USDA Zone 4b: The Warmer Half of Zone 4
Minimum winter temperatures: −25°F to −20°F (−31.7°C to −28.9°C)
USDA Zone 4b is the warmer published half of Zone 4 on the 2023 Plant Hardiness Zone Map. Per USDA ARS and PRISM’s 1991–2020 average annual extreme minimum temperature bands, 4b covers −25°F to −20°F (−31.7°C to −28.9°C). University of Maine Cooperative Extension, updated November 17, 2023, states that same 4b band. It is still inside the whole-number Zone 4 tag, but milder than 4a for the rose canes and evergreen desiccation the Zone 4 page already describes. It is not Zone 5. Zone 5a’s published floor is −20°F to −15°F. Lilac, panicle hydrangea, hardy apples, and the rest of the Zone 4 palette still apply, including the own-root rose and soil-mound step that page already names. Half-zone lines are mapped products, not Arden guesses. Wisconsin Horticulture’s note on the 2023 map describes zone 4b, with zone 4a, as covering most of northern Wisconsin. That is one published example, not a city directory. Confirm your ZIP on planthardiness.ars.usda.gov before you copy a neighbor’s “Zone 4” calendar.
Related USDA zones
Half-zone temperatures come from the 2023 USDA Plant Hardiness Zone Map (ARS / PRISM). Confirm your ZIP on the official map before treating city averages as gospel.
Typical regions
Warmer mapped cells inside Zone 4. State guides on this site that already list 4b include New York, Michigan, Arizona, Washington, Colorado, and Oregon. Wisconsin Horticulture’s 2023-map note describes 4b across most of northern Wisconsin, together with 4a. Verify the USDA map for your address — a state span is not your parcel.
Challenges & solutions
- 01
Reading 4b as Zone 5
4b’s floor is −25°F to −20°F, not the −20°F to −15°F Zone 5a band. Plants rated only for Zone 5 still belong on a Zone 5 page. Use the Zone 5, 5a, or 5b guide only when the USDA map returns that band.
- 02
Own-root roses still need the mound
The warmer half reduces how often canes see the −30°F floor, compared with 4a. It does not cancel the Zone 4 note to plant own-root roses and mound soil over the crown. Hybrid teas stay the poorer choice against Explorer, Parkland, and Canadian Artist shrub roses.
- 03
Copying a 4a wrap onto every 4b bed
4a mulch habits are for a −30°F floor. 4b’s published floor is five degrees warmer. Still mulch after the ground freezes, because the Zone 4 page already says snow cover is less reliable than in Zone 3. Do not invent a statewide tomato date from the letter b.
- 04
Evergreen desiccation, same step
The Zone 4 page already says water arborvitae and broadleaf evergreens before the ground freezes and use a burlap screen on the windward side. 4b does not retire that step and does not add a new spray.
- 05
Shrub and fruit lists do not move
Keep the Zone 4 shrub list (lilac, spirea, panicle hydrangea, rugosa rose) and the fruit list (hardy apples, pears, plums, tart cherries). Crape myrtle, southern magnolia, camellia, peaches, and sweet cherries stay off a typical Zone 4 site, including 4b.
Seasonal guide
- spring
- May into early June. Whole Zone 4 last frost is typically mid-May. 4b leans toward the earlier end of the variation the Zone 4 page already names (late April in some southern or coastal cells, early June in higher or interior cells). Use your NOAA station. Wait until after that date before tomatoes. Do not invent a single statewide date from the half-zone letter.
- summer
- June through August. Warm days, cool nights, peak bloom around mid-July on the Zone 4 calendar. The half-zone difference is winter, not July.
- fall
- First frost on the whole Zone 4 page is late September. Plant spring bulbs and garlic in October, then mulch after the ground freezes.
- winter
- November through April. Milder than 4a, and still not Zone 5: the published average extreme minimum is −25°F to −20°F, not a night-by-night forecast and not a −20°F floor. Mound own-root roses the way the Zone 4 page already says. Prune dormant fruit in February–March.
