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The short answer
Harvest or cover broccoli heads when the forecast falls to 30F (−1C), and cover whole plants at 28F (−2C). Experts give varying limits for the plant itself, from about 26F (−3C) down to brief spells at 20F (−7C) for plants hardened by cool weather.
Readers also askHow cold can cabbage tolerate?
Forecast ladder Warm grown plants Harvest or cover After a freeze Common questions
How cold can broccoli tolerate? The forecast ladder
Established broccoli plants handle light frost well and usually take 26F (−3C) with little damage, but the heads are damaged first. North Carolina State University puts it plainly: broccoli heads are more frost sensitive than broccoli plants. So will frost kill broccoli? A light one won’t, and the University of Minnesota notes that the plants keep growing after the first light frosts of fall. The trouble starts in the upper 20s F (about −2C), and it starts at the top.
The table matches each forecast low to the alert you’re likely to see from the National Weather Service. Alert rules are set by each local office, so these are the definitions its office in Aberdeen, South Dakota uses.
| Forecast low | Alert you may see | What happens to broccoli | What to do |
|---|---|---|---|
| 33 to 36F (1 to 2C) | Frost advisory | Light frost doesn’t damage it, and the plant keeps growing | Nothing yet. Note which heads are nearly ready |
| 30F (−1C) | Freeze watch or freeze warning, for lows below 32F (0C) | Heads near harvest are at risk | Cut any head close to full size, or cover it |
| 28F (−2C) | Freeze warning. The Weather Service calls 28F (−2C) or lower for an extended period a hard freeze | Heavy frost can turn the florets mushy | Cover whole plants overnight and uncover them in the morning |
| 25 to 26F (−4 to −3C) | Freeze warning | Plants usually take 26F (−3C) with little damage, but need covering below 25F (−4C) | Cover every plant, with two layers if you have them, and cut any usable head |
| 20F (−7C) | Freeze warning | Only plants hardened by cool weather endure it, and only for a brief spell | Harvest every usable head. We wouldn’t count on the plants |
Experts give varying ranges for the plant itself, from 26F (−3C) with little damage down to brief spells at 20F (−7C) for plants that have hardened in cool weather. We’d cover the whole plant at 28F (−2C), because the head goes first and a cover costs you nothing but a trip outside before dark.
Two things the forecast won’t tell you. Texas A&M University says plant surfaces open to a clear sky can be as much as 5 degrees F (about 3C) colder than the recorded air temperature, so treat the forecast low as the best case for an open bed. And the alerts don’t run all fall: the Aberdeen office’s page says it stops issuing frost and freeze alerts for the season in mid October. A late crop can meet a hard freeze with no warning headline at all, so watch the number, not the alert.
Why broccoli grown in warm weather is weaker
Broccoli that has had a spell of cool weather tolerates more cold than plants that went straight from warm weather into a freeze. North Carolina State gives an example. Several days in the upper 60s or low 70s F (about 20 to 23C) followed by a plunge into the upper 20s F (about −2C) does more damage than several days in the 40s or 50s F (4 to 15C) followed by nights in the mid 20s F (about −4C). The colder night does less harm, because the plants had time to adjust.
Texas A&M explains why: plants acclimate to freezing weather when they get consistent, gradually colder weather, and warm spells in between can undo it. The University of Missouri says properly hardened cole crops, the cabbage family that broccoli belongs to, can take temperatures well below freezing for short periods. Missouri adds a detail most guides leave out: young plants tend to be more cold tolerant than older ones.
Feeding matters too. North Carolina State says plants heavily fertilized with nitrogen, which pushes lush, tender growth, are damaged more by cold than plants fed at light to moderate levels. After a warm spell or a heavy feed, we’d act one rung higher on the ladder than the forecast suggests.
Harvest early or cover?
If the head is close to full size, cutting it before a hard freeze is safer than covering it. Texas A&M says to cut the main head when it’s about 4 inches (10cm) across, and North Carolina State says heads are ready at 4 to 8 inches (10 to 20cm), before the buds start to open. A head that size has little left to gain and a lot to lose, so we’d cut it on the afternoon before a forecast of 30F (−1C) or lower.
