Most of us want to have trees and shrubs close to us. Research shows that proximity to plants makes us feel good.* When they’re set off in some way, like marking both sides of a passageway, potted trees and shrubs give us a sense of order and rightness. You can do this with a pot of pansies or impatiens, but the effect isn’t quite as soaring.
There are many reasons to plant trees or shrubs in pots (ceramic, plastic, lightweight foam, terra-cotta, or otherwise). Besides the sensual pleasure of having woody plants nearby, marking entrances, and themselves creating a visual focal point, you may seek this alternative if deer are ravaging the plants in your garden. In pots close to your house, deer-vulnerable shrubs like arborvitaes enjoy a safe(r) haven. You can also festoon potted trees with holiday lights. One client kept three trouble-free junipers for holiday lights in pots for five years before transplanting them out into the landscape, where they now make a beautiful buffer between neighbors.
In highly urban situations, where there is often a sea of concrete, a potted tree brings an island of green. There are other challenges pots can rise to. My client Mina had a deck beyond which was a hillside of tangled ground cover that neither of us had the hubris to try to clear. Instead, we brought trees and shrubs into her living space on the deck. She wanted to have the feeling of a multi-layered garden (trees, shrubs, perennials, and annuals), but with a clustered collection of pots. We wanted the woody plants in the deck garden to return each year for a good many years to justify their expense.
The trees and shrubs we picked for Mina had to be tough characters, because I feared they wouldn’t get watered enough by this busy lady. So they had to be species that are tolerant of dry conditions. The woody specimens also had to be sufficiently cold hardy to overwinter in their pots outdoors. We didn’t want to have to move these heavy pots indoors every fall, nor did we want to be bothered with wrapping things up in ugly burlap. Nothing depresses me more than seeing shrubs wrapped in burlap for six or more months of the year. Why bother? If they’re not equipped for our winters, I don’t plant them. If they’re that vulnerable to deer damage, I don’t plant them. Tough Enough
You can see your USDA Hardiness Zone at Planthardiness.ars.usda.gov. I live in Zone 6a, which means that the average extreme minimum low in the winter is -10 to -5 F. A woody plant’s stems are just as hardy in a pot as in the ground, but a plant’s root systems are significantly less cold hardy than its above-ground parts. When you plant in pots, the roots are exposed to colder ambient temps. In the earth, roots enjoy the temperature moderation provided by soil. A rough guideline is that your plant selections for pots need to be hardy to at least two zones colder than your USDA zone. So in my case, I’d want to use woody plants that are hardy to at least Zone 4a for any pots that I want to overwinter outdoors.
Here are some evergreen and deciduous woodies that have worked well for me. Most are hardy to Zone 4a. It’s helpful to buy trees that have been propagated and grown right here in the Hudson Valley, by the way, because their local provenance ensures that they are adapted to our winters. Evergreen
Boxwood, arborvitae, junipers, and spruces Deciduous
Bald cypress (dwarf)
Elderberry (like Black Lace)
Ninebarks (like Summer Wine)
Black locust (like Twisty Baby)
Hydrangea paniculata (like Limelight)
Lilacs (not all, but many—check the label for hardiness zone)
You may also have spots around your home that are microclimates where you can get away with using plants a half or whole zone less hardy—in my case, to Zone 5a or 5b. (Microclimates can occur due a number factors, such as garden beds that hug the south side of your house, or that are surrounded by warmth-radiating pavement such as concrete or asphalt, or those that abut fences, walls, and rocks that protect plants from winter winds.) That opens up delightful possibilities like Knockout Roses. Like all facets of horticulture, testing winter-hardiness limits can be regarded as a nifty experiment. This should be your mantra: “If I am learning, I am a successful gardener.” Repeat. Mixed Media and Water Me Well
Cornell Urban Horticulture Professor Nina Bassuk says we should choose soil-less potting mix over “topsoil” or field soil of any kind. (She points out that simply calling something “topsoil” is a meaningless designation, by the way—anyone can call their product that, even if it’s junk.) Soil-less media like those using peat or coir (ground coconut hulls) are highly porous and designed to allow water to drain freely out of pots, while field soil in containers perches—that is, hangs on to water too tightly.
A good mix will feel light and friable in the bag. Don’t be surprised that it’s actually a bit hydro-phobic at first: it takes a certain amount of water saturation to penetrate all that pore space. Once your trees and shrubs are potted up, water them deeply once or twice a week during the growing season. Less frequent but deeper watering is more effective than frequent shallow watering. Smaller pots will need to be watered more often than larger ones.
Bassuk says that the pots should be watered well before going into the winter. For one thing, well-hydrated woody plants are less prone to desiccation by winter winds. At the beginning of winter, she recommends moving the pots as close to the house as possible and ganging them together so the sides are touching. “The warmth of each pot insulates its neighbor,” she says. “You could also stack straw bales around them to further insulate them.”
The best containers for overwintering are salt-glazed pottery and plastic. Ceramic, lightweight foam, and terra cotta pots are the most likely to crack under the freeze-and-thaw pressure of our winters. Time to Move Out
If a tree or shrub is well cared for in its container, it may outgrow its space. This will take a long time in the case of dwarf woodies. (By the way, “dwarf” means “grows very slowly, but doesn’t necessarily stay small”—for plants that stay little, see the “miniatures.”) You can prune the stems of shrubs and multi-stem trees to keep top growth in check, but this is not advisable for trees with one central leader.
For smaller potted plants whose roots have fully colonized the pot and clearly want to break out, you can transplant the plant into a larger pot. More brutally, you can root prune an outer ring of roots and then replant in the same pot, but this kind of root reduction is stressful on the plant.
For vigorously growing woodies, I transplant into successively larger pots and then at some point make the decision to move the plant into the landscape. Oftimes this is after many years of service as a containerized woody element of a mobile, elevated, and elegant pot garden.
Resources Cornell Urban Horticulture Institute The Phantom Gardener
*Green Cities: Good Health
is an extensive research database on the benefits of proximity to trees and shrubs to our health. Depts.washington.edu/hhwb