolved with the application of iron sulfate; this is the cheapest and most widely available type of iron fertilizer. Apply liberally, and scratch into the soil around the plant. Generally, there is plenty of iron in the soil, itís just not in a form that is available to the plant. If thatís the case, you can add chelated forms of iron. A chelating agent is a synthetic organic substance that can maintain iron (as well as copper, manganese and zinc) in a nonionized, water-soluble form that is readily absorbed by plants. Scatter dry granules within the plant's drip line, then water thoroughly so the chelate soaks into soil around roots. Leaves should start to green up in two to three weeks.

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Last Modified: 08/06/2013