Plants for zone 4b
24Hand-matched picks that survive −25°F to −20°F (−31.7°C to −28.9°C) winters.
- Hostas Hosta spp.
Hostas are the go-to shade perennial. Huge, textured leaves ranging from electric chartreuse to deep blue-green to variegated. Low-maintenance once established, and they come back bigger and better every year.
Part shade Moderate - Roses Rosa spp.
Modern disease-resistant roses have eliminated most of the old "rose-growing is hard" mythology. Knock Out and Drift series bloom all season with zero spraying. Traditional hybrid teas still reward gardeners willing to work for them.
Full sun Moderate - Ferns Various genera
Ferns bring texture and movement to shady spots where most other plants refuse. Unfurling fiddleheads in spring feel like watching evolution replay. Native species are nearly indestructible once established.
Part shade Moderate - Astilbe Astilbe x arendsii
Astilbe fills the summer gap in shade gardens with airy, feathery plumes in pink, red, white, and lavender. Deer and rabbit resistant. A foolproof companion for hostas and ferns.
Part shade High - Ornamental Grasses Various genera
Ornamental grasses add sound, movement, and four-season structure. Seed heads catch morning light. Winter silhouettes add interest when everything else has died back. Almost maintenance-free once established.
Full sun Low - Coneflower Echinacea purpurea
Coneflowers are drought-tolerant prairie natives that bloom for months. Pollinators swarm them. Seed heads feed finches in winter. Perfect for low-maintenance, ecologically friendly gardens.
Full sun Low - Black-Eyed Susan Rudbeckia fulgida
Black-eyed Susans are the reliable workhorse of the sunny border. Golden petals around dark chocolate centers. Blooms from midsummer until frost. Spreads gracefully without being invasive.
Full sun Low - Russian Sage Perovskia atriplicifolia
Russian sage creates a lavender-blue haze from mid-summer through fall. Silvery foliage, aromatic leaves, and airy bloom spikes. Completely drought-tolerant once established.
Full sun Low - Sage Salvia officinalis
Common sage is both a staple culinary herb and an attractive ornamental. Silver-green leaves, short purple bloom spikes, and a sharp, savory flavor for meats and stuffings. Lives 5+ years with good care.
Full sun Low - Clematis Clematis spp.
Clematis delivers the biggest, showiest blooms of any climbing vine. Different pruning groups bloom at different times — plant several for flowers spring through fall. The secret: "head in the sun, feet in the shade."
Full sun (roots shaded) Moderate - Heuchera (Coral Bells) Heuchera spp.
Heuchera is the foliage rock star of shade gardens. Modern cultivars come in lime, caramel, peach, burgundy, silver, and nearly black. Delicate bloom spikes add charm. Easy, evergreen in mild climates.
Part shade Moderate - Peonies Paeonia lactiflora
Peonies can live 100+ years in the same spot. Spectacular late-spring blooms in pink, white, red, and coral. Plant once, enjoy for generations. The key is getting the planting depth right — too deep and they'll never bloom.
Full sun Moderate - Daylilies Hemerocallis spp.
Daylilies are nearly indestructible. They bloom in sun or part shade, tolerate poor soil, resist deer, and multiply quickly. Thousands of cultivars in every color except blue.
Full sun to part shade Moderate - Tulips Tulipa spp.
Tulips open the spring garden with jewel-tone cups in every color imaginable. Plant bulbs in fall for April–May bloom. Most modern hybrids fade after 2–3 years — treat them like annuals or choose species tulips that return reliably.
Full sun Low - Daffodils Narcissus spp.
Daffodils are the most dependable spring bulb. Deer ignore them, squirrels won't dig them, and they multiply quietly year after year. Plant once in fall and enjoy for decades.
Full sun to part shade Low - Iris Iris germanica
Bearded iris deliver some of the most elaborate blooms in any garden — ruffled falls, upright standards, and fragrance in shades no paint can match. Plant rhizomes at the surface, not buried, for healthy long-lived clumps.
