Overwintering Tender Plants: Techniques for Mountain Gardeners
Dear Friend,
Every gardener in the Southern Appalachians eventually comes face to face with the question: What do you do with your tender plants when the mountain winter settles in? It’s one of those challenges that separates casual gardening from true plant stewardship — knowing how to keep a favorite plant alive through months of cold, low light, and fluctuating humidity, so it can thrive again next year.
In our part of the world, we have a reasonably mild winter compared to higher elevations, but we still see hard freezes, prolonged cold snaps, and the occasional ice storm. That’s enough to threaten or outright kill many tender perennials, tropical ornamentals, and even certain shrubs if left unprotected.
Today, I want to walk you through overwintering plants. We’ll break it down by plant type, method, and environmental requirement so you can match the right strategy to the right species. My goal is that by the end of this letter, you’ll have a clear, step-by-step plan for overwintering success — whether your collection includes a few potted geraniums or a greenhouse full of tropicals.
Step 1: Understanding Tenderness
In horticultural terms, a “tender” plant is one that cannot survive sustained freezing temperatures without damage or death. The threshold varies widely — for example:
Tropical species such as Canna indica or Hibiscus rosa-sinensis are damaged below ~50°F and will die quickly if exposed to frost.
Subtropical plants like Citrus sinensis or Olea europaea can tolerate brief dips into the 20s or 30s, but not prolonged freezes.
Tender perennials such as Dahlia pinnata or Colocasia esculenta may die back to their storage organs (tubers, corms, rhizomes) in cold weather but can be preserved if those structures are kept frost-free.
Here in Asheville, the average first frost falls around October 15–20, and the average last frost is mid-April. That gives us roughly six months of potential frost exposure — too long for most tender species without intervention.
Step 2: Evaluating the Plant’s Winter Needs
Overwintering success depends on matching the storage environment to the plant’s biological requirements. The key factors to assess are:
Temperature tolerance (absolute minimum and optimal winter range)
Light requirements (active growth vs. dormancy)
Moisture needs (dry storage vs. evenly moist)
Air circulation (to prevent fungal issues)
Root zone protection (especially for plants in containers)
For example, a Pelargonium × hortorum (zonal geranium) may overwinter indoors at 50–60°F with moderate light and low water, while a Dahlia tuber should be lifted, dried, and stored in a cool, dark place at 40–50°F with minimal moisture to prevent shriveling.
Step 3: Choosing the Overwintering Method
Here are the most reliable methods for mountain gardeners, with technical details and plant examples.
A. Bringing Indoors (Active Growth)
Best for: Tropical and subtropical species that require year-round foliage to survive — citrus, hibiscus, ferns, certain orchids, potted herbs like rosemary and basil.
Temperature: 55–65°F optimal, never below 50°F for tropicals.
Light: 10–14 hours/day. Supplement with full-spectrum LED grow lights if natural light is insufficient.
Humidity: Maintain 40–60% RH to prevent leaf drop; use pebble trays or humidifiers.
Pest Management: Inspect thoroughly before bringing indoors. Apply insecticidal soap or horticultural oil if pests are detected; isolate for two weeks to monitor.
Watering: Reduce from summer frequency; allow upper soil layer to dry between waterings to prevent fungal gnat outbreaks.
B. Cool Storage (Dormancy)
Best for: Tender perennials with underground storage organs — dahlias, cannas, elephant ears, gladiolus.
Temperature: 40–50°F (consistent).
Light: None required; store in total darkness.
Moisture: Keep slightly damp — too dry and tubers shrivel; too wet and rot develops. Use peat moss, coarse vermiculite, or wood shavings as packing material.
Procedure:
Dig after first frost blackens foliage.
Gently remove soil and allow 1–3 days to cure in a shaded, airy spot at ~60°F.
Trim stems to 2–4 inches.
Label with variety and storage date.
