Bok choy, a variety of Chinese cabbage, is a fast-growing leafy green prized for its crisp stalks and tender leaves. Classified as a cool-season crop, it thrives in mild temperatures and possesses cold tolerance. Bok choy is frost hardy; the plant can generally withstand temperatures slightly below freezing without sustaining permanent damage. This resilience makes it a favored vegetable for both early spring and late fall gardens.
Understanding Bok Choy’s Cold Tolerance
The cold a bok choy plant can endure depends on the severity and duration of the temperature drop, distinguishing between a light frost and a hard freeze. A light frost (28°F to 32°F) is typically well-tolerated by mature plants. This exposure initiates vernalization, causing the plant to convert starches into sugars. The accumulation of sugars improves the flavor, giving the leaves and stalks a sweeter taste.
The tolerance limit is crossed when temperatures fall consistently below approximately 25°F. This sustained cold is defined as a hard freeze, which freezes the water within the plant’s cells. This leads to irreversible cellular damage, wilting, and the death of the plant. While the ideal growing range is 45°F to 75°F, this frost hardiness allows for an extended growing season outside of summer heat.
Timing Planting and Harvest
Planting must be timed to avoid both the high heat of summer and the threat of severe freezes. For a spring harvest, seeds can be sown directly into the garden two to four weeks before the average date of the last expected frost. Planting this early takes advantage of the cool, moist conditions bok choy prefers for germination and growth.
Gardeners must be mindful of temperature fluctuations during the spring, as a sudden cold snap followed by rapid warming can trigger premature flowering, or bolting. Bolting causes the plant to divert energy to forming a seed stalk, making the leaves bitter and the stalks woody. Nighttime temperatures consistently dropping below 55°F can initiate this stress response in young plants.
The fall growing season is often preferred because cooling temperatures are less likely to induce bolting than the erratic weather of spring. Seeds should be sown in mid-to-late summer, allowing the crop to mature as the days shorten and temperatures drop. Harvest should occur before the onset of sustained hard freezes, though a light frost beforehand enhances the flavor. Succession planting (small batches of seeds planted every two weeks) helps ensure a continuous supply and reduces the risk of losing the crop to unexpected weather.
Protecting Bok Choy from Severe Cold
While mature bok choy can survive a light frost, protection is needed if the forecast predicts a hard freeze below 28°F or if the harvest is extended into colder months. Physical barriers are the most effective way to insulate the plants. Lightweight fabric row covers, or frost cloth, can be draped over the plants or supported by hoops to create a protective microclimate.
These covers trap latent heat radiating from the soil and shield the leaves from icy winds and frost crystals. Cloches (bell-shaped covers typically made of glass or plastic) can be placed over individual plants for concentrated protection. For long-term winter harvest extension, installing a temporary cold frame or hoop house offers a robust environment, safeguarding the crop against repeated freezes. Applying a thick layer of organic mulch around the base of the plants also helps insulate the soil and roots.