Cold hardiness & minimum temperature

Is Scarlet Monkeyflower (Mimulus cardinalis)cold hardy? Hardiness zone & min temp

Also called Scarlet Monkeyflower, Scarlet Monkey Flower, Cardinal Monkeyflower.

More about scarlet monkeyflower

About Scarlet Monkeyflower

Mimulus cardinalis · also called Scarlet Monkeyflower, Scarlet Monkey Flower · flowering

Mimulus cardinalis is a perennial native to moist stream banks and seeps in western North America, from Oregon south to Baja California, producing vivid scarlet-orange tubular flowers from spring to autumn that are strongly attractive to hummingbirds. It requires reliably moist to wet, humus-rich soil and grows best in dappled shade in hot climates or full sun where summers are cool. The most critical care requirement is consistent soil moisture — plants wilt dramatically and may die in a single dry day during summer. Toxicity to cats and dogs is not confirmed by the ASPCA; treat with caution.

Cold limit: USDA 6-10 · RHS H4 (-15°C to 30°C)

Watch for — Crown and Root Rot: Despite needing wet soil, standing water around the crown in cold or poorly aerated conditions leads to fungal rot — grow in flowing or well-aerated marginal positions rather than stagnant boggy ground.

What scarlet monkeyflower's hardiness rating actually means

Yes — scarlet monkeyflower is genuinely cold hardy. Rated RHS H4 and USDA 6-10, it lives outdoors all year and needs winter cold rather than protection from it. 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 6-10 — 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

Minimum survivable temperature is roughly about −10 to −5 °C. Scarlet Monkeyflower is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for scarlet monkeyflower as it gets too cold:

It tolerates winter lows to about −10 to −5 °C once established.
Below its rated zone, the visible damage is browned or blackened top growth and, in the worst case, a killed crown or root.
First-year, newly planted, or container-grown specimens are noticeably less hardy than established garden plants — the roots are exposed.

Can scarlet monkeyflower go outside or overwinter — and where?

Plant it out within USDA 6-10 and it overwinters with little or no help.
It does not want to come indoors — a warm winter room actually weakens a hardy plant by denying it dormancy.
The real risks in its range are waterlogging, wind-rock on young plants, and a late hard frost on new growth — not ordinary winter cold.

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 scarlet monkeyflower can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H4 figure above.

Scarlet Monkeyflower hardiness — frequently asked questions

Is scarlet monkeyflower cold hardy?

Yes — scarlet monkeyflower is genuinely cold hardy. Rated RHS H4 and USDA 6-10, it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Scarlet Monkeyflower is hardy across USDA 6-10; it belongs in the ground or a frost-proof container, not on a windowsill, and many types actively need a cold winter to perform.

What is the minimum temperature scarlet monkeyflower can survive?

Minimum survivable temperature is roughly about −10 to −5 °C. Scarlet Monkeyflower is built for winter — once established it takes hard frost and snow in its stride.

What hardiness zone is scarlet monkeyflower?

Scarlet Monkeyflower is rated USDA 6-10 and RHS H4 — Hardy in an average winter across much of the temperate world.

Can scarlet monkeyflower survive winter outside?

Plant it out within USDA 6-10 and it overwinters with little or no help. It does not want to come indoors — a warm winter room actually weakens a hardy plant by denying it dormancy. The real risks in its range are waterlogging, wind-rock on young plants, and a late hard frost on new growth — not ordinary winter cold.

What happens to scarlet monkeyflower below its minimum temperature?

It tolerates winter lows to about −10 to −5 °C once established. Below its rated zone, the visible damage is browned or blackened top growth and, in the worst case, a killed crown or root. First-year, newly planted, or container-grown specimens are noticeably less hardy than established garden plants — the roots are exposed.