What to Plant This Month: Frost Dates, Not Your Zone

If you have ever searched what to plant this month and landed on a table that gives your growing zone one date, that table is guessing.

It is not your fault for trusting it. The table looks official. It has zones in the left column and months across the top, and it is the single most common format on the internet. The problem is that nobody who measures this data publishes it that way. The USDA does not. NOAA does not. No cooperative extension office does. They all publish frost dates by weather station, city or state, because that is how frost actually behaves.

So this page does it the honest way. You look up two numbers for where you live, write them down, