Without fail, all citrus lose their vibrancy throughout the cool season, particularly if you live somewhere cold. Without fail, citrus owners will rush into a garden centre concerned about their sick plants and buy up an array of citrus fertilisers to address the problem. So, first thing is first, don’t be concerned by a winter yellowing off.
Because citrus fruit in winter the plants can become exhausted from their production. Add to that the colder temperatures that restrict the flow of water through the plants’ roots and you have plants in mild distress - but it's natural. As soon as spring and the warmer temperatures arrive, the flow of water increases through the plants veins. Most fruit will then be freed from the branches and they begin to regain their vibrancy.
However an inspection of the yellowing off may alert you to a host of other problems your citrus may be suffering. Gall wasp swellings, curled leaves, leaf miner track marks, scale (oval black bumps) and possibly a few dedicated colonies of ants, are all afflictions that your citrus can do without. Knowing how and when to address them is much like piecing together a puzzle.