Marantas defintely go into a state of semi-dormancy each winter. When that occurs, you must allow the soil to approach dryness between controlled waterings. Soil should be kept evenly moist during the remainder of the year when the plant is in its active growth cycle. With this plant, it's VERY important to withhold ALL fertilizer during that period of semi-dormancy. There are some plants that have an adverse reaction to water that is heavily treated with floride, and (based on my experience) I happen to think Maranta is one of them. If your water is heavily treated, allow it to set in an open container for 24 hrs (at a minimum) before watering your Maranta. Because I believe Marantas to have an adverse reaction to floride, I prepare a soil mix for them that does NOT contain perlite. I'm linking to a U of Florida site that may be helpful. (Pay special attention to the section titled 'Physiological Problems'.) The site exists primarily for commercial growers of foliage plants, but I refer to it often when I need really helpful assistance. If you explore the site, you'll find good info on lots of house plants. Here is a link that might be useful: Maranta Info

That sounds very similar to a problem I am having, which I have diagnosed on my own as SAD virus. I am planning to do some re-plugging with SAD resistant variety soon. Google it.

The most likely cause of your problem is a dried out root ball. Pot grown rhododendrons are very prone to this - there are many examples on this forum. The light peat based medium used by the grower is very hard to rewet once it dries out. The problem is made worse if at planting the roots are not cut, raked with a cultivator or otherwise unmatted. The best thing to do is to dig up the plant, tease out the roots - don't be afraid to really do this thoroughly - and soak in a bucket until the root mass is wet. Then replant. If you notice signs of wilting over the summer, water at the base of the plant with a slow trickle for 30 to 60 minutes. Don't soak the whole bed. That just makes it harder for the roots to move out of the original medium and into the soil around it. You may have to use the trickle technique again next year, too.

I would clean up the weeds and small plants around the trunk and give the tree more space. And then, I would add some fresh and clean top soil or potting soil on top of the roots to cover the roots up, about 1/2 to 1 inch, and sprinkle some water to work the soil into the roots. Air bubbles and empty spaces between roots can kill the root hairs on the roots. The soil will fill up those empty spaces. Root hairs are the main tissue to absorb water and nutrients to the tree. Do not add too much of water as we want to avoid water log and root rot. I avoid using organic top soil or organic potting soil as most of them contain manure or other unknown organic substances, especially when the tree is weak right now. If I have some potting plants, I would move them near the tree to create micro-climate to keep it warm in winter, but not too close. Too close could create too much moisture, which promote fungal growth. I also would not let their branches touching each other to avoid scratching each other to create wounds. Move those potting plants away in other part of the year for fresh air and sun light. I would feed it with half strength of Miracle Gro solution for the tree to reduce the transplanting shock, just for one time at this time. Winter is not its time for fertilizing. Too much of fertilizer will promote new growth, which will be killed by cold weather. I agree with Grant Yang's assessment. Michelia alba is a tropical tree. The tree drops some leaves is responding to the cold weather. The roots of Michelia alba are quite brittle. Once it was planted in about the right level with the ground, do not dig it up again. The roots should not be exposed to the air too. Cover the roots up with clean top soil. Bay Area is in rainy season right now. We do not need to water them unless the top soil is dry by a few sunny days. Good luck. I transplanted my 5 years old Michelia alba from pot to ground 3 years ago. It is under a tall Russian Olive tree. I live in East Bay. It is hot in summer and cold in winter. From time to time, it is quite windy too. I have to stick it to keep it straight. I plan to prune it next year to keep it about 12 feet tall.