My parents had a beautiful tri-colored Japanese maple they kept in a pot on their deck for many years, but because of where they lived, they had to overwinter it somehow. There are several ways one can overwinter potted trees and shrubs you want to display year after year. The first thing to consider is the type of plant you are overwintering. Tropical and subtropical species must go indoors in a well-lit location where temperatures are tolerant. Temperate species like the aforementioned maple actually need to remain in colder locations. I do not recommend moving your favorite potted rose to the safety of your south facing window. Temperate plants, to differing genetic degrees, require a rest. Forced to remain in leaf year round or even for longer and longer periods naturally (as is the trend with climate change), plants become “exhausted”. Leaf buds of temperate species enter a dormant period when their leaves fall. The buds then must go through a chilling period as a requirement to break dormancy in spring. The duration and temperature threshold vary by species.
There are many methods, with varying levels of effectiveness, one can use to overwinter potted woody ornamentals. As mentioned in last week’s blog post, all our potted material at Wellfield is moved to a “sheltered” location, away from direct prevailing winds, and is healed in with a healthy dose of woody chips. Another option is a non-heated, attached garage with a little natural light and access to radiant house heat. A little light is necessary as the buds are timing the day length with their chemical stopwatch (as discussed previously), so complete darkness is not ideal. The key in our garage scenario is soil moisture management. The soil should be allowed to dry out for long periods of time, as the roots are still alive and growing (slightly). The root zone should not sit in water-logged soil, either. Think of the natural freeze and thaw cycles it would be exposed to if growing in your garden. Typically the top few inches go through periods of high moisture (thaws) and low moisture levels (freezes). You want to imitate this process.
Eventually, the Japanese maple outgrew my parent’s ability to move it in and out of the garage each year, and it found a permanent home in their backyard garden. With a little bit of awareness and care, they enjoyed the maple for years on their deck. Armed with this information, you can enjoy your favorite potted plants for years, too.
Josh Steffen, Horticulture and Facilities Manager