Cold hardiness & minimum temperature

Is Annual Phlox (Phlox drummondii)cold hardy? Hardiness zone & min temp

Also called Annual Phlox, Drummond's Phlox, Texas Phlox, Pride of Texas.

More about annual phlox

About Annual Phlox

Phlox drummondii · also called Annual Phlox, Drummond's Phlox · flowering

A Texas native annual that produces an abundant, long-lasting display of trumpet-shaped flowers in shades of red, pink, lavender, white, and bicolour from spring into early summer. It is easy to grow in any well-drained soil in full sun and is an excellent choice for containers, borders, and wildlife gardens. Cool-season bloomer that fades in high summer heat.

Cold limit: USDA 2–11 (grown as cool-season annual) · RHS H6 (7°C to 30°C)

Watch for — Summer heat collapse: Flowering declines sharply in temperatures above 30°C. Treat as a cool-season annual; start seeds early and allow plants to finish before midsummer heat arrives.

What annual phlox's hardiness rating actually means

Hardiness works differently for annual phlox: 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 H6 means: Hardy throughout the UK and northern Europe. On the US scale that maps to USDA 2–11 (grown as cool-season annual) — 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 annual phlox 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 annual phlox 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 annual phlox can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H6 figure above.

Frost protection for borderline annual phlox

Annual Phlox 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.

Annual Phlox hardiness — frequently asked questions

Is annual phlox cold hardy?

Hardiness works differently for annual phlox: 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. Annual Phlox is grown 2–11 (grown as cool-season annual); you sow after the last frost and harvest before the first one, then start again next year.

What is the minimum temperature annual phlox 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 annual phlox?

Annual Phlox is rated USDA 2–11 (grown as cool-season annual) and RHS H6 — Hardy throughout the UK and northern Europe.

Can annual phlox 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 annual phlox 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.