Cold hardiness & minimum temperature

Is Ageratum houstonianum 'Blue Horizon' (Ageratum houstonianum 'Blue Horizon')cold hardy? Hardiness zone & min temp

Also called Blue Horizon Ageratum, Cut-flower Floss Flower.

More about ageratum houstonianum 'blue horizon'

About Ageratum houstonianum 'Blue Horizon'

Ageratum houstonianum 'Blue Horizon' · also called Blue Horizon Ageratum, Cut-flower Floss Flower · flowering

Ageratum houstonianum 'Blue Horizon' is a tall, cut-flower floss flower bearing dense clusters of fluffy lavender-blue blooms on long, sturdy stems. An F1 hybrid grown as a warm-season annual, it flowers from summer to frost, attracts butterflies, and is prized for bouquets. It needs full sun to part shade, steady moisture and fertile, free-draining soil.

Cold limit: USDA 10-11 (grown as an annual in zones 2-9) · RHS H2 (18-27°C)

What ageratum houstonianum 'blue horizon''s hardiness rating actually means

Hardiness works differently for ageratum houstonianum 'blue horizon': 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 H2 means: Tender — survives a frost-free greenhouse or a very mild, sheltered spot. On the US scale that maps to USDA 10-11 (grown as an annual in zones 2-9) — 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 ageratum houstonianum 'blue horizon' 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 ageratum houstonianum 'blue horizon' 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 ageratum houstonianum 'blue horizon' can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H2 figure above.

Frost protection for borderline ageratum houstonianum 'blue horizon'

Ageratum houstonianum 'Blue Horizon' 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.

Ageratum houstonianum 'Blue Horizon' hardiness — frequently asked questions

Is ageratum houstonianum 'blue horizon' cold hardy?

Hardiness works differently for ageratum houstonianum 'blue horizon': 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. Ageratum houstonianum 'Blue Horizon' is grown 10-11 (grown as an annual in zones 2-9); you sow after the last frost and harvest before the first one, then start again next year.

What is the minimum temperature ageratum houstonianum 'blue horizon' 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 ageratum houstonianum 'blue horizon'?

Ageratum houstonianum 'Blue Horizon' is rated USDA 10-11 (grown as an annual in zones 2-9) and RHS H2 — Tender — survives a frost-free greenhouse or a very mild, sheltered spot.

Can ageratum houstonianum 'blue horizon' 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 ageratum houstonianum 'blue horizon' 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.