Many warm-season crops, such cantaloupe, capsicums, chillies, cucumbers, eggplants, lima beans, pumpkins, squash, tomatoes, and watermelon, are native to subtropical climates, and are therefore very frost-sensitive.
They prefer temperatures of 21°C (70°F) to 35°C (95°F), and need daytime temperatures above 15.5°C (60°F).
A week of daytime temperatures below 12.5°C (55°F) may stunt their growth, and when overnight temperatures fall below 8°C (46.5°F), plants may be damaged, especially seedlings, as they’re more frost-tender than mature plants.
When are Warm Season Vegetables Most Likely to be Damaged by Cold?
Frost occurs on clear (cloudless), windless nights in early spring when the air temperature drops to 2°C (35.5°F) or less. Frost damage may occur at any stage of plant development, but is most damaging at the seedling or flowering stages.
- Young plants and seedlings are vulnerable to frost damage in early spring, the start of their growing season. To avoid this problem, gardeners plant their warm season vegetable seedlings 1-2 weeks after the last frosts have passed.
- Mature plants are vulnerable to cold damage late in their growing season, in autumn, when overnight temperature can drop quite low.
How Cold Affects Warm Season Plants
When the temperature falls to freezing (0°C or 32°F) and below, the water inside the plant cells freezes, causing the cells to die. When this occurs, plants will appear visibly frost-burnt.
As temperatures drop towards the minimum required for growth, the metabolic rate of plants reduces, and as a result their growth slows down. If the temperatures remain low for extended periods, plant may decline in quality, and may eventually die.
Increasing Plant Frost Tolerance with Seaweed Extract
Gardeners have traditionally protected plants from frosts using various methods, such as greenhouses, row covers, cloches and growing near sun-facing walls.
Frost protection ensures that vegetable seedlings get off to a good start with minimum plant stress, and mature plants can keep cropping late into their growing season, extending their productive period and increasing yields.
One very simple way to increase the cold tolerance of plants is by using seaweed extract, which is essentially dried and liquefied seaweed.
Research over the last few decades has shown many significant benefit to using seaweed extract in agricultural applications.
The benefits of seaweeds application in agricultural field are numerous and diverse such as stimulation of seed germination, enhancement of health and growth of plants namely shoot and root elongation, improved water and nutrient uptake, frost and saline resistance, biocontrol and resistance toward phytopathogenic organisms, remediation of pollutants of contaminated soil and fertilization.(Nabti, 2016)
The compounds in seaweed extract work to enhance various phases of plant growth, increase pest and disease resistance, as well as reduce plant stress.
Seaweeds affect agricultural crops for enhancement of plant growth, seedling growth, both root hair and secondary root development. It can also improve nutrient incorporation, fruit setting, resistance properties against pests and diseases, improving the stress management (drought, salinity and temperature).(Mukherjee, 2020)
How Do Seaweed Extracts Work to Help Stressed Plants?
One of the major factors that cause major crop losses worldwide are abiotic stresses of plants. These are non-living factors that have a negative impact on plants, such as drought, salinity, temperature extremes, flood, heavy metals, ultraviolet radiation, and other environmental extremes.
Many abiotic factors such as drought, salinity, and temperature are manifested as osmotic stress and cause secondary effects like oxidative stress, leading to an accumulation of reactive oxygen species (ROS) such as the superoxide anion (O⁻ ₂) and hydrogen peroxide (H₂O₂). These are known to damage DNA, lipids, carbohydrates, and proteins and also cause aberrant cell signaling.(Khan, 2009)
The collective studies on the subject to date suggest that seaweed products produce abiotic stress tolerance in plants, and the bioactive substances derived from seaweeds impart stress tolerance, and enhance plant performance.
In studies, plants sprayed with seaweed extracts show enhanced salt and freezing tolerances. It was also found that seaweed extracts contain compounds known as betaines, including gamma-aminobutyric acid betaine, 6-aminovaleric acid betaine, and glycine betaine, which have been shown to enhance leaf chlorophyll content of plants.
Do we understand how all the chemistry works?
Not quite. What is understood is that a class of compounds found in seaweed extracts, known as cytokinins, play an important anti-stress role in plants.
Cytokinins are one of the major plant hormones that regulate numerous aspects of plant growth and development. They are primarily involved in performing cell division (production of new cells required for growth) in plant roots and shoots. These hormones helps promote cell growth, development, and differentiation, affecting apical dominance and delay in leaf senescence (leaf drop).
The chemistry of bioactive compounds in the seaweed and the physiologic mechanism of action of the compounds that impart this tolerance are largely unknown. However, a number of reports suggest that the beneficial antistress effects of seaweed extracts may be related to cytokinin activity.
Cytokinins mitigate stress-induced free radicals by direct scavenging and by preventing reactive oxygen species (ROS) formation by inhibiting xanthine oxidation.
