Answer: Both magnolia and musa (banana) are hardy enough to winter outdoors in your central California garden. The top of the banana plant may freeze down to the soil in extremely cold weather, but the roots are hardy and will sprout new stems if that happens. Since your plants are above ground, you'll need to either bury the planters in the ground to help insulate the roots, or wrap some protection around the planters so the soil and roots do not freeze solid. You can place each planter inside a larger container and stuff newspapers or other insulating material between the two containers to help insulate the roots. Or, you can take both plants indoors for the winter.
Yellowing of leaves and browning edges might be weather related, or could indicate under watering or over fertilizing. It's natural for older leaves to yellow and die, especially during this time of year, but browning edges accompanied by yellowing leaves indicates a cultural problem.
Try leaching excess salts from the planters by flooding the soil several times to wash it out. This will also help rehydrate the soil, just in case there are some bone-dry areas around the roots. (This sometimes happens with plants in containers and can be avoided by immersing the containers in a larger container of water until the soil is thoroughly wet. I do this once a month during the summer months.)
Hope the above helps your plants regain their health!
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