9 Ways To Protect Vulnerable Perennial Plants From Winter Damage
As our gardens prepare for a winter rest, a little extra protection for favorite borderline perennials may be in order. With extra care, from easy mulch to added insulation, winterizing measures give a good foundation for them to withstand cold weather. Explore winter protection for those too-special-to-lose perennials with gardening expert Katherine Rowe.
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Sometimes, favorite perennials are worth a little extra attention to see them through the winter. Tender or marginal selections vary depending on climate, and even hardy specimens like roses and hydrangeas benefit from protection against frigid winters.
Too valuable to lose, taking extra steps to winterize can be as easy as adding extra mulch. For good security, we’ve got the best measures to set up your marginal perennials for overwintering success.
Inventory Tender Perennials
Vulnerable perennials are those that may not survive the winter in your growing area. Maybe they’re right on the cusp of your USDA hardiness zone or sensitive to winter extremes like persistent drying winds or repeated freezing and thawing.
Because we love the recurrent ornamental and ecosystem value perennials offer, it’s worth trying to overwinter the more tender selections. In overwintering, the primary goal is to protect the roots and crown from extreme temperatures, frost, winds, and sun. It’s really the roots that determine the plant’s survival and strong reemergence come spring.
Marginally hardy perennials include:
- New plantings (those planted near autumn’s first frost without established roots)
- Those in pots and containers
- Those for which your USDA zone is in the low range of their perennial limits
- Selections not adapted to severe or beyond-average conditions in cold climates
When temperatures drop and daylength shortens, many herbaceous perennials prepare for dormancy and stop actively growing. Make sure not to fertilize into fall beyond the active growing season. The plant needs to direct energy to the roots rather than producing new shoots, stems, and blooms that may take on damage in cold. Here are a few more tips for the best ways to winterize marginal perennials.
Don’t Cut Back in the Fall
While fall is prime for cutting back many plants, some of our more tender growers are best left standing through dormancy. Their foliage and stems offer winter insulation for the crown, especially helpful for marginally hardy types in cold climates.
Cut back hardy, woody perennials as usual by about a third to prepare for overwintering. Pruning in fall directs energy to the roots rather than the shoots. Cut back reliable, dormant frost-proof perennials to six inches above the soil level.
There’s a balance between cutting back herbaceous perennials to prevent fungal disease and leaving woody specimens standing. Go ahead and cut back perennials like tall garden phlox, monarda, yarrow, and hostas.
In cold climates, common vulnerable perennials that benefit from winter protection include:
- Hardy chrysanthemums
- Caryopteris
- Hardy geraniums
- Russian sage
- Red hot poker
- Heuchera
- Verbena
- Roses (zone 6 and lower)
- Some hydrangeas (species and variety-dependent)
Water Through Frost
Water is beneficial as conditions turn frosty. Perennials absorb water as they continue to grow until the ground freezes. Once heavy frosts set in and the soil freezes, they stop actively growing as roots are unable to uptake water. In winter, they depend on insulating snowpack and freeze-and-thaw cycles for moisture in cold climates. In warmer climates, regular rainfall is usually sufficient to keep soils from drying out completely.
Plants that enter winter dormancy rely on moisture leading into winter. Hardy roots and less stress entering the cool season are the foundation for successful overwintering, and fall is prime for developing robust roots. The season brings a reprieve from summer conditions and active upper growth. They settle in to promote root growth before cold weather.
And, dry soil freezes more quickly, which can damage roots. When water freezes, it produces heat, adding insulation to the plant. Normal moisture supports turgidity (water-filled roots, stems, and leaves) and strengthens tissues against frost damage by allowing stress-free natural processes to continue until dormancy.
Water Early
A good rule of thumb is to water early to mid-day in the fall to avoid dropping or freezing nighttime temperatures. During cold snaps, daytime watering allows moisture absorption without damaging roots in freezing soils. In warm conditions, it helps with water retention and absorption before evaporation as the day warms. It also prevents damp conditions that promote fungal diseases.
