Vetiver is known for its versatility and tolerance to a wide variety of environmental conditions including drought, flooding, fire, poor soil, salt, and contaminants, making it a great choice for bioengineering applications in many areas of the world. As a tropical grass, it also has a relatively strong tolerance to cold. When the temperature starts to drop, its top growth becomes dormant while the underground portion remains alive. Soil acts as an insulator storing heat, keeping the subsurface warmer than the air or surface temperature. When the surface freezes, vetiver roots will continue to grow until about 5°C, at which point they become dormant.
Based on previous research and experience, vetiver dies when the corm freezes. Thus, air temperature can get very cold temporarily and the plants will be able to survive. In Australia, vetiver growth was not affected by severe frost at -14°C and it survived for a short period at -22°C in northern China. For more information on vetiver’s cold tolerance, visit Dr. Paul Truong’s powerpoint slides on the topic and read his latest publication.
Like soil, aboveground biomass and snow can be very good insulators for the plant’s base. For example, in February 2021, temperatures in Texas USA abnormally dropped to below freezing for ten consecutive days. We at Leachate Management Specialists have a field of vetiver planted on top of a closed landfill in College Station, Texas where air temperature dipped to -14°C. Soil temperature at 5 cm below ground dropped to 5°C and 6.7°C at 15 cm. During this period, it also snowed, so that acted as additional insulation. Our vetiver there were full-grown plants already in dormancy for the winter. By the following Spring, the plants started regrowing like normal and returned to a healthy field.
So what happens to YOUNG vetiver when cold temperatures arrive? They don’t have the aboveground and belowground biomass for protection, so the corm is more exposed and vulnerable to the cold. In 2004, Liyu Xu in China completed a study of 2-month-old vetiver and found that 90% survived when soil at 5 cm belowground was 2°C but only 50% survived when that depth was -1°C. In 1999, Robert Adams in the USA compared twelve different cultivars, all similar in DNA to the Sunshine or Monto. He tested 6-month-old plants and found that differences in cold tolerance existed between the cultivars and soil freezing at 15 cm below ground may have been the critical factor for survival.
To provide supplemental research to the Adams study, we wanted to compare cold tolerance of some of the same cultivars at two different locations in the USA. We first planted four of the best performing cultivars in Collegeville, Pennsylvania (PA) and Little Rock, Arkansas (AR) in 2018. The plants were one month old before dormancy arrived. In PA, the soil temperature at 5 cm and 15 cm leveled out at about 3°C and air temperature dropped to -22°C, but there was snow cover during that coldest period which insulated the soil. Meanwhile in AR, the soil temperature dipped to 5°C several times and air temperature hit -5.3°C. By the spring, it was clear that none of them survived at either location.
In 2019, we again planted the same cultivars, but gave them more time to grow before winter. In PA, 5-month-old vetiver had a 50% survival rate when soil temperature at 5 cm dropped to 3°C and air temperature was -14°C (with lower snow totals). The cultivars varied in survival. That same winter in AR, all of the cultivars survived near a building with the area’s soil temperature briefly down to 5°C a couple of times and minimum air temperature was -7.8°C.
In 2020, two of the best performing cultivars were planted again in PA and had about two months to grow before dormancy. None of them survived the winter when soil temperature at 5 cm and 15 cm underground leveled out at 3°C and air temperature dropped to -16°C while there was snow on the ground.
The results of this study of young vetiver cultivars support the earlier conclusions that survival depends on if the corm or ground freezes, although cultivars may differ and younger plants are more susceptible with exposed bases and may still die if soil under the surface doesn’t freeze. It also appears snow and biomass help insulate the plants from freezing. Additional information and photographs about this trial are found here.