Growing trees and shrubs in containers offers flexibility for decorating patios or balconies. However, winter presents a challenge because the roots of containerized plants face conditions far harsher than those planted in the ground. Survival often depends on intervention and preparation. With careful selection and insulation, a container-grown tree can endure freezing temperatures and thrive again in the spring.
The Core Vulnerability: Why Container Roots Freeze
A tree’s root system is inherently less tolerant of cold temperatures than its above-ground branches and trunk. While dormant woody stems may withstand temperatures well below zero, roots can be damaged or killed near or just below the freezing point. In-ground trees benefit from the earth’s massive insulating volume, which keeps the soil temperature stable and often above freezing, even when the air is frigid.
This insulating effect is lost when a tree is grown in an above-ground container. The root ball is exposed to ambient air temperatures on all sides, causing the soil to freeze solid and rapidly reflect temperature fluctuations. Container soil temperatures can plummet significantly lower than in-ground soil temperatures. Fine feeder roots, which handle water uptake, are particularly susceptible, with some varieties dying at temperatures as high as \(23^\circ\text{F}\).
The entire root system, including mature roots, can be killed when the soil temperature falls below approximately \(5^\circ\text{F}\) to \(10^\circ\text{F}\). This extreme exposure causes direct freeze damage and a secondary injury called desiccation. When the root ball is frozen, the tree cannot absorb moisture. Exposed foliage, especially on evergreens, continues to lose water to dry winter winds and sun, leading to desiccation.
Determining Survivability: Hardiness Zones and Selection
Successful overwintering begins with selecting the right tree for your climate, guided by the USDA Plant Hardiness Zone Map. This map indicates the average annual minimum winter temperature for a region. A plant’s listed hardiness zone refers to the coldest zone where its roots can typically survive when planted directly in the ground.
Because a container provides significantly less protection, a general guideline is to select a tree hardy to two zones colder than your own climate. For example, a tree grown in Zone 6 should ideally be hardy to Zone 4 or lower to survive the winter outdoors in a pot. This extra margin of cold tolerance compensates for the lack of soil insulation.
Preparing the tree for dormancy is necessary before the hard freeze arrives. Ensure the tree is fully hydrated with a deep watering before the soil freezes completely, as water is needed for cellular protection during dormancy. The container material also plays a role. Thick-walled wood or fiberglass provides better insulation than porous terracotta or thin plastic, which are also susceptible to cracking during freeze-thaw cycles.
Outdoor Winter Protection Strategies
For trees remaining on a patio or deck, several protective measures can mimic the earth’s insulation. One simple method involves grouping containers tightly together in a sheltered location, preferably against a south- or east-facing wall. Clustering pots shields inner containers from drying winds and allows the plants to share a slight thermal benefit.
A physical windbreak should be erected on the exposed sides of the grouping to reduce moisture loss and temperature fluctuation. This barrier can be constructed from burlap stretched between stakes or bales of straw stacked around the perimeter. Reducing wind exposure prevents the rapid cooling of the container walls.
Insulating the pots is a direct way to protect the vulnerable root ball. This is often accomplished by wrapping the container exterior with insulating materials. For more robust protection, a protective corral can be built around the pots using chicken wire or snow fencing. This enclosure should then be completely filled with a thick layer of loose insulating material, covering the containers fully up to the base of the trunk.
The insulation should aim for a depth of six to twelve inches over the pot’s surface and around the sides to keep the root temperature above \(20^\circ\text{F}\). Ensure the insulating material is dry before application to prevent moisture-related fungal issues. For evergreen species, a burlap screen or anti-desiccant spray can be applied to the foliage to limit water loss while the roots are frozen and unable to function.
Alternative Storage: Utilizing Sheltered Spaces
Moving the containerized tree into an unheated, sheltered space is often the most reliable method for winter survival. Ideal locations include unheated garages, sheds, or cold storage cellars, where temperatures remain cool but consistently above freezing. The goal is to keep the tree dormant, with the optimal temperature range falling between \(30^\circ\text{F}\) and \(45^\circ\text{F}\).
In an unheated garage, the mass of the surrounding house moderates the outside temperature, preventing extreme cold that would kill the roots outside. Deciduous trees, having dropped their leaves, can tolerate dark conditions. However, broadleaf and coniferous evergreens still require some light, making a garage with a window or a cold frame a better option.
Another effective storage method is sinking the container directly into the ground in a garden bed. This involves digging a hole large enough to accommodate the pot up to its rim and backfilling the space around the container with soil or mulch. This technique restores the earth’s natural temperature-buffering properties, protecting the root ball from freezing solid.
Whether stored in a garage or buried, soil moisture must be monitored periodically throughout the winter. Dormant trees require minimal water but can still dry out, so light watering every two to three weeks may be necessary to prevent desiccation. Check the soil with a finger and water only when the top two inches feel dry. Avoid over-watering, which can lead to root rot in cold conditions.