To ensure the survival and future blooming of herbaceous perennials that go dormant in winter, especially those previously part of your summer container displays, it’s crucial to protect them during the colder months.
Hardy perennials possess roots that remain dormant until the following spring when they start their new growth phase. Examples of such perennials include hostas, Shasta daisies, heucheras, astilbe, lady’s mantle, and daylilies.
To successfully overwinter these plants, the key is to maintain their dormancy while providing a winter environment that aligns with their hardiness zone. It’s important to note that perennials in containers are more vulnerable to severe cold and the effects of freezing and thawing compared to those planted in the ground.
Therefore, if you have a perennial that’s hardy to your zone and it’s in a container, it typically requires additional protection to thrive through winter.
That said, you have three different options as far as overwintering your perennials go:
If your potted perennial is currently in a suitable location, it’s often best to leave it there for the winter. When deciding whether to keep it outdoors, consider the container’s size and its ability to withstand winter conditions.
Additionally, the plant’s hardiness should ideally exceed your area’s zone by at least one, preferably two zones. For instance, if you reside in Zone 5, the herbaceous perennials in your containers should ideally be hardy to Zone 4 or even lower.
Large containers have an advantage because they contain more soil, which provides better insulation for the plant’s roots and helps maintain a consistent soil temperature.
However, it’s important to be cautious of sunlight hitting the container’s sides, especially if it’s dark-colored. This exposure can lead to alternate freezing and thawing cycles, which might fool the plant into believing it’s spring, triggering premature growth when it’s actually just a warm day in February.
To improve the chances of borderline-hardy plants or those in smaller containers surviving the winter, consider relocating them to an unheated garage or shed. During this period, the plant is dormant, so it doesn’t require light for photosynthesis.
However, it’s important to periodically check the soil moisture every couple of months to ensure it doesn’t become bone-dry. Be cautious not to overwater, though, as excessive moisture can lead to plant rot or premature dormancy breakage.
As late winter or early spring approaches, and the plant begins to resume growth, reintroduce it to normal outdoor growing conditions gradually. This can be done by exposing the plant to outdoor elements for increasing periods of time, helping it acclimate and avoid shock.
Locate an area where you can bury the plant, along with its pot, into the ground. This method helps provide improved insulation for the plant’s roots. An unused space in a vegetable garden often works well for this purpose. Submerge the pot and plant into the soil, ensuring that the top of the pot is level with the surrounding ground.
To offer additional protection during the winter, cover the plant with a layer of winter mulch, approximately two to three inches (5 to 8 cm) thick. Suitable mulch options include shredded bark or leaves.
This mulch layer helps maintain a stable temperature around the plant’s roots and protects them from harsh winter conditions.
When spring arrives, carefully remove the mulch, and then lift the container out of the ground to allow the plant to resume its growth.