C. Unheated Greenhouse or Cold Frame
Best for: Hardy marginal perennials, cool-season annuals, and containerized shrubs just outside their hardiness range.
Temperature: Protect from freezing; use thermal mass (water barrels, masonry) and frost blankets to buffer extremes.
Light: Full winter sun; avoid shading from nearby structures.
Ventilation: Critical on sunny days to prevent overheating and humidity spikes.
Moisture: Water sparingly — roots are less active in cool conditions, but do not allow complete drying.
D. In-Ground Protection (Mulching & Wrapping)
Best for: Semi-hardy shrubs and perennials where digging is impractical — figs (Ficus carica), rosemary (Rosmarinus officinalis), certain salvias.
Method:
After first frost, apply 6–12 inches of mulch (straw, shredded leaves) around root zone.
For stem protection, wrap with burlap or frost cloth, securing against wind.
Avoid using plastic directly on plants — traps moisture and causes rot.
E. Root-Pruning and Potting for Indoor Storage
Best for: Large tender specimens grown in-ground (e.g., tropical hibiscus, brugmansia) that can be reduced in size for indoor overwintering.
Timing: Begin root pruning 4–6 weeks before frost, then lift and pot into containers with a well-drained mix.
Care Indoors: Treat as potted tropicals; monitor closely for pest outbreaks.
Step 4: Pest & Disease Prevention During Storage
Overwintered plants are prone to pest flare-ups because predators are absent and stress weakens natural defenses. Implement the following:
Quarantine all plants entering an indoor or greenhouse environment for two weeks.
Monitor weekly for spider mites, aphids, scale insects, mealybugs, and whiteflies.
Apply systemic insecticides (imidacloprid, for non-edible plants) in early fall to control hidden infestations.
Use biological controls in greenhouses — Encarsia formosa for whitefly, Phytoseiulus persimilis for spider mites.
Sanitize tools and surfaces with a 10% bleach or 70% isopropyl alcohol solution before and after handling plants.
Step 5: Acclimatization (Hardening In and Out)
Just as you harden off seedlings in spring, you must reverse the process when bringing plants indoors in fall:
Reduce sunlight exposure gradually over 7–10 days to prevent leaf scorch.
For plants going dormant, reduce water by half during the last two weeks outdoors to slow metabolic activity.
In spring, reverse this process, reintroducing plants to outdoor light and wind slowly to avoid shock.
Step 6: Special Considerations for Asheville & the Blue Ridge
Our microclimates vary dramatically here — a sheltered cove may stay 5–10°F warmer than a ridgeline garden during a cold snap. When planning overwintering strategies, consider:
Elevation — Above 3,000 feet, expect earlier frosts and deeper freezes.
Wind exposure — Strong winds accelerate desiccation and freeze damage; windbreaks are invaluable.
Soil moisture — Wet winter soils encourage root rot in marginally hardy plants; improve drainage before winter.
Case Studies from the Nursery
At Unicorn Farm, we overwinter a variety of tender plants for both display and client installation. Here are two examples:
1. Brugmansia
Lifted from field planting in early October.
Root-pruned and potted into 7-gallon containers.
Overwintered at 55°F in a high-light greenhouse bay.
Watered lightly once every 10–14 days.
Treated preventatively for spider mites at intake.
2. Dahlia
Dug after first frost blackened foliage.
Cured in a shaded, airy shed for 3 days.
Stored in coarse vermiculite at 45°F.
Checked monthly for rot or shriveling.
The Payoff
Overwintering tender plants isn’t just about saving money — though it certainly reduces replacement costs in spring. It’s about preserving genetics (especially rare cultivars), maintaining mature plant size for immediate impact, and extending your horticultural range far beyond what our winters would normally allow.
With the right technique matched to the right plant, success rates of 85–95% are achievable. And each spring, when you reintroduce those healthy, overwintered plants to the garden, you’ll feel the satisfaction of having shepherded them through the cold, dark months.