Winterizing lavender plants grown outdoors in Wisconsin is essential due to the harsh cold, strong winds, and often significant snow. Wisconsin typically falls into USDA Hardiness Zones 3b to 5b, meaning average minimum winter temperatures can plummet. Successful winterization focuses on protecting the plant's roots and preventing winter injury.
Here's how to winterize lavender plants in Wisconsin:
1. Choose the Right Lavender Variety
This is the most critical first step for success in Wisconsin's climate. Many lavender types are not cold-hardy enough.
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English Lavender (Lavandula angustifolia) and Lavandin Hybrids (Lavandula x intermedia) are your best choices.
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'Munstead' is highly recommended for its excellent cold hardiness, often surviving down to Zone 3 with sufficient snow cover.
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'Hidcote' is another very reliable and cold-hardy English variety.
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'Phenomenal' is a popular lavandin hybrid known for its strong cold hardiness (often to Zone 4) and resilience.
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'Grosso' (a lavandin) is also quite hardy and frequently used for essential oil.
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2. Ensure Excellent Drainage (Crucial for Winter Survival)
Lavender roots are extremely susceptible to rot, especially in cold, wet conditions. This is paramount for Wisconsin winters.
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Amend Soil Heavily: If your soil is heavy clay (common in parts of Wisconsin), extensively amend the planting area with coarse sand, pea gravel, or small, sharp aggregate to dramatically improve drainage.
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Plant High/Mound: Consider planting lavender in raised beds or on slight mounds, an inch or two above the surrounding soil level. This helps excess water drain away from the crown (base of the plant).
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Location: Select a planting spot that does not accumulate standing water after rain or snowmelt.
3. Pruning Timing (Critical for Cold Climates)
Unlike warmer regions, do NOT perform a heavy prune on your lavender in the fall in Wisconsin.
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Light Trim Only (Late Summer/Early Fall): You can deadhead spent blooms in late summer (around August to early September) to tidy the plant. If the plant is very overgrown, you can lightly trim back a few inches of green growth, but avoid any significant shaping or deep cuts.
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Why avoid fall pruning? A heavy fall prune stimulates new, tender growth that will not have time to harden off before the freezing temperatures arrive. This new growth is extremely vulnerable to frost damage and can severely weaken or kill the entire plant.
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Main Prune in Spring: Your primary, shaping prune must be done in early spring, after the danger of the last hard frost has passed and you see clear signs of new green growth emerging from the base (typically late March to April in Wisconsin, but observe your specific plant and local conditions). At that time, prune back about one-third of the plant, removing any stems that clearly died back over winter. Remember the golden rule: only cut into green, leafy stems, never into old, bare, woody parts.
4. Mulching for Root Insulation
Mulch is vital in Wisconsin to insulate the lavender's roots from extreme cold and damaging freeze-thaw cycles.
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When to Apply: Apply mulch in late fall, after the ground has begun to cool or even lightly freeze. Applying it too early can trap warmth and prevent the plant from entering full dormancy.
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Type of Mulch: Use a coarse, well-draining organic mulch that won't hold excessive moisture against the plant. Good options include:
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Straw
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Pine needles
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Small gravel or pea gravel (excellent for both drainage and insulation)
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Application: Apply a generous layer, about 4-6 inches thick, around the base of the plant, extending out to the drip line.
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Crucial Caution: Keep the mulch 2-3 inches away from the crown (base) of the plant stem. Piling mulch directly against the stem can trap moisture, lead to rot, and create a haven for rodents that can damage the plant. The goal is to insulate the roots, not to suffocate or rot the crown.
5. Snow Cover (Natural Protection)
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Embrace Snow: In regions like Wisconsin with consistent snow, heavy snow cover is actually highly beneficial. Snow acts as an excellent insulator, protecting the plant from both extreme cold and dehydrating winter winds.
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Avoid Compacting: Try not to compact heavy snow directly on top of the plants when shoveling, as too much weight can break branches.
6. Wind Protection
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Sheltered Location: Plant lavender in a spot that offers some natural protection from harsh winter winds, such as near a south-facing building wall or among other established evergreens.
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Temporary Windbreak: If your lavender is in a very exposed, windy location, consider setting up a temporary burlap screen or windbreak on the windward side of the plant. Stake the burlap into the ground around the plant, ensuring there's still adequate air circulation.
7. Fall Watering (Minimal)
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Ensure the plant goes into winter well-hydrated but not waterlogged. As fall progresses and temperatures drop, gradually reduce watering.
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Once the ground begins to freeze solid, cease watering completely. The plant is dormant, and excess moisture can freeze around the roots, causing damage.
8. Container-Grown Lavender
If you are growing lavender in pots in Wisconsin, they are significantly more vulnerable to cold than in-ground plants.
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Move Indoors: The safest option is to move potted lavender to an unheated garage, shed, or a cool basement with a window. Ideal temperatures should remain between 35-45°F (2-7°C).
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Light: Provide as much natural light as possible, or supplement with grow lights.
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Water Sparingly: Water very rarely, only when the top 1-2 inches of soil are completely dry.
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Bury the Pot: In some cases, you can dig a hole in the ground and bury the entire pot (up to the rim) in a well-draining spot in your garden. This leverages the insulating power of the earth.
By carefully selecting cold-hardy varieties, ensuring proper drainage, mastering pruning timing, and providing ample root insulation, you can significantly increase the chances of your lavender plants successfully overwintering and thriving again in the Wisconsin climate.