Pruning lavender in Wisconsin, which spans USDA Hardiness Zones generally from 3b to 5b, is significantly different from pruning in a hot climate. The key difference is the harsh winter.
In colder climates like Wisconsin, the primary goal of pruning is to ensure the lavender plant can survive the winter by avoiding stimulating tender new growth that won't harden off before freezing temperatures arrive.
Here's a breakdown of when and how to prune lavender in Wisconsin:
1. The Main Pruning: Early Spring (Most Important Prune)
This is the most critical time to prune lavender in Wisconsin.
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When: Wait until early to mid-spring, after the last hard frost has passed and you begin to see definite signs of new green growth emerging from the base of the plant (typically April to early May, depending on your specific location in Wisconsin and the year's weather).
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Crucial: Do NOT prune heavily in the fall in Wisconsin. This is different from warmer climates. Fall pruning can stimulate new growth that won't have time to harden before winter, making the plant very vulnerable to cold damage and potentially killing it.
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Why:
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Removes Winter Damage: It allows you to assess and remove any stems that have died back or been damaged over the harsh winter.
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Promotes New Growth: Once new growth is visible, you can safely cut back older growth to encourage bushiness and a strong framework for the current growing season.
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Maximizes Blooms: Pruning now sets the plant up for a robust flush of flowers in the summer.
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How to Do It:
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Assess the Plant: Wait for visible green growth. If a branch looks dead (brittle, snaps easily), it likely is. If it bends, there's still life.
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Cut Back by One-Third: Cut back about one-third of the plant's current year's growth.
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Aim for Green: Always make your cuts above green foliage. Never cut into old, bare, woody stems that have no leaves, as lavender generally does not regrow from these. Leave at least 2 inches (5 cm) of green growth above any woody parts.
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Shape: Aim for a compact, mounded shape. This improves air circulation and sunlight penetration.
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Use Sharp, Clean Tools: Always use sharp, sterilized pruning shears to make clean cuts, which heal faster.
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2. Light Deadheading/Harvesting (Summer)
This is a very light "pruning" and is often done as you harvest flowers.
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When: As your lavender flowers throughout June and July, you can lightly deadhead spent flower spikes.
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Why:
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Encourages Re-blooming: Some varieties might produce a small second flush of flowers.
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Tidies the Plant: Keeps the plant looking neat.
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How to Do It:
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Snip off the faded flower stalks just above the first set of leaves or where the flower stem meets the main foliage.
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Avoid deep cuts: Do not cut into the main green foliage or woody stems at this time. This is just a light trim of the flower stems.
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Important Pruning Tips for Wisconsin Lavender:
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The Golden Rule: NO FALL PRUNING: This is the most crucial difference for cold climates. Heavy pruning in the fall will stimulate new, tender growth that will almost certainly be killed by Wisconsin's severe winter freezes, potentially leading to the death of the entire plant.
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Patience in Spring: Lavender can be slow to show signs of life in spring in cold climates. Don't assume your plant is dead just because it looks brown in April. Wait until May or even early June before deciding if it needs to be completely replaced. Look for small green shoots emerging from the base.
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Protection: In Wisconsin, it's often a good idea to plant cold-hardy lavender varieties (like 'Munstead' or 'Hidcote' – English lavenders) and ensure they have excellent drainage. A winter mulch of gravel (not wood mulch) around the base can help moderate soil temperatures, but ensure it doesn't hold excess moisture.
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Air Circulation: Good air circulation is important year-round, but especially in spring to dry out the plant quickly after snowmelt and spring rains. Pruning helps achieve this.
By focusing your main pruning efforts in early spring, you'll help your lavender survive Wisconsin's winters and thrive for many years.