Fruit trees are like energetic teenagers, they burn through energy, produce copious amounts of love and but if they're tired, they'll definitely show it!
Citrus trees are evergreen, which means that they keep their leaves all throughout the year. The rootstock that we use for our Citrus trees is Trifoliata; it has a deciduous growth cycle. During Winter the roots will reduce the amount of nutrition it provides. This can leave the evergreen part of the tree hungry for food. Leaves with a yellow border or mottled yellow indicate a nutrient deficiency. Just like a hungry teenager, give them food and they'll perk up!
If the leaves are a yellow tone during winter, wait until Spring to give your citrus tree a good feed with fertiliser to encourage new growth. This is because the deciduous rootstocks won't take up a lot of food during the winter months. Fertilising citrus in the cooler months can be tricky, as it is best to avoid encouraging a flush of tender new growth that could easily get frost or cold damaged. A great way to improve the nutritional elements in your soil in winter is to feed your tree with a mix of dolomite, lime and gypsum, which will increase nutrition and improve leaf appearance without stimulating a flush.
If it is nutritional deficiency, complete a pH test on your soil and respond based on the result.
It is worthwhile noting the type of yellowing of your citrus leaves:
Light green to yellow leaves all over the tree indicates a Nitrogen deficiency – Treatment Blood and Bone/Organic Matter
Yellowish-green blotch at the leaf base that spreads outward - Magnesium deficiency – Treatment Dolomite or Epsom Salts.
Yellow leaves with green veins (generally soil high in alkaline - common in coastal areas) - Zinc/Iron deficiency - Treatment Zinc deficiency: foliar spray (zinc sulphate) on the spring flush leaves. Iron deficiency – you will need to look at your soil and reduce the pH (talk to your local nursery).
References: QLD Department of Agriculture, Fisheries & Forestry and of course of FST team.
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