I've had some experience with Pittosporum variegatum in cooler climates, and you're right—they do prefer warmer areas. However, with a few precautions, you can often successfully grow them in regions that occasionally dip into cooler temperatures like yours.
One of the key strategies is site selection. As you mentioned, placing them in a sheltered spot is a wise move, especially where they get some wind protection. Winds can exacerbate the cold damage because they strip away whatever warmth the plant is trying to conserve. If your sheltered spot also has a southern exposure, that'll further enhance warmth and light during the shorter winter days, which can help.
Mulching is another effective technique for winterizing. Apply a good layer of organic mulch around the base of your plants—about 2-3 inches thick. This not only helps to moderate the soil temperature but also retains moisture, which is crucial in winter when the risk of drought stress can sometimes be underestimated.
If you find that the temperatures are dropping below what your Pittosporum can handle, even occasionally, consider using a frost cloth or horticultural fleece on nights when a freeze is expected. Just remember to remove it during the day to let the plant breathe and get sunlight.
Regarding potting, it's definitely a viable option if you want extra flexibility. Growing them in pots allows you to bring them inside or into a greenhouse if a particularly harsh cold spell is forecasted. Just ensure the pots are large enough for the roots to grow comfortably, and remember they will need more attentive watering and feeding.
In terms of maintaining those lovely variegated leaves, make sure they receive enough light, as insufficient light can cause the variegation to fade. However, be cautious of too much direct sunlight during the hottest parts of the day, which can sometimes scorch the leaves, depending on your specific climate conditions.
One thing to keep in mind is that environmental stress, like cold, can impact leaf color and density. Keeping your plants healthy with regular watering during dry spells, and perhaps a balanced slow-release fertilizer in spring, can help mitigate stress effects.
If you're interested in exploring further care tips or regional adaptations, checking out the Royal Horticultural Society's website or similar resources might offer more tailored insights, especially regarding hybrid varieties or related species that might be better adapted to your conditions.
Have you considered overwintering any other less-than-hardy plants, or do you have any ideas about integrating these practices into a broader garden plan? It'd be interesting to see how others have adapted traditionally warmer climate plants to cooler environments.