Let water penetrate the soils with a thorough session. Frequent, shallow water doesn’t support strong root development. Water deeply and then hold off for a few days to encourage roots to dig deep into the soil. Deep roots hold water longer, boosting drought resistance, physical stability, and ability to withstand cold and hot temperature extremes.
Mulch for Protection
Mulch insulates roots by regulating temperature and aids in moisture retention. In the winter, it protects against fluctuating extremes and icy situations. Mulching protects roots during frost heaving when soils naturally freeze and thaw due to temperature and moisture changes.
Add a two-to-three-inch layer for the best protection. Borderline specimens benefit from extra mulch around their dormant crowns. Leaf mulch, leaf mold, weed-free straw, compost, bark, and aged wood chips are good options.
Taking advantage of autumn’s natural leaf drop is a valuable resource across the garden bed. In addition to protection, leaves support soil health and provide nutrition as they decompose, among other benefits.
Add Insulation
Extra insulation is helpful for the tender growers you know are iffy. Protect the roots and crown with 8 to 12 inches of mulch, covering the lower stems and roots in a mound of soil, compost, shredded leaves, or other preferred material. Evergreen boughs work well as a blanket of insulation, too.
A wire cage filled with leaves (chicken wire or similar) forms a cylindrical frame and insulating cushion to last all season. Wrapping with burlap or frost cloth can help during temperature extremes and cold snaps.
Overwintering Pots and Containers
There are several ways to overwinter perennials in pots and containers. Potted specimens lack the benefit of surrounding soil mass, which provides insulation to in-ground plantings. Their above-ground situation exposes pots to surrounding air temperature with little protection. The soil is prone to freeze and thaw cycles, which can lead to root disturbance and damage.
Potted selections, then, need to be hardier than their in-ground counterparts to survive outdoors. A good rule of thumb is to leave those that are two zones lower than your growing zone (for example, a zone 7 gardener could opt to leave a perennial hardy to zone 5 outdoors in a pot).
Coverage
In mild climates (zone 8 and warmer), a simple thermal blanket or burlap cover can provide enough insulation to protect the potted plant from frost, wind, and cold snaps. In colder climates, more insulation is helpful for winterizing.
Cluster or huddle pots with the most tender perennials and smaller pots in the group’s center. Surround them with the hardier specimens and larger containers. If you have the space and elbow grease, consider digging a hole for the pot to sit in over the winter.
Insulate pots with mulch like shredded bark, woodchips, straw, or leaves. Mulch heavily all around the group, or use bagged leaves for easy removal in the spring. Evergreen boughs and thermal blankets are other insulating options.
Shelter
An enclosed space offers the best protection against winter elements and the best chance of survival. Bring potted perennials into an unheated space like a garage, basement, shed, or cold frame when possible. They benefit from a little natural light in these spaces and require occasional watering without natural moisture from rain or snow. The ideal indoor temperature range is between 30-40°F (-1-4°C).
Maintenance
Stored perennials need water periodically. It’s best to check every few weeks to prevent soil from drying out completely. About once a month is usually sufficient. Ensure the pot has good drainage and err on the dry side rather than keeping the soil too wet. Overwatering during less active growth can lead to fungal issues.
Wrap
Just as vulnerable in-ground varieties benefit from added insulation, containers left outdoors benefit from an extra wrap in cold conditions. Wrap the pots and exposed plant parts in frost cloth, fleece, thermal, or frost blankets. Some gardeners wrap their pots (not plants) in layers of plastic bubble wrap, insulating foam, or burlap for the season.
Prepare for Spring
In late winter and early spring before new growth emerges, winterized perennials are ready for uncovering. Remove boughs, wire cages, and thick mulch so that it’s away from crowns. Cut back dead upper stems. As sunlight warms the roots and temperatures increase, fresh growth appears – or at least that’s the goal with all the special treatment!
Overwintered potted perennials may break dormancy earlier than their garden counterparts. Take care to harden them off for spring by gradually removing winter protection.
