The dwarf Meyer lemon tree planted in our yard (it came with the house) has had yellow leaves on it all winter, despite fertilizing, which prompted me to look for the reasons a lemon trees leaves go yellow.

I found five main reasons that a lemon trees leaves may turn yellow:


Age

As a lemon trees leaves age, they turn yellow and drop from the tree.  Since the bulk of the leaves on the tree have turned yellow, I’d guess age is not the issue here. 
Overwatering

This has possibilities as the past two years we have seem above average rainfall in the Canberra region.  Canberra Airport recorded 959.6mm of rainfall during 2010, well above the historical average of 616.8mm and the fourth highest rainfall on record. Likewise August 2011 had slightly above average rainfall.

Under Watering

I don’t think this is the case given 2 above.

Cold Weather

Certain types of citrus trees are prone to their leaves turning yellow and dropping simply because the weather turns cold outside.  Meyer Lemons are particularly prone to this phenomenon.   No one would deny that the Canberra region has a cold winter, so this may be contributing to my yellow leaved Lemon tree.

Nutrient Deficiency

Yellow leaves on a lemon tree send most of us straight to the fertilizer.  Since my tree hadn’t seen any fertilizer for at least 2 years that was my first instinct.  I later learned why gardeners recommend fertilizing in spring and autumn…a lemon tree will not absorb fertilizer during the winter when it’s cold!

But, Why Does My Lemon Tree Have Yellow Leaves?

My best guess would be a combination of the cold weather (the photo above was taken on July 11, 2011) and a nutritional deficiency. In which case a good dose of fertilizer and warmer weather will remedy the situation...