Cold hardiness & minimum temperature

Is Bok Choy 'Black Summer' (Brassica rapa var. chinensis 'Black Summer')cold hardy? Hardiness zone & min temp

Also called Black Summer bok choy, dark bok choy.

More about bok choy 'black summer'

About Bok Choy 'Black Summer'

Brassica rapa var. chinensis 'Black Summer' · also called Black Summer bok choy, dark bok choy · edible

'Black Summer' is a fast, uniform bok choy with very dark green leaves and crisp pale stalks, bred for slow bolting and heat tolerance so it crops reliably through warmer weather. A cool-season Asian green, it matures in roughly six to eight weeks and is ideal for successional sowing in spring, summer, and autumn for stir-fries and steaming.

Cold limit: USDA Grown as a cool-season annual in zones 2-11 · RHS H4 (13-24°C)

What bok choy 'black summer''s hardiness rating actually means

Hardiness works differently for bok choy 'black summer': it is grown as a seasonal crop, not overwintered. The question is not "what zone" but "how long is your frost-free growing window". Its RHS rating of H4 means: Hardy in an average winter across much of the temperate world. On the US scale that maps to USDA Grown as a cool-season annual in zones 2-11 — the zones where it can be left outdoors year-round.

New to these scales? The USDA hardiness zone map explained covers how the zone numbers work, and you can find your own zone with the zone finder.

Minimum temperature — and what happens below it

As an annual crop, its "minimum temperature" is the first hard frost — that is the end of the plant's life, not a survivable low. Many types are also damaged by light frost (around 0 °C).

Concretely, for bok choy 'black summer' as it gets too cold:

Light frost (around 0 to −2 °C) damages or kills tender summer crops outright; cold-hardy types take a few degrees of frost.
The plant does not "survive winter" — its life cycle simply ends, by design, when frost arrives or it finishes cropping.
A surprise late spring frost can also kill young transplants set out too early, before the season even starts.

Can bok choy 'black summer' go outside or overwinter — and where?

Time it to your frost dates: sow or plant out after the last spring frost, and aim to harvest before the first autumn frost.
In short-season zones, start it indoors or under cover to stretch the effective growing window.
Hardier crops in this group can be sown for an autumn or overwintered harvest in mild zones — check the specific crop.

Work back from your local frost dates with the frost-date calculator: the last spring frost and first autumn frost are what really decide when bok choy 'black summer' can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H4 figure above.

Frost protection for borderline bok choy 'black summer'

Bok Choy 'Black Summer' is right on a hardiness edge in many gardens, so if you are pushing it, these measures buy it the margin it needs:

Use fleece, cloches or a cold frame at each end of the season to dodge a borderline frost and add growing weeks.
Have row cover ready for an unexpected late spring or early autumn frost.
Know your local last- and first-frost dates and count back the crop’s days-to-maturity to schedule the sowing.

Bok Choy 'Black Summer' hardiness — frequently asked questions

Is bok choy 'black summer' cold hardy?

Hardiness works differently for bok choy 'black summer': it is grown as a seasonal crop, not overwintered. The question is not "what zone" but "how long is your frost-free growing window". A seasonal crop, not a perennial. Bok Choy 'Black Summer' is grown Grown as a cool-season annual in zones 2-11; you sow after the last frost and harvest before the first one, then start again next year.

What is the minimum temperature bok choy 'black summer' can survive?

As an annual crop, its "minimum temperature" is the first hard frost — that is the end of the plant's life, not a survivable low. Many types are also damaged by light frost (around 0 °C).

What hardiness zone is bok choy 'black summer'?

Bok Choy 'Black Summer' is rated USDA Grown as a cool-season annual in zones 2-11 and RHS H4 — Hardy in an average winter across much of the temperate world.

Can bok choy 'black summer' survive winter outside?

Time it to your frost dates: sow or plant out after the last spring frost, and aim to harvest before the first autumn frost. In short-season zones, start it indoors or under cover to stretch the effective growing window. Hardier crops in this group can be sown for an autumn or overwintered harvest in mild zones — check the specific crop.

How do I protect bok choy 'black summer' from frost?

Use fleece, cloches or a cold frame at each end of the season to dodge a borderline frost and add growing weeks. Have row cover ready for an unexpected late spring or early autumn frost. Know your local last- and first-frost dates and count back the crop’s days-to-maturity to schedule the sowing.