Climate: what temperatures chiles need

Chiles are warm-season plants from the tropics and subtropics. Cold stalls them, frost kills them, and too much heat makes them drop their flowers. Between those limits there is a fairly wide band where they grow well. Soil and feeding are covered in the Soil and Nutrients chapters.

Temperature ranges by stage (germination, growth, fruit set)

Minimum night temperature

Transplant once night lows stay above 10 °C (50 °F)³

Minimum daily average

About 17 °C (62.5 °F), derived from the Chile Pepper Institute’s transplanting temperatures²

Heat limit for fruit set

Blossoms may not set above 32 °C (90 °F)¹

Best growing range

Days 18–29 °C (65–85 °F), nights 16–21 °C (60–70 °F)¹

Every guide consulted here agrees on the broad shape: warm days, mild nights, no frost. The exact figures vary a little from source to source, so the table gives a representative figure for each stage with its source.

Germination is covered in detail in the Seed Starting chapter. The short version is that chile seed wants warmth: the RHS suggests around 21 °C (70 °F) in a heated propagator or on a warm windowsill.⁷

The night minimum: 10 °C (50 °F)

The University of Minnesota advises transplanting outdoors once night lows are above 10 °C (50 °F).³ That is the floor for planting out and slow, steady growth. Other sources are more cautious: the RHS says chiles can usually cope with a minimum night temperature of about 12 °C (54 °F) but grow better above 15 °C (59 °F),⁷ and the Chile Pepper Institute suggests transplanting when nights are around 13 °C (55 °F).² Treat 10 °C (50 °F) as the lowest acceptable night, not the ideal. Flowers need warmer nights still, as the next section explains.

The daily average: about 17 °C (62.5 °F)

None of the sources gives a minimum average daily temperature directly; they give day and night temperatures instead. The Chile Pepper Institute says to transplant when daytime temperatures average 21 °C (70 °F) and nights are around 13 °C (55 °F).² A day’s average sits roughly halfway between its daytime and night-time temperatures, so take the daytime figure and subtract half the day-to-night swing: 70 °F minus half of 15 °F gives 62.5 °F, or about 17 °C.² This guide uses 17 °C (62.5 °F) as the lowest daily average for active growth. Two other guides point to the same number: the low ends of NMSU’s ideal day and night ranges, 18 °C (65 °F) and 16 °C (60 °F),¹ and Ohio State’s 21 °C (70 °F) days with nights above 13 °C (55 °F)⁵ both average out to 62.5 °F.

Fruit set

Flowering is the most temperature-sensitive stage. NMSU says blossoms may not set if temperatures drop below 16 °C (60 °F) or rise above 32 °C (90 °F).¹ The Chile Pepper Institute singles out night temperature as the key to flower set, ideally 18–27 °C (65–80 °F).² Oregon State notes that some cultivars will not even flower below 16 °C (60 °F).⁶ Cool weather also slows ripening: NMSU says mature green fruit is ready 45–55 days after pollination under ideal conditions, and later in cool weather.¹

Heat stress and flower drop

This guide takes 32 °C (90 °F) as the daytime high at which flowers drop and fruit fails to set. Four pepper guides name that figure. NMSU says blossoms may not set above 32 °C (90 °F).¹ Utah State says flowers abort and fruit does not set on days above 32 °C (90 °F).⁴ Ohio State links periods of extreme heat above 32 °C (90 °F) to the loss of flowers and small fruit,⁵ and Minnesota lists temperatures above 32 °C (90 °F) among the things that weaken growth.³

Some sources put the start of trouble lower. The RHS says temperatures over 30 °C (86 °F) can reduce fruiting,⁷ and the New England Vegetable Management Guide expects poor fruit set and blossom drop when days rise above 29 °C (85 °F).⁸ Read those as the point where yields start to slip; 32 °C (90 °F) is where the pepper-specific guides agree fruit set fails.

Hot nights make it worse. The University of Maryland says very hot weather, especially with evening temperatures above 24 °C (75 °F), will slow or stop pod production,¹⁰ and the Chile Pepper Institute says fruit will not set when nights are above 30 °C (86 °F).²

Heat has one upside. The Chile Pepper Institute reports that New Mexican pods ripening at 30–35 °C (86–95 °F) have twice as many capsaicinoids, the compounds that make chiles hot, as pods ripening at 15–22 °C (59–72 °F).²

Helping plants through a heat wave

• Water evenly. Dry soil makes plants drop flower buds, flowers and small fruit,⁶ and heat dries soil fast. The Nutrients chapter covers watering during fruiting.
• Give afternoon shade. The University of Maryland suggests a spot with some late-afternoon shade, and cooling plants with a water spray during hot, dry weather.¹⁰
• Vent covers. Plants under row covers need ventilation once the air passes 27 °C (80 °F).⁴

Cold damage

Chiles are highly susceptible to frost, so NMSU advises waiting until after the last spring frost to transplant.¹ Cold short of frost also does harm: Ohio State notes that frost and cold nights can damage or kill flowers, leading to stunted or deformed fruit and a smaller crop.⁵ Harvested fruit suffers chilling injury below 7 °C (45 °F), which may not show until the fruit warms up again.⁸

