by: Curtis Swift, Ph.D., High Altitude Lavender, Colorado
Have you suffered loss of your lavender planting due to winter injury? There are a number of reasons this could have occurred. Did you plant the wrong cultivar or did you lose plants even when the cultivar you selected was said to tolerate the USDA hardiness zone you live in?
Species and Cultivar Selection
Selecting the proper species and cultivar of Lavandula for your site is critical. Talking with other growers from areas with similar winter climatic conditions will help you avoid the mistake of planting Lavandula that do not belong in your area. For example, if you really want to grow Lavandula stoechas in Colorado then consider growing this tender lavender in a heated greenhouse, otherwise don’t plant it. Even when you have selected what you believe should be successful in your area, there are steps you can take to help ensure survival.
Pruning
Keep in mind plants that are succulent suffer more winter damage. This can be due to late season shearing/shaping or an excessive amount of plant available nitrogen. Succulent plant are unable to properly prepare for winter and suffer the consequences. Even the process of collecting cuttings late in the season can slow the winter acclimation process. Pruning, shearing, shaping tends to stimulate growth and this is something you don’t want to do as winter approaches. Growers in areas where winters are mild can continue to shape and shear their plants but those of us who have cold winters will suffer less damage if we quit cutting plants by mid-August.
Nitrogen
Nitrogen is critical to the overall health of the plant and if a soil test shows an inadequate level, nitrogen should be applied, but only at the proper time. Without nitrogen the plant is unable to develop adequate roots, carry on its many metabolic processes, or produce and store the sugary energy reserves it needs to carry the plant through winter into spring. The key is not applying excessive amounts and applying nitrogen at the proper time. The last application of nitrogen fertilizer for the season should be made after the first harvest but not too late. This recommendation is important whether you use a synthetic or organic product. Slow release synthetic fertilizers may still release nitrogen into the fall months causing succulent growth, therefore using them is not encouraged.
Mulching
Covering the soil with a mulch layer of organic matter, such as wood chips, hay or other plant material, will help keep the soil moist through winter and reduce frost heaving responsible for tearing apart roots. Dehydrated or damaged roots means the stems and leaves above are unable to receive the water and nutrients needed for survival and can stimulate early spring growth. Much of the winter damage I see in Colorado lavender fields is on the south or south-west side of the plants. Buds and stems on these exposures are more likely to break dormancy earlier than plant tissues on the north or east of the plant in part due to the lack of adequate soil moisture on those sides of the plant. Lavandula has a vascular system where roots on one side provide water and nutrients to the above ground parts on the same side. Insufficient soil moisture (or damaged roots) can cause that portion of the plant to come out of dormancy and be damaged by spring freezes. Keeping the soil cold and moist well into spring helps reduce winter damage.
Ground Cover
When considering a fabric ground cover for your lavender planting, the need to cover the fabric with mulch may even be more important. Black fabric mulches tend to heat the air underneath (between the soil and fabric) warming the soil and encouraging the plant to break dormancy too early in the spring.
Soil Preparation
Of course no discussion of enhancing winter survival would be complete unless soil preparation was mentioned. The deeper and more extensive the root system, the more prepared the plant will be for winter. This ca not occur unless you have spent the time and money to prepare the soil properly. This may mean the addition of organic matter to enhance drainage and opening up the soil to permit root penetration. But soil preparation has been discussed previously so I won’t dwell on that topic. (link to Lavender 101 article on Soil Preparation – http://www.uslavender.org/2013/10/soil-preparation-for-lavender/)
Frost Blanket
The final step you can take to enhance winter survival is placing a frost blanket/barrier over the plants. You might decide to use a fabric on those cultivars you suspect are not as hardy as they need to be for your site. I have applied this protective layer as early as September when I planted a new field. I did not remove it until after the last spring frost and had no damage to the plants. I have also applied fabric as late as January. The key to success of preventing winter injury is leaving the fabric on until after the last possible freeze in the spring.