We planted a lemon tree five and a half years ago. Today it's about the exact same size as ever.
I've tried using citrus fertilizer, but to no avail. We've had dry summers and wet summers, but nothing changes. I'm in Melbourne, Australia
What's going on? Is there anything else I should tell you?
I would suggest:
Lack of sunlight
I notice it's right next to a fence corner and suspect there's not enough sunlight getting through to this one for it to thrive. I'm not sure what to suggest as a solution for that.
Possible magnesium deficiency (pretty common in Australian citrus trees) or some other mineral deficiency in soil
I also notice, on close inspection of the picture, that there is a leaf which has the classic yellowed edging typically associated with magnesium or zinc deficiency.
I found out just a few months ago after asking what was causing the yellow leaves on our tree here:
Are you absolutely positive it's getting the nutrients it needs via the citrus fertiliser you used? Perhaps that leaf I'm looking at is an anomaly.
Genetically determined dwarfism
On reflection, if neither of the above make sense, and given how fecund and otherwise healthy this tree of yours looks, I really start to think @bstpierre might be right in thinking you've got a genetic dwarf, whether or not it was labelled as such.
Lack of environmental stimulus to grow/reproduce (highly unlikely)
Another obscure possibility, which I find hard to believe but I have heard ABC talkback radio (Adelaide) gardener Jon Lamb mention at least twice, is that if you give a plant really optimal conditions it just "enjoys life" rather than competing by growing tall or producing fruit in order to pass on its genes before it dies. If you have really paid attention to the tree and done soil tests and whatnot to be sure there's no mineral deficiency then this may be the (admittedly bizzare) answer. Given that you said your wife already tried pruning it though, I really do doubt it.
Even a Meyer lemon will grow 12-15 feet high: since you have full size fruit on your apparently 4 foot tree, you definitely have a hybrid that will only grow about 4 inches/year versus a Ponderosa lemon that will average a foot/per year. However; any vegitation growing above the root spread will rob it of both nitrogen & nutrients. You'll notice a slight growth acceleration by clearing & controlling that area. Don't mulch: Just compost a 2" top layer per year, but no closer than 6" from the stump. Fresh wooded mulch robs the tree of nitrogen & nutrients ... until it decomposes to compost after 2-3 years ! Max sun light is essential but not critical for standard size fruit trees. However; dwarfs easily tolerate a minimum of 6 hours of mid day sun.
