Unless you live in a unusual microclimate in the city, lemon trees are not the best choice. Even up here, during the winter, the leaves turn slightly yellow because the soil is too cold inhibiting the uptake of nitrogen.

What citrus needs is: Warmth, water, and fertilizer. All in abundance.

Thanks villandry. I gave it a lot of fertilizer this weekend, but I suspect you are correct. It's on the roof, were it's typically very windy and none too warm, except on rare days.