A smaller head is worth covering instead. The plant keeps growing through light frost, so a cover buys it the time it needs to size up.
How to protect broccoli from frost
To protect broccoli from frost, drape a frost cloth, a sheet or an old blanket loosely over the plants late in the afternoon, before sundown, and take it off in the morning. Texas A&M explains the timing: a cover doesn’t insulate so much as hold in the heat rising from the ground, so it should go on after the soil has soaked up a full day of sun.
The University of Missouri says row covers give 4 to 8 degrees F (about 2 to 4C) of frost protection, depending on thickness, and North Carolina State notes that frost cloth is rated by the degrees of protection it provides, so check the label. In a severe frost, Texas A&M suggests two layers. Weigh the edges down with soil or pins: the University of Minnesota warns that a row cover loosened by strong wind can do more damage than the plants would have suffered without it. Our guide to garden fabric row cover explains fabrics and hoops.
What you can use
Garutom frost cloth, 10 x 33 ft (3 x 10m)
Why this one. It’s a frost cloth rather than plastic, which is what the advice above calls for, and a piece 10 by 33 feet (3 by 10m) will cover a long bed of broccoli with cloth to spare for weighing down the edges.
Where it falls short. A single layer of row cover buys only a few degrees, so check its rating before you rely on it below 28F (−2C), and add a second layer or harvest the heads when the mid 20s F (about −4C) are forecast. It’s also more cloth than a few plants need, so expect to cut it to size.
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What to do with broccoli after a freeze
A broccoli head with mushy florets after a freeze is lost, but the plant usually isn’t. North Carolina State says to cut the damaged head off, compost it and leave the plant in the ground. We’d judge each head by hand once it has thawed: firm is worth keeping, mushy goes on the compost.
Don’t pull the plant on the strength of a ruined head. It isn’t finished yet.
What we would skip
Clear plastic laid on the plants. Texas A&M says clear polyethylene isn’t the best material to drape over plants, and it can damage them where it touches. A sheet or frost cloth does the job better.
Tying the cover tight around the stems. Texas A&M says it cuts down the heat from the ground that the cover is there to catch. Let it hang loose.
Common questions
Why has your broccoli turned purple?
Cold weather can turn broccoli heads and leaves blue or purplish, and North Carolina State says that’s just a reaction to cold, not a sign of damage or lost quality. Some varieties are purple anyway, so check the seed packet before you blame the weather.
Will your broccoli grow side shoots after the head freezes?
Usually, yes. North Carolina State says side shoots will still likely form after you cut off a frost damaged head, just below the cut where the leaves join the main stem, and in mild weather they can keep coming for several weeks. How and when to harvest broccoli.
Is broccoli or cauliflower hardier?
Experts disagree. Purdue University lists broccoli as hardy and cauliflower as semi hardy, while Texas A&M groups the two together and says to protect or harvest both when temperatures below 30F (−1C) are predicted. We’d treat cauliflower as the more tender of the two and never leave it uncovered below that.
How cold can carrots tolerate?
Experts disagree. Purdue puts carrots in its semi hardy group, to harvest or protect when the mid to upper 20s F (about −4 to −2C) are forecast, while Texas A&M says carrots survive the low 20s F (about −6C) and even the teens F (about −10 to −7C), especially with some protection. We’d go with the cautious figure and protect them in the mid 20s F (about −4C). How cold can carrots tolerate.
The plant will see out most fall nights on its own. The head would rather spend them in your fridge.
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Checked against: North Carolina State University, “Tips for Harvesting Broccoli”. North Carolina State University, “Basics of Broccoli Production”. North Carolina State University, “Frost and Freeze Protection for Vegetables”. University of Minnesota, “Growing broccoli in home gardens”. Texas A&M University, “Harvesting Tips for Winter Vegetables”. Texas A&M University, “Protecting Landscapes and Horticultural Crops from Frosts and Freezes” (EHT-048). Texas A&M University, “Broccoli”. University of Missouri, “Cole crops harmed by hot weather”. Purdue University, “Cold Temperature Tolerance of Vegetables”. National Weather Service, Aberdeen office, “Fall Frost/Freeze Decision Support Page”. National Weather Service, glossary entry for hard freeze.