Full sun Low - Salvia Salvia nemorosa
Perennial salvias produce upright spikes of blue, purple, or pink that bees and hummingbirds find irresistible. Drought-tolerant, deer-resistant, and they bloom again after a hard cutback in mid-summer.
Full sun Low - Garden Phlox Phlox paniculata
Tall garden phlox delivers fragrant, mounded flower heads from mid-summer through fall. Classic cottage-garden plant. Modern mildew-resistant varieties like "David" and "Jeana" finally make phlox easy to grow well.
Full sun to part shade Moderate - Bee Balm Monarda didyma
Bee balm is a hummingbird and bumblebee magnet. Shaggy red, pink, or purple blooms from mid-summer onward. North American native. Modern mildew-resistant varieties like "Jacob Cline" make it dependable.
Full sun to part shade Moderate - Sedum Sedum spp.
Sedums combine succulent toughness with four-season beauty. Upright "Autumn Joy" blooms pink-to-rust from late summer through winter. Creeping types cover hot, dry slopes where nothing else survives. Pollinators love the late-season flowers.
Full sun Low - Hellebore Helleborus spp.
Hellebores bloom when nothing else does — late winter into early spring. Nodding flowers in cream, pink, plum, and near-black. Evergreen foliage, deer-resistant, long-lived. The quiet hero of the shade garden.
Part shade to shade Moderate - Shasta Daisy Leucanthemum × superbum
Shasta daisies deliver the clean white-and-yellow daisy look everyone recognizes. Easy, long-lived, and excellent for cutting. "Becky" is the landscape standard — tall, sturdy stems that don't flop.
Full sun Moderate - Catmint Nepeta × faassenii
Catmint produces clouds of small blue-purple flowers from late spring into fall. Aromatic silver-green foliage deters deer. Drought-tolerant and shrubs off to revive after a mid-summer haircut. "Walker's Low" is the landscape standard.
Full sun Low - Rhododendron Rhododendron spp.
Rhododendrons combine massive spring bloom clusters with evergreen four-season presence. They want cool roots, acidic soil, and dappled shade. Get the siting right and they live for decades.
Part shade Moderate
Frequently Asked Questions
01 What temperatures define USDA Zone 4b?
On the 2023 USDA Plant Hardiness Zone Map, Zone 4b is the half-zone whose average annual extreme minimum temperature falls between −25°F and −20°F (−31.7°C to −28.9°C). University of Maine Cooperative Extension (updated November 17, 2023) states the same band. Those bounds come from USDA ARS / PRISM — not from an Arden-estimated midpoint.
02 Is Zone 4b the same as Zone 5?
No. Zone 5a, the colder half of Zone 5, is −20°F to −15°F. Zone 4b stops at −20°F. A plant rated only for Zone 5 is not guaranteed because a neighbor’s label says “Zone 4.” Use the Zone 5 guide when the map returns Zone 5.
03 How do I know if I am in 4a or 4b?
Enter your ZIP on the official 2023 USDA Plant Hardiness Zone Map (planthardiness.ars.usda.gov). This site’s state guides already list 4b among the spans for New York, Michigan, Arizona, Washington, Colorado, and Oregon. Wisconsin Horticulture’s 2023-map note describes 4b across most of northern Wisconsin. A state span is not your parcel. The map’s station-based half-zone is.
04 Can I grow the same shrubs in 4b as in 4a?
Often, for plants already rated for whole Zone 4: lilac, spirea, panicle hydrangea, and hardy apples were already inside the band. The warmer half is easier on rose canes than 4a, and the Zone 4 page already says to mound own-root roses. 4b does not make crape myrtle, camellia, or a peach a safe bet.
05 Should I use a Zone 4 or Zone 4b planting calendar?
Use 4b timing when the USDA map places your ZIP in 4b — especially last frost and the −25°F floor — and keep whole-number Zone 4 guidance for species tags that only publish a single digit. The half-zone page plus your NOAA station beat a generic “Zone 4” calendar.