Reactive oxygen species (ROS) are a common factor in many abiotic stresses such as salinity, ozone exposure, UV irradiation, temperature extremes, and drought(Khan, 2009)
Even though we can only partly explain how seaweed extracts work to support plant growth and reduce plant stress, we do know for sure they they do work!
Application Rate of Seaweed Extract for Frost Protection
For frost protection, seaweed extract is applied as a foliar spray (on the leaves of the plants) at a rate of 3-5 mL per litre of water when used in hand sprayer or backpack sprayer; or 20mL to 50mL per 9L watering can. (Note: one teaspoon = 5ml)
A foliar application of seaweed extract provides crops with an additional 2-3°C extra frost tolerance, and will keep plants protected for a period of 10-12 days.
Seaweed extract will start to provide protection after 36 hours from application, but if the product also contains potassium (most do), the potassium will provide protection within 6 hours of application.
Most commercial seaweed extracts include potassium, an essential plant macronutrient which plays an important role in plant protection. This nutrient not added intentionally, it’s there because it’s left over from the industrial process that uses potassium hydroxide (caustic potash) to break down the dried seaweed raw materials.
If we look at the description of an agricultural seaweed extract product that is sold for crop protection, AgroBest Demeter, we see the role potassium plays.
With its seaweed base and the additive of potassium, crops will be protected both long term and short term.
The potassium provides protection to the plant within 6 hours of application, and the seaweed will continue protection after 36 hours to keep plants protected for a period of 10-12 days.
Demeter will provide approximately 2-3°C extra frost tolerance, whilst also providing nutritional benefits essential to plant quality.
The manufacturer of the product states that the degree of protection will vary between crops, and the seaweed extract also helps plants that are already frost affected to recover.
Stone fruit, apples and pears will receive approximately 2-3°C extra frost tolerance, while canola and cereal crops see additional protection of up to 4°C. Grapes can reach up to 2.5°C extra protection and in the case of vines application should be made each 7-10 days. Results indicate that potatoes will have an additional 2°C of protection, or for crops which have already been frost affected Demeter will aid with the recovery of the crop.
The Materials Safety Data Sheet (MSDS) shows the ingredients and their proportions:
- NAME, CAS NUMBER, CONCENTRATION
- Potassium 7440-09-7 <15%
- Phosphorous 7723-14-0 <5%
- Seaweed n/a 25 – 45%
- Other ingredients (considered non-hazardous) n/a Balance to 100%
The description of the home garden. seaweed extract product. Seasol, is far less detailed in its use for frost protection, which is typical of gardening product information aimed at the general public.
Use Seasol regularly every two weeks throughout the year (30mL concentrate per 9 litres of water – standard watering can) can protect foliage. Frost damage is caused when frozen plant cells thaw out and rupture. With regular use, it can lower the temperature at which plant cells freeze. It’s a bit like antifreeze for your garden, providing plants with a better chance of standing up to frost and colder temperatures.
The application rates of the agricultural and home products are essentially identical, because they’re the same thing, it doesn’t matter which is used. Either spray the foliage or mix into a watering can and water over the plants, for 2-3°C extra frost tolerance.
Watering vegetable and herb seedlings with seaweed extract straight after planting is a really good practice. It stimulates new root growth, which helps the plants establish faster, and become more resilient, while also increasing tolerance to stress caused by extreme weather and environmental conditions.
More articles on Garden Pests, Diseases and Problems
- Nabti, Elhafid & Jha, Bhavanath & Hartmann, Anton. (2016). Impact of seaweeds on agricultural crop production as biofertilizer. International Journal of Environmental Science and Technology. 14. 10.1007/s13762-016-1202-1.
- Mukherjee, A., Patel, J.S. Seaweed extract: biostimulator of plant defense and plant productivity. Int. J. Environ. Sci. Technol. 17, 553–558 (2020). https://doi.org/10.1007/s13762-019-02442-z
- Khan, Wajahatullah & Menon, Usha & Subramanian, Sowmyalakshmi & Jithesh, Mundaya & Rayorath, Prasanth & Hodges, D. & Critchley, Alan & Craigie, James & Norrie, Jeffrey & Prithiviraj, Balakrishnan. (2009). Seaweed Extracts as Biostimulants of Plant Growth and Development. Journal of Plant Growth Regulation. 28. 386-399. 10.1007/s00344-009-9103-x.
- Colorado State University Extension, Master Gardener – CMG GardenNotes #720, Vegetable Planting Guide, Revised October 2014
- Purdue University Cooperative Extension Service – HO-203, Effects of Cold Weather on Horticultural Plants in Indiana, Larry A. Caplan
- South Dakota State University Extension – Fall Frost Tolerance of Common Vegetables, Updated September 05, 2019, Rhoda Burrows