Choosing plants · Seasons and climate · Beginners
Hardiness zone vs heat: why your zone is not enough
Your USDA zone only measures winter cold. Learn what it leaves out (summer heat, humidity, chill hours) and how to check a plant against all of them.
Updated · 5 min read
Your USDA hardiness zone tells you one thing: how cold your winters get on average. It says nothing about summer heat, humidity, rainfall or how long winter lasts. That is why a plant "rated for your zone" can still fail. A Japanese maple can be happy in Seattle and scorched in a hot inland summer with similar winter lows, and lavender rated for your zone can rot in Florida's wet, humid summer. To choose plants that last, check the zone first, then check heat, humidity and, for some plants, winter chill.
What the zone actually measures
The USDA Plant Hardiness Zone Map (planthardiness.ars.usda.gov) is based on the average annual extreme minimum temperature: the coldest night of each year, averaged over 30 years. The current map, released in 2023, uses data from 1991 to 2020.
- Each zone covers a 10°F band. Zone 7 is 0°F to 10°F.
- Each zone is split into two 5°F half zones. Zone 7a is 0°F to 5°F; 7b is 5°F to 10°F.
- A plant tag that says "zones 5–9" means the plant normally survives winter lows down to zone 5 (−20°F to −10°F). The upper number is a rough guide to where it stops doing well, and that is often about heat, not cold.
You can look up your zone by ZIP code on the USDA site or with our free zone by ZIP tool. Zones vary within a ZIP code: hilltops, valley bottoms, cities and coasts can differ by half a zone or more.
What the zone leaves out
| Factor | What it does to plants | Where to look |
|---|---|---|
| Summer heat | Long runs of hot days and warm nights stress many temperate plants; they stop growing, scorch or decline | American Horticultural Society (AHS) Plant Heat Zone Map; your state extension service |
| Humidity and summer rain | Wet foliage and soggy roots in heat cause fungal disease and root rot, especially in Mediterranean and alpine plants | Extension service plant lists for your region |
| Winter wet | Cold plus waterlogged soil kills plants that would survive the same cold in dry soil | Drainage of your own site |
| Chill hours | Many plants need a period of winter cold to flower or leaf out properly | Extension service; fruit tree nurseries list chill requirements |
| Late frosts | A warm spell followed by frost damages early flowers and new growth | Your last frost date |
| Microclimate | South walls, paving and slopes run hotter; low spots collect cold air | Your own observation |
The AHS heat zone map divides the US into 12 zones by the average number of days per year above 86°F (30°C), the temperature at which many plants start to suffer heat stress. Some tags and catalogs list both, for example "USDA 5–9, AHS 9–1". Where a heat zone is given, check it.
In the West, Sunset's climate zones combine winter lows with summer heat, humidity, elevation and ocean influence, and are widely used by western nurseries.
Example 1: Japanese maple in Seattle
Japanese maples (Acer palmatum) are generally rated hardy to about zone 5 or 6 through zone 8 or 9 depending on variety. Seattle (zone 8b or 9a, depending on the neighborhood) is within the range for most varieties, and so are parts of Texas and the inland Southeast with similar winter lows.
The difference is summer. Seattle has mild summers with few days above 86°F and cool nights. In hot, dry inland summers, the thin leaves of many Japanese maples lose water faster than the roots can supply it, and the edges brown and crisp ("leaf scorch"), especially in afternoon sun and wind. The same tree can be grown in hot regions, but usually with afternoon shade, steady water, mulch, and a variety known to handle heat.
Same zone, very different outcome.
Example 2: lavender in Florida
English lavender (Lavandula angustifolia) survives cold winters well; it is commonly rated to zone 5. Many Florida gardeners in zones 8 and 9 see a tag that includes their zone and expect success.
What usually kills it there is not winter but summer: high humidity, frequent heavy rain and warm nights. Lavender comes from dry, sunny hillsides with fast-draining soil. In wet heat its roots and crown rot and the plant collapses, often in its first summer. In humid climates, gardeners have better luck with very sharp drainage (raised beds, gritty soil, gravel mulch), some of the hybrid lavenders, or treating lavender as a container plant or an annual.
Example 3: not enough cold
The opposite problem shows up in mild-winter areas. Tulips, herbaceous peonies, lilacs and many apple, pear and cherry varieties need a stretch of winter cold to flower well. "Chill hours" are commonly counted as hours between about 32°F and 45°F. In the Gulf Coast, Florida and southern California, many of these plants flower poorly or not at all. Tulip bulbs are often pre-chilled in a refrigerator and grown as annuals there, and fruit nurseries sell "low-chill" varieties for warm areas. Check the chill hours your area typically gets with your extension service before buying fruit trees.
A quick check for any plant
- Zone. Is your zone inside the plant's range, not at the very edge? If you are at the edge, give the plant a sheltered spot.
- Heat. If you live where summers are long and hot, does the plant appear on your extension service's or local nurseries' recommended lists? Is an AHS heat zone given?
- Humidity and drainage. Is it a Mediterranean or dry-climate plant going into humid summers or heavy clay? Plan for drainage or pick something else.
- Chill. Does it need winter cold to flower or fruit, and does your area get enough?
- Your spot. A south-facing wall can be a zone warmer; a frost pocket can be a zone colder.
For more on matching a garden style to your climate, see how to choose a landscape style for your climate zone.
FloraVista checks both your zone and a list of regions where each plant struggles with heat before it adds a plant to your list: start with a photo.
Questions and answers
- What does a USDA hardiness zone measure?
- Only the average coldest winter night of the year, based on 30 years of data. Each zone is a 10°F band, split into 5°F half zones labeled a and b.
- Why did a plant rated for my zone die?
- Often because of something the zone doesn't measure: summer heat, humid nights, wet winter soil, or not enough winter cold. Poor drainage and planting late in the season are other common causes.
- What is the AHS heat zone map?
- A map from the American Horticultural Society that groups areas by the average number of days a year above 86°F. Some plant tags and catalogs list a heat zone range next to the hardiness zone.
- What are chill hours?
- The number of hours in winter spent at cool temperatures, commonly counted between about 32°F and 45°F. Many fruit trees, tulips, peonies and lilacs need enough of them to flower well.
Work it out for your own bed
Free tools
- Plant spacing calculatorBed size and spacing in, number of plants out. Square or triangular grid.
- Hardiness zone by ZIPZIP in, 2023 USDA zone, frost dates and planting windows out.
- Pet toxicity checkerSearch by name to see whether a garden plant is listed as toxic to dogs or cats.
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