Answer: A few things come to mind. First, moving plants indoors from outdoors, although necessary when cold temperatures arrive, can be a real shock to their system because the conditions are usually so different. Yellowing and/or dropping leaves is a typical reaction. Being in the garage for a week would contribute to the shock.
Although it's true that container plants need more fertilizer and watering than those in the ground, be careful not to overdo. Continued fertilizing while the plant is under stress could promote leaf loss. Also, dropping leaves can be a sign of overwatering. Don't fertilize for a month or two, but continue watering at a slightly reduced level.
Salts in fertilizer and salts in the water can build up over time in containers. At least once a month, water very slowly and deeply, allowing the water to drain out the bottom of the pot and leach any salts away.
Magnolias prefer a well-drained, organic soil of a slightly acidic pH (5.5-6.5), and moderate, consistent soil moisture.
Check the leaves for signs of magnolia scale, a tiny insect that sucks juice from leaves and stems. If you find them, use a light horticultural oil such as Oil-Away from Gardens Alive (ph# 812/537-8650, firstname.lastname@example.org) to control them.
Next spring, slowly acclimate your plant to moving back outdoors by putting it outside in a sheltered location for a couple hours at a time, gradually increasing the time period. In fall, do that in reverse, rather than abruptly moving from outside to inside, which will help lessen the shock. I hope this info helps save your magnolia!
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