Cold hardiness & minimum temperature

Is Purple Choy Sum (Brassica rapa var. parachinensis 'Purple')cold hardy? Hardiness zone & min temp

Also called purple choy sum, purple Chinese flowering cabbage.

More about purple choy sum

About Purple Choy Sum

Brassica rapa var. parachinensis 'Purple' · also called purple choy sum, purple Chinese flowering cabbage · edible

Purple Choy Sum is an ornamental-yet-edible flowering cabbage with striking purple stems and leaf veins, topped by yellow buds. Grown for sweet, tender stems and shoots, it matures in about 40-55 days, holds colour best in cool weather, and crops cut-and-come-again, adding vivid colour to beds and stir-fries alike.

Cold limit: USDA Cool-season annual in zones 2-11; colour and flavour are best below 24°C · RHS H3 (light frost tolerant; tender to hard freezes) (12-24°C)

What purple choy sum's hardiness rating actually means

Hardiness works differently for purple choy sum: 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 H3 means: Half-hardy — comes through mild UK winters outside but is killed by a hard freeze. On the US scale that maps to USDA Cool-season annual in zones 2-11; colour and flavour are best below 24°C — 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 purple choy sum 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 purple choy sum 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 purple choy sum can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H3 figure above.

Frost protection for borderline purple choy sum

Purple Choy Sum 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.

Purple Choy Sum hardiness — frequently asked questions

Is purple choy sum cold hardy?

Hardiness works differently for purple choy sum: 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. Purple Choy Sum is grown Cool-season annual in zones 2-11; colour and flavour are best below 24°C; you sow after the last frost and harvest before the first one, then start again next year.

What is the minimum temperature purple choy sum 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 purple choy sum?

Purple Choy Sum is rated USDA Cool-season annual in zones 2-11; colour and flavour are best below 24°C and RHS H3 — Half-hardy — comes through mild UK winters outside but is killed by a hard freeze.

Can purple choy sum 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 purple choy sum 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.