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Regional Calendars, Climate & DataHeat Stress Calendar for Crops
Heat can hurt crops as much as frost, but it is easier to plan around than to fix. A heat stress calendar maps the hot weeks to the crop stages that are most sensitive.
Key points
- Heat hurts most at flowering, so keep sensitive stages out of the hottest weeks.
- Use shade, mulch and early-morning irrigation.
- Watch night temperatures as well as daytime highs.
- Protect people, animals and pollinators in hot weather.
How does heat hurt crops?
Hot weather raises water demand, can scorch leaves, and interferes with flowering and fruit set in many crops. Hot nights add stress because plants lose energy to respiration when they should be recovering. Cool-season crops such as lettuce and spinach bolt in heat and turn bitter.
Different crops have different limits, and sensitivity depends on stage. Check extension crop guides for how heat affects the crops you grow.
Soil temperature matters too. Hot soil can stop seed from germinating, as with lettuce in midsummer, and can stress roots of transplants. Check soil with a thermometer before sowing cool-season crops in late summer.
Which growth stages are most sensitive?
Flowering and pollination are among the most heat-sensitive stages in crops such as tomatoes, peppers, beans, corn and squash, and heat during that period can reduce fruit or grain set. Seedling stages are also vulnerable, because small plants have shallow roots and little reserve.
Grain and fruit fill can be affected by heat and water stress, reducing size and quality. Mark the likely flowering period of each crop on your calendar and compare it with your usual hot weeks.
- Seedlings: shallow roots, easily scorched
- Flowering: heat can reduce fruit or grain set
- Fill: heat and dry soil reduce size and quality
How do you time plantings around the heat?
Aim for sensitive stages to fall outside your hottest weeks. That might mean planting early so flowering happens before peak heat, or planting later so flowering follows it. Many gardeners plant cool-season crops in early spring and again in late summer for fall harvest.
For example, if your hottest weeks usually come in July, you might plan tomatoes and peppers to set fruit before then, and sow the fall brassicas in late summer when heat starts to ease. That is an example of timing, not a recommended date.
Succession planting across several weeks spreads risk, because a single heat wave then catches only part of your plantings at a sensitive stage.
What protects crops during hot weeks?
Shade cloth during afternoon peaks, mulch to cool the soil, windbreaks to reduce hot drying winds and well-timed irrigation all reduce stress. Choose varieties described as heat tolerant, and give plants room for airflow.
Water deeply in the early morning rather than lightly in the heat of the day, and follow local water rules, which vary by community and can tighten in drought. For irrigated crops, schedule by soil moisture and crop stage.
Reflective mulch, shade structures and overhead cooling are used in some systems, and your extension office can say which fit your crops and region. Check water use rules before using any method that increases water demand.
How should you work and harvest in hot weather?
Heat affects people too. Schedule field work for the cooler part of the day, drink water, take shade breaks and watch for signs of heat illness in yourself and your crew. Employers and farm owners should learn the heat safety rules that apply in their state.
Harvest early in the morning where possible, since produce picked cool holds quality longer. Keep harvested produce shaded and cool before storage or sale.
Plan breaks, shade and drinking water before the heat arrives instead of improvising during it, and make sure everyone on the crew knows the warning signs and what to do if someone feels unwell.
What about livestock, pollinators and nights?
Hot weather also affects animals and pollinators. Provide shade and water for livestock, and keep water sources available for bees and other beneficial insects. If you must use any pesticide, follow the label, pre-harvest intervals and pollinator protections, and apply only as allowed in your state.
Warm nights matter too: tomatoes and some other crops may fail to set fruit when nights stay warm. Watch minimum temperatures as well as daytime highs.
How do you build a heat stress calendar?
List your crops down the side of a sheet and the months across the top. Mark flowering and fill periods, then shade the weeks that your records show as hot. Where the marks overlap, plan shade, water or a change of planting date.
Update the sheet each year with your own records of hot weeks and results. Your county extension office can suggest heat-tolerant varieties and timing for your area.
Review the sheet each fall with notes on which weeks were hottest and which crops struggled, so next year's plan adjusts to what happened.
Sources and further reading
Last reviewed: November 2026. Educational information, ranges only. Confirm timing with your county extension office.