Frost and Cold Wave: How to Protect Field Crops and Garden Plants

Quick answer: Frost protection in crops means keeping plant tissue above freezing on cold, clear, still nights. The simplest steps are a light irrigation in the evening when frost is forecast, covering nurseries and young plants overnight, and, for mustard, gram and rajma, a spray of 0.1% sulphuric acid or 500 ppm thiourea as advised in agromet bulletins.

Key Takeaways

• IMD declares a cold wave in the plains when the minimum temperature is 10 C or below with a departure of 4.5 to 6.4 C below normal, or when it falls to 4 C or below.
• Frost usually forms on clear, windless nights. Damage comes from ice forming in plant tissue, which pulls water out of cells.
• Moist soil stores and releases more heat than dry soil, so a light evening irrigation before a frosty night protects crops.
• Mustard, gram, rajma, potato, tomato, peas, papaya, banana and young fruit plants are sensitive. Wheat generally tolerates cold well.
• Cover nurseries and small plants at night and remove the cover in the morning. FAO notes that smoke does not protect crops and pollutes the air.

What is frost and how is it different from a cold wave?

Frost is the formation of ice on or inside plants when the temperature at plant level drops to 0 C or lower. A cold wave is a spell of abnormally low temperature declared by IMD over a region. A cold wave raises the risk of frost, but frost can also form on a single clear night without a declared cold wave.

According to the IMD cold wave FAQ, a cold wave is considered in the plains when the minimum temperature is 10 C or less and the departure from normal is 4.5 to 6.4 C, or when the actual minimum is 4 C or less. A severe cold wave means a departure of more than 6.4 C or an actual minimum of 2 C or less. For hilly areas, the threshold is a minimum of 0 C or less.

In India, frost risk is highest from mid-December to late January in Punjab, Haryana, Rajasthan, UP, MP, Bihar and Gujarat, and on clear nights in hill states.

How does frost damage crops?

Frost damages crops mainly through ice formation between plant cells, not through cold alone. As the FAO frost protection manual explains, this ice draws water out of the cells and dehydrates them, and the injured tissue dies when it thaws.

On clear, calm nights, the ground and leaves lose heat quickly to the open sky. The coldest air settles in low-lying fields and near the soil surface, which is why young plants and nurseries are hit first. Cloud cover or wind usually keeps temperature at plant level a little higher.

Signs of frost damage

• Leaves look water-soaked or glassy in the early morning, then turn brown or black by afternoon.
• Flowers and young pods of mustard, chana and peas dry up or drop.
• Potato and tomato foliage turns black and limp at the top of the canopy.
• Papaya and banana leaves become papery and torn; young mango and guava shoots dry back.

Which crops are most at risk from frost?

Crops in flowering and pod-filling stage, vegetables with soft leaves, and tropical fruit plants are most at risk. Wheat generally tolerates winter cold and often benefits from a cool spell, although severe frost at heading can still harm it.

How to protect field crops from frost, step by step

Most protection must be done before sunset on the night frost is expected. Watch IMD district forecasts and agromet bulletins from mid-December and act when minimum temperature is forecast near 4 C or lower with clear skies.

• Check the forecast every afternoon. Clear sky, no wind and a minimum near 0 to 4 C mean high frost risk.
• Give a light irrigation in the evening, or run sprinklers. Moist soil holds and releases more heat through the night. Avoid heavy flooding.
• Do not hoe or loosen the soil during frost spells. A loose surface reduces heat flow from the lower soil to the surface.
• For mustard, gram and rajma, apply the protective spray recommended in agromet advisories (details below) before the cold spell.
• Cover nurseries with polythene at night and remove the sheet in the morning so seedlings do not overheat.
• Wrap young fruit plants with sarkanda, straw, polythene or gunny bags, leaving the south-east side open for sunlight.
• Plant windbreaks such as tall crops or trees on the north and north-west side of orchards and vegetable plots for future seasons.

How to plan ahead so frost does less damage

Passive protection means choices made weeks or months before a frost night that lower the risk. FAO lists site selection, cold-air drainage, plant selection, nutrition, covers and soil management among these methods.

• Avoid planting sensitive vegetables and nurseries in low-lying patches where cold air collects on still nights.
• Time sowing so that flowering of mustard and chana does not fall in the coldest weeks of late December and early January where possible.
• Keep the soil surface firm and moist rather than freshly dug; FAO notes that soil near field capacity transfers and stores heat better.
• Keep the orchard floor free of tall weeds before winter, since a dense weed cover keeps the soil surface colder at night.
• Keep old sheets, straw, sarkanda and poles ready so covers can go up quickly on a forecast night.

These steps cost little and make the emergency measures on a frost night far more effective.

Does spraying sulphuric acid or thiourea protect crops from frost?

Yes, agromet advisories recommend these sprays for frost-sensitive crops. The agromet advisory for cold conditions advises spraying mustard, gram and rajma with dilute sulphuric acid at 0.1% (1 litre of H2SO4 in 1,000 litres of water) or thiourea at 500 ppm (0.5 g per litre).

Thiourea is easier and safer for most farmers to handle. If you use sulphuric acid, always add acid slowly to water, never water to acid, wear gloves and eye protection, and use a plastic sprayer. Follow the product label and local agriculture officer advice.

Balanced nutrition before winter also helps plants handle stress. A seaweed-based biostimulant such as seaweed extract fertiliser and adequate potassium from a source like sulphate of potash (NPK 0-0-50) support plant vigour. These do not replace irrigation or covers on a frost night.

Does smoke protect crops from frost?

Smoke is widely used in villages, but the FAO frost manual states that smoke does not help frost protection and pollutes the air. It advises against using it.

The warming people feel near a fire is local and short-lived. On a clear night the heat rises and is lost, and the smoke adds to the winter pollution already present in north India. Burning crop residue for smoke can also attract penalties in states where residue burning is banned. Irrigation and covers give more reliable protection.

How to protect garden and potted plants from frost

Frost often forms at 0 to 2 C on clear nights, so home gardeners in north India should act when the forecast minimum drops below about 4 C. Tender plants like tomato, chilli, basil, tulsi, money plant, adenium and succulents need the most care.

• Move pots under a verandah, shed or roof overhang in the evening, or cluster them against a warm wall.
• Water the soil lightly in the afternoon, not late at night, so it holds heat but leaves are dry by evening.
• Cover beds with old cotton sheets, jute bags or frost cloth resting on sticks, and anchor the edges. Avoid plastic touching leaves.
• Remove covers mid-morning once the sun is out.
• Mulch the soil surface with compost or enriched vermicompost to keep roots warmer and feed the plant gently.
• Avoid heavy fertiliser in December; soft new growth is more prone to frost burn.

What to do after frost damage

Do not rush to cut damaged parts. Wait a week to see how far the damage goes, because some browned leaves recover and dead tissue gives some cover to the growth below during the next frost.

• Give a light irrigation after the cold spell if the soil is dry.
• Once warm days return, prune dead shoots of fruit plants back to healthy wood.
• Scout potato and tomato for late blight, and mustard for aphids, which often rise after cold, cloudy weather.
• If the crop is insured under PMFBY, report localised damage to your insurer or agriculture office within the time limit given in your policy.

What to do next

From mid-December, check the IMD district forecast every afternoon and keep irrigation water, covers and a sprayer ready. Mark your mustard, chana and vegetable fields that are at flowering, since those need protection first.

For garden plants, decide now which pots can be moved and keep a few old sheets or jute bags handy. Dealers stocking winter crop-care products can contact Katyayani Organics for bulk supply.