Cold soil holds plants back even when the air is fine. The University of Maryland says planting before the soil reaches 18 °C (65 °F) makes plants “just sit there”.¹⁰ Cool soil delays emergence and slows growth.¹

Timing and protection

• Harden off with care. Minnesota suggests bringing plants back indoors if nights drop below 13 °C (55 °F) while they are hardening off.³
• Warm the soil. NMSU recommends black plastic mulch to warm the soil early in the season, switching to organic mulch once it warms.¹ Utah State advises holding off on straw or grass clippings until the soil is warmer than 24 °C (75 °F).⁴
• Use row covers early, and take them off in heat: Ohio State says to remove them at 27 °C (80 °F) or higher to prevent heat damage.⁵

Transplanting comes after your last spring frost, whatever month that falls in. In the Southern Hemisphere the seasons are reversed, so spring planting falls in what northern calendars call autumn; either way, go by your local frost date and the temperatures above, not the calendar. The Zones chapter explains how to find your frost dates.

Humidity, rain and disease

Chiles like some humidity. The RHS says they prefer a humid atmosphere and suggests damping down the greenhouse daily in warm weather.⁷ Types differ here: the Chile Pepper Institute notes that New Mexican varieties grow better in the southwestern US, while bells and habaneros grow better in more humid regions.²

Wet weather is a different matter, because rain and wet leaves spread disease:

• Bacterial spot favors high temperatures of 24–30 °C (75–86 °F), high humidity and frequent rain or overhead watering. It spreads by splashing water, tools and hands.⁹ Use drip irrigation or a soaker hose, water in the morning so leaves dry quickly, and stay out of the plants while they are wet.⁹
• Cool, wet conditions promote other diseases; Utah State advises against overhead watering late in the day.⁴
• Phytophthora root rot follows over-watering and makes plants wilt and die suddenly. NMSU recommends drip irrigation and raised beds to reduce it.¹ The Soil chapter covers drainage and raised beds.

Sun and day length

Chiles need full sun. Oregon State and Ohio State both call for at least 6–8 hours of direct sun a day,⁶,⁵ and the University of Maryland says peppers prefer 8–10 hours.¹⁰ Sun matters for the crop as well as the plant: the Chile Pepper Institute says more sunlight improves fruit set,² and Minnesota notes that fruit ripened in cool or cloudy weather is less flavorful.³

In cooler climates, the RHS recommends your warmest, sunniest spot, such as against a south-facing wall, or a greenhouse, polytunnel or sunny conservatory for the best crop.⁷ In the Southern Hemisphere the warm wall faces north.

Strong sun can also burn fruit. Sunscald leaves pale patches on fruit exposed to direct sun in hot, dry weather,⁴ especially when there are too few leaves to shade it.³ Oregon State singles out hot, late-afternoon sun.⁶ Plenty of healthy foliage over the fruit is the best protection.

None of the extension guides used here gives a day-length requirement for chiles; they talk about hours of direct sun and temperature instead. Indoors, seedlings grow well under lights left on for 14–16 hours a day.⁴

How the species differ (C. annuum, C. chinense, C. baccatum, C. pubescens)

The temperature figures above come from general pepper guides, which are written mostly with C. annuum types in mind. None of the sources gives separate temperature thresholds for the other species. What they do describe is where each species comes from and how much warmth and season it needs. Oregon State, growing at the cool end of the chile range, is the most useful here.

• C. annuum has the largest number of cultivars and the most variation of the garden peppers:⁶ bells, jalapeños, New Mexican types, cayennes and many more. The Chile Pepper Institute notes that New Mexican varieties are suited to the dry southwestern US.²
• C. chinense (habaneros, Scotch bonnets, superhots) originates in the Amazon region. Oregon State says it can be difficult to grow outdoors there, manageable in the warmest spots with plastic mulch, and most productive under row covers, in a high tunnel or in a heated greenhouse.⁶ It is also slow: the University of Maryland says habanero types ripen 90–120 days from transplanting.¹⁰
• C. baccatum (ajíes, Lemon Drop, Bishop’s Crown) originates in the lowlands of southern South America. Oregon State says it can be grown outdoors with season-extension aids such as row covers but benefits from a greenhouse.⁶
• C. pubescens (rocoto, manzano) was domesticated in the Andes and is adapted to high altitudes in the tropics. Oregon State says it needs a long growing season, and a high tunnel or heated greenhouse to ripen there.⁶

You will often read that rocoto tolerates cool weather better than other chiles. None of the extension sources checked for this guide says so, so it is not relied on here. Plan for rocoto as you would for any chile, and protect it from frost, which all chiles are highly susceptible to.¹

If your season is short or cool, start with C. annuum. Save C. chinense, C. baccatum and C. pubescens for a long, warm season or a covered space. The Zones chapter shows how to estimate how long your chile season is.