By Curtis Swift, Ph.D.
Submitted on December 31, 2015
We cover our fields with frost blankets each winter to ensure we can continue to provide a consistent supply of essential oils, buds, and hydrosol products. Reports from around the nation show considerable losses of lavender plants occur each winter and we don’t want winter loss to be a problem with our fields hence the frost blanket cover.
Mesa Lavender Farms’ fields, at an altitude of approximately 4600 feet, are located in the semiarid region of Western Colorado. This area has relatively dry, mostly snow-free winters, and bright winter sunlight. Our average last killing spring frost (28°F) is ~April 15 however this area can have killing frosts into the middle of May so protection is needed that late into the spring.
This photo shows the crew starting the process of covering the field in Mack Colorado. One year plants are covered with three-foot-wide fabric. Other plants except those on the far right, are covered with 6-foot wide fabric.
HAL grows cultivars of Lavandula angustifolia, commonly called English or Common Lavender, as well as Lavandin. The latter, Lavandula x intermedia, is produced by crossing Lavandula angustifolia and Spike Lavender, Lavandula latifolia. Lavandula angustifolia evolved in a colder environment than Spike Lavender and thus has greater winter hardiness than the resulting hybrid cultivars of the two species. Because of this greater perceived winter hardiness of the L. angustifolia cultivars, Mesa Lavender Farms currently only covers its Lavandins.
The lowest winter temperature recorded by the National Weather Service at the airport in Grand Junction since 1962 was -12° F. A grower friend, however, has indicated -25° F. was a problem for several days at his Grand Junction nursery. In the Grand Valley of western Colorado, winter damage to Lavandula angustifolia and Lavandula x intermedia is most likely not due to low temperatures but rather to sublimation (to be covered below) and plants being simulated into active growth too early in the spring and suffering the consequences of heavy spring frosts.
Mesa Lavender Farms’ fields receive on average 8 inches of precipitation each year with the highest rate of 1.2 inches occurring in September. Precipitation in the form of rain and snow in December, January, and February typically average 0.6 inches per month. With the intense winter sun and dry air, soil moisture losses through evaporation can exceed rain and snowfall levels.
Plants in dry soil are known to break dormancy earlier in the spring than they otherwise would if the soil had adequate moisture. Frost blankets help retain soil moisture by reducing evaporation. We ensure our plants go into the winter months with what we hope is adequate soil moisture but when winter is especially dry we pull aside the frost blankets in February to water the planting beds. We seldom have snow cover at that time of year so rolling the fabric off the plants, while not easy, is usually possible. After watering the frost blankets are pulled back over the plants and pinned into place.
When frost blankets are not used, evaporative losses of soil moisture and early spring warming of the soil can occur. As with dry soil, warm soil causes plants to break dormancy earlier in the spring than is healthy. The protection provided by frost blankets helps correct these problems.
HAL’s plants often begin growth under the frost blankets in early spring and this new growth is susceptible to late spring frosts consequently the blankets are typically not removed until mid-May. A quick trim of the plants after removing the blankets helps correct any leggy shoots and damaged areas.
How soon do we apply our frost blankets?
With fall planted lavender (planting in August or September), frost blankets are placed over the plants immediately after planting and not removed until mid-May the following year. Past experience has shown great success with this process. In the fall the frost blankets trap in soil warmth which allows root growth to continue later than otherwise would occur. This ensures the plants became better rooted before the soil turns too cold for root growth. Once the soil freezes the blankets help keep the ground cold longer into the spring. We ensure the soil is moist going into the winter even if we need to roll the fabric off the plants and water the planting beds. After watering the fabric is placed back over the plants and pinned to the ground.
How late have we applied our frost blankets?
We like to have the fields covered in December but time does not always permit this task to be accomplished that early. We may not be able to apply blankets until mid-January. When possible we prefer to apply blankets shortly after a snowstorm. The snow trapped under the blankets will eventually melt and help improve the soil’s moisture content. Snow on unprotected plants can dry out and kill plant tissue through the sublimation process.
Most of this field of ‘Grosso’ Lavandin has been covered. The four rows of 5-year-old plants in the foreground will be covered with twelve-foot wide fabric with two rows under each blanket. The remaining one and a half rows of two-year-old plants seen in the background will be covered with 6-foot wide fabric. The rolls of fabric we use are 250 feet long. With the 16 rows in this field, each 420 feet long 6,720 linear feet of fabric is required.
Sublimation is a condition where snow changes from a solid to a gas without passing through the liquid phase. As sublimation occurs moisture is pulled from plant tissues and any soil the snow touches. The drying of the soil results in roots losing moisture and dying.
These photos show an unprotected plant at the top and winter damaged plant below. The damage is most likely due to sublimation.
When frost blankets are placed over plants or soil covered with snow, it affords protection from the intense sun and snow is less likely to undergo sublimation. In the spring the blanket-protected snow melts and seeps into the ground helping keep the soil cold and the plants dormant.
Depending on the thickness of the frost blanket, rainfall may run off the blanket helping prevent excessive soil moisture levels which are devastating to lavender. While high rainfall levels typically are not a problem in our area such conditions can be problematic in other areas where lavender is grown. The proper thickness of fabric used as a frost blanket, especially if the plants are on a raised bed, will help shed water and help prevent damage due to excess soil moisture levels and root rot pathogens such as the fungal-like organism Phytophthora.