About 3 weeks ago, I planted 4 tropical hibiscus (3 gal) alongside a side of a fence (northern exposure) and noticed this morning that 2 of them are starting to have yellowing leaves, which are starting to drop. Can someone help me figure out what might be the cause, since there could be many: 1) under-watering, 2) planting related, or 3) cold weather related?

1. Under-watering: I haven't been watering them too much--maybe 2 times a week because they are in the shade for most of the day and the weather has been pretty balmy for the most part (50s-60 at night, 70s-80s during the day). I was a bit worried about the lack of direct light at first, but I thought they might be okay because they will get more sun as the year progresses and the position of the sun changes. I am fairly confident that they will be in full sun when the summer gets here; the fence just happens to shade them in the wintertime.

2. Planting: I did plant them according to the directions on the plant label, except now that I am better educated about planting hibiscus AFTER having planted them already, I am considering digging them up, amending the soil with mushroom compost & better soil, and replanting. When I planted them, I just used topsoil and mixed it with the sandy soil that is typical of South Florida; the hole I dug for each plant was fairly large and appropriate according to the label.

3. Cold weather: We also had a cold snap at the end of last week, where the temperature dropped to slightly above freezing. They seemed to be okay afterwards, though some of the blooms were a bit mangled. It started blooming well again earlier this week, and now their leaves are yellowing and starting to fall off. Could the cold temps have caused this? It seems a bit late--I would think the leaves would have yellowed shortly after the cold weather rather than a week later.

Maybe I am underwatering? Everything seemed to be going well for the last 3 weeks! :(

Thanks!