Are your citrus leaves turning green with yellow veins in winter?
When gardeners see this colour change in their citrus leaves, they often wonder if this is due to a nutrient deficiency, and if so, what they can do to fix it.
The abnormal yellowing of leaf tissue is called chlorosis, which is caused by a lack of the green pigment chlorophyll which is essential for photosynthesis.
Leaf chlorosis can be caused by various factors, plant nutrient deficiencies being one possible cause. Other possible causes include poor drainage, damaged or compacted roots and high (alkaline) soil pH.
Yellow Vein Chlorosis
When your citrus tree leaves displays yellow veins while the rest of the leaf remains a normal green colour, this condition is referred to as yellow vein chlorosis.
Usually, yellow vein chlorosis occurs during the autumn and winter period due to reduced nitrogen uptake by the roots from the soil in low temperatures. Citrus tree nitrogen uptake is generally lowest during dormancy, it increases during flowering and reaches its peak during fruit set. It doesn’t matter how much nitrogen fertilizer is present in the soil, it is less available to the citrus tree in cold weather and as such the tree displays the signs of nitrogen deficiency.
Nitrogen is classed as a mobile nutrient, which means that plants and trees can move it from one part of their structure (leaves and branches, etc.) to another, away from places where it’s no longer needed and into new growth.
Since the nitrogen in the soil is less available in the cold seasons, citrus trees will mobilize nitrogen reserves from older tissues, redirecting them during the spring ﬂush into new leaves and ﬂowers. When part of the nitrogen of the leaf is translocated back into the tree because of inadequate nutrition, the result is yellow-vein chlorosis.
When It’s Not Nutrient Deficiency
Keep in mind that yellow vein chlorosis can also be caused by girdling of branches, roots, or the tree trunk itself. Girdling (ring-barking) is the removal of a strip of bark right around a branch or trunk of a woody plant.
Look for obvious signs of physical damage to branches or the base of the trunk at soil level. Bark may be eaten by pest animals such as rodents (rats, rabbits, etc.) or damaged with powered gardening equipment such as line trimmers. Damage to the roots may occur due to root rot from waterlogged soils. Check drainage during the wet seasons and keep mulch away from the base of the trunk to prevent collar rot.
How do you distinguish whether the problem is related to girdling or cold weather nutrient deficiency? Unlike nutrient deficiency related yellow vein chlorosis, this type of damage will also cause leaf drop, fruit drop, dieback, and possibly the eventual death of the tree.
Treating Yellow Vein Chlorosis
If your citrus tree has cold weather induced yellow-vein chlorosis, what can you do?
Ideally, nothing! If you’ve been feeding your citrus at the right times of the year, typically at the start of spring and autumn at the very minimum (or as per the feeding directions on the fertilizer packaging) and you’re using a balanced fertilizer, then the tree will take care of itself when the weather warms up and it can better access the nitrogen available in the soil. Let Nature do the work, that’s the Permaculture approach!
If you’re obsessing about doing something, then you can use a foliar fertilizer which is sprayed on the leaves, where it’s absorbed directly. It’s the quickest method of getting nutrients into plants. Foliar nutrition is useful in condition where the tree’s ability to take up nutrients is decreased and it needs the extra nutrition, such as prolonged periods of drought, wet conditions or cold weather.
University of Florida, IFAS Extension – Publication #HS876, Citrus Problems in the Home Landscape, Mongi Zekri and Robert E. Rouse
University of Florida, IFAS Extension – Publication #HS-797, A Guide to Citrus Nutritional Deficiency and Toxicity Identification, Stephen H. Futch and David P. H. Tucker
University of California, Davis – California Fertilization Guidelines, Citrus