Bok choy, a member of the Chinese cabbage family, is a highly valued cool-season crop prized for its crisp stalks and tender leaves. This leafy green thrives in the moderate temperatures of spring and fall, easily tolerating cooling weather. While its natural cold hardiness allows it to withstand a light frost, the plant’s resilience has a definitive limit. A sudden or prolonged drop in temperature below freezing can quickly turn a productive crop into a total loss. Successfully growing and harvesting this vegetable depends on understanding the specific temperatures it can handle and knowing how to intervene when a deep freeze is imminent.
Understanding Bok Choy’s Cold Hardiness
Bok choy is robust enough to survive a light frost, which typically occurs when air temperatures briefly dip into the 32°F to 36°F range. This minor exposure to cold is not only tolerated but is often beneficial, as the plant responds by converting starches into sugars. This process, a natural defense mechanism, enhances the vegetable’s sweetness and crisp texture, making it particularly flavorful for a fall harvest. However, the plant’s cellular structure is not built to withstand a hard freeze, generally defined as temperatures falling below 28°F for an extended period.
A hard freeze causes water inside the plant’s cells to expand and form ice crystals, leading to the rupture of cell walls. This damage results in the characteristic water-soaked, mushy appearance of a frozen plant upon thawing. Newer and younger bok choy plants are significantly more susceptible to this irreversible damage than mature, established ones.
Another risk associated with cold weather is premature flowering, known as bolting. If nighttime temperatures consistently hover below 50°F to 55°F for a sustained period, the plant perceives this as a long winter and enters survival mode. This stress triggers the plant to rapidly send up a central flower stalk to produce seeds, effectively ending the leaf and stalk production and often leading to a bitter taste. Gardeners must manage cold exposure carefully, balancing the benefits of a light frost with the risks of a hard freeze or prolonged cold snap that causes bolting.
Protective Measures Against Freezing Temperatures
When a hard freeze is forecast, preventative action is crucial to protect bok choy from cell damage. One effective preparation is to thoroughly water the plants during the warmest part of the day before the freeze event. Moist soil retains significantly more heat than dry soil, releasing that warmth slowly overnight and moderating the temperature immediately surrounding the plants.
Physical barriers are the most reliable defense against a hard freeze. Floating row covers made from medium to heavy-weight spun-bonded fabric can provide an insulation layer that raises the temperature beneath by 6°F to 10°F. For maximum protection, the covers must be placed over hoops or a frame to prevent the fabric from touching the leaves, as contact can transfer cold and cause localized damage. The edges of the cover must also be secured tightly to the ground to trap the residual heat escaping from the soil.
Applying a thick layer of organic mulch, such as straw or shredded leaves, around the base of the plants helps to insulate the root zone. For smaller plantings, temporary cloches made from five-gallon buckets or large plastic containers can be placed over individual heads. These protective measures must be implemented before sundown to capture any warmth accumulated during the day.
Assessing and Harvesting Freeze-Affected Plants
After a freeze has passed, it is important to wait until the plants have fully thawed before assessing the damage, typically by mid-morning. Freeze-damaged leaves will exhibit a distinctive water-soaked, translucent, or wilted appearance. The tissue will feel soft and mushy to the touch, and the color may have shifted to a dark, grayish-green.
While the outer leaves bear the brunt of the cold, the inner core of the bok choy head is often insulated and protected. To determine viability, the damaged outer leaves and stalks should be peeled away and discarded. If the innermost leaves appear firm, crisp, and retain their healthy color, they are still safe to harvest and consume.
For plants that have survived with only minor damage, harvesting should be completed immediately. Damaged greens have a reduced shelf life, so they should be used right away. When harvesting, you can either cut the entire plant off cleanly at the soil line or use the “cut-and-come-again” method by taking only the largest outer leaves and leaving the inner core to potentially produce new growth.