Why should we prune lavender in the first place
While the lavender plant is perfectly capable of soldiering on sans pruning, there lies a distinct advantage in adhering to an annual pruning regimen.
A common reason home gardeners desire to implement a lavender pruning regimen is aesthetics—left unpruned, lavenders mature to develop woody, leafless growth in the center of the plant, giving them an untidy, leggy appearance that deviates from the idealized cultivated ‘mound’.
A second reason is to promote more blooms. Whether you enjoy the look of fuller plants, or are cultivating the blooms for harvesting, more blooms are in the interest of people growing lavender for production purposes as well as aesthetics.
However, because pruning lavender keeps the plant’s shoots in a juvenile state, it can affect the plant’s ability to withstand cold winter temperatures. Cutting back too hard can potentially result in killing the plant when done improperly. To prevent injury, gardeners need to make sure they know both the time of year and the correct techniques to cut back lavender so that it remains a tidy mound that can be enjoyed over many years.
When to prune lavender
The cadence for pruning lavender depends on three major factors: the maturity of the plant, cold hardiness of the specific variety being grown, and the gardener’s desire to support wildlife habitat.
For the young, newly planted lavender, the first year is about fostering robust root growth. Pruning isn't necessary during this phase. The objective during this time is to have as much leaf canopy to support healthy root growth, and to allow the young shoots to mature and harden off so they can survive their first winter with minimal damage. If desired for aesthetics, only a very light pruning after flowering is recommended (see section below).
After the first year, gardeners can begin a regular pruning regimen. The safest time of year to prune any lavender species is early in the Spring after all danger of frost has passed (check to verify your area’s first frost date). Pruning promotes shoot growth, and pruning during this period allows the shoots ample time to toughen up before winter's chill. Avoid postponing pruning into the late spring or summer, this risks accidentally pruning off desired flower buds.
It is often recommended to give lavenders a light prune in the late summer right after flowering. However, because this may not give new shoots sufficient time to harden off before winter, gardeners living in harsher winter climate areas should consider their lavender’s cold hardiness before choosing to prune in the late summer. Less hardy lavender species, such as Lavandula stoechas, may suffer winter die-back when pruning after flowering if you live in USDA Cold Hardiness Zone 8 or colder. The same could happen with Lavandula latifolia for those living in Zone 6 or colder. You can find your cold hardiness zone here: https://planthardiness.ars.usda.gov/. Once you know your zone, you can use the table below to identify whether the lavender you are growing might be susceptible to damage from a late season pruning. Common lavender species are listed in order from most to least cold hardy.
Gardeners interested in supporting wildlife habitat may choose to refrain from a late season prune altogether. When pollinated lavender flowers are allowed to set seed, they produce nutlets that can serve as a food source for many birds species well into the winter months.
How to prune lavender
To prune any plant, including lavender, its best to understand some basics about plant anatomy. It is aways best to make your pruning cut at shoot internodes—the stem segments in between ‘nodes’ or leaf buds of the plant. For selective pruning at internodes, it is best to use a pair of hand pruners such as secateurs. Because these internodes occur quite frequently on lavender shoots, lavenders may be pruned using shears if desired.
For lavenders, it is important to note that plants can be killed by cutting back hard down to the leafless woody stems. As lavender plants age, leaf buds in the center of the plant senesce and loose their ability to produce new leaves. When shoots are cut back to these leafless wood segments, they are unable to produce new shoots, resulting in the death of the plant. When hard pruning, always use the last few leafy nodes as a guide and make your cut at the internode just above the last few leafy nodes.
Mature plants can sometimes exhibit a flush of leafy growth at the base of the plant, indicating new emerging shoots. If this is the case, older shoots may be cut back to the base to allow for these new shoots to replace them.
When pruning lavenders, it is also helpful to understand the difference between a flowering and a non-flowering shoot. Flowering shoots have a terminal cluster of flowers, technically known as a verticillaster inflorescence, at the terminus of the shoot. Non-flowering shoots lack this inflorescence and are typically shorter in length.
When light pruning in the late summer, a good rule of thumb is to cut back all shoots (both flowering and non-flowering) approximately one inch (1”) of shoot length below the tip of non-flowering shoots. Avoid pruning back more than a few inches. If the plant is pruned too hard at this time, it may suffer winter damage.
The ‘hardest’ pruning should be done in the spring. Remove any dead, diseased, or winter damaged shoots. Aim to remove approximately one third (1/3) of the canopy at this time.
Happy pruning!
Resources
Klingaman, G. (2012, 05 04). Plant of the Week: Spanish Lavender. Retrieved from U of A Division of Agriculture: https://www.uaex.uada.edu/yard-garden/resource-library/plant-week/lavender-spanish-5-4-12.aspx#:~:text=The%20flower%20is%20distinctive%20amongst,like%20bracts%20at%20the%20top.
Lavandula angustifolia. (n.d.). Retrieved from Missouri Botanical Garden Plant Finder: https://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?taxonid=281393
Lavandula latifolia - Medik. (n.d.). Retrieved from Plants for A Future: https://pfaf.org/user/Plant.aspx?LatinName=Lavandula+latifolia
Lavandula stoechas. (n.d.). Retrieved from NC State Extension: https://plants.ces.ncsu.edu/plants/lavandula-stoechas/
Lavandula x intermedia. (n.d.). Retrieved from NC State Extension: https://plants.ces.ncsu.edu/plants/lavandula-x-intermedia/
Lavender, the Useful Plant. (2022, 06 01). Retrieved from UT Institute of Agriculture: https://utgardens.tennessee.edu/lavender-the-useful-plant/