Can broccoli survive winter frost?
Written by
Paul Reynolds
Reviewed by
Prof. Samuel Fitzgerald, Ph.D.Cold-hardy varieties of broccoli, such as Purple Sprouting, can withstand moderate frosts of about 20°F (-7°C) if well prepared. I harvested broccoli in December, after the mid-month snows in zone 5, using just a row cover and a few hoops. The vegetative parts of the plant begin to die back once temperatures drop below 40°F (4°C). The starches in the heads of broccoli plants are converted to sugar by frost, causing the heads to sweeten naturally. Stalks below 15°F (-9°C) kill any broccoli, prepared or not.
Variety Selection
- Purple Sprouting: Tolerates 15°F (-9°C)
- Waltham 29: Survives 20°F (-7°C)
- Rudolph: Handles repeated light frosts
- Avoid heat-sensitive hybrids like Green Magic
Protection Methods
- Row covers: Add 5-8°F warmth
- Straw mulch: 6-inch layer insulates roots
- Cloches: Trap heat around individual plants
- Remove covers during sunny days
Flavor Enhancement
- Harvest 24-48 hours after frost
- Cold concentrates sugars in florets
- Purple types develop richer color
- Morning harvest preserves sweetness
Install barriers before your first frost. Hoops support row fabric above plants, keeping them out of contact. I use sandbags to hold mine in place. After the soil has frozen a bit, apply at least 6 inches of straw mulch around the stem. This creates an insulating pocket of air. Ventilate the covers, as they may become warmer than 40°F (4 °C) inside, as they will mold.
Harvest your broccoli heads for peak sweetness after they have been exposed to frost. Wait 24 to 48 hours after frost for optimal conversion of starch to sugar, and harvest the heads in the morning while the weather is still cold (and before the temperature really starts to warm). I process mine immediately after picking. Heads touched by frost keep better than heads harvested in summer and still have some crispness a week or two later.
Learn your zone-specific limits as well: Zones 3-4 should harvest by October 15. Zones 5-6 can harvest through November. Warmer zones of 7-10 may over winter plants. I use probes to track soil temperature when air temperatures drop below 25°F (4°C). Frozen soil kills roots faster than air frost.
Prepare for extreme cold events. When the forecast indicates temperatures below 20°F (-7°C), I harvest every last 4-6" head immediately - there's no sense in leaving those that are not going to survive. I leave at least 2" of stem when cutting, but that doesn't seem to matter in the long run. Sometimes it tells the plant to send up side shoots in the spring if this is done. Make sure to mulch heavily over the cut stems.
Read the full article: When to Plant Broccoli for Best Results
