Perennial know-how

Zone 4 or Zone 5? Reading Minnesota's new hardiness map

Every perennial tag has a zone number on it, and in 2023 the USDA redrew the map underneath those numbers — the first update in over a decade. The headline: most of the Twin Cities metro, including a good chunk of Anoka County, officially moved from Zone 4b into Zone 5a. Here's what that actually means for your garden.

What a zone number really is

A hardiness zone is one thing only: the average coldest night of winter over the last 30 years. Zone 4 means lows averaging −30 to −20°F; Zone 5a means −20 to −15°F. It says nothing about snow cover, wind, wet feet in spring, or how long a cold snap lasts — all of which matter just as much to whether a perennial comes back.

Where we sit

The new 5a boundary runs up through the metro to about Ham Lake — which puts East Bethel and the greenhouse almost exactly on the line between 4b and 5a. Ten miles can genuinely change your zone around here, so it's worth looking up your exact zip on the USDA map. Northern Minnesota remains solidly Zone 3, and most of the state outside the metro and the south is still Zone 4.

How to read a plant tag

The rule is simple: you can plant anything rated for your zone or colder. A Zone 4 gardener can grow every Zone 3 plant on the bench. A tag that says "Zones 5–9" is the one that deserves a hard think north of the metro.

Should you garden like it's Zone 5 now?

Our honest take: buy like a Zone 4 gardener, experiment like a Zone 5 one. The map is a 30-year average — one polar vortex doesn't read averages, and a bare, snowless January is harder on roots than a well-blanketed −25°F night. If a Zone 5 plant tempts you, give it the best microclimate you've got: near the house foundation, out of the wind, with sharp drainage and a good mulch. Treat it as an experiment, not the backbone of the bed.

• Snow is insulation. A reliable snow blanket keeps soil far warmer than the air — open, windswept sites are effectively a zone colder.
• Containers lose about two zones. Roots above ground feel the full cold, so a Zone 4 perennial wintering in a pot is living a Zone 2 life. Sink pots or shelter them.
• Fall planting has a deadline. Get perennials in by mid-to-late September so roots establish before freeze-up — or plant in spring and skip the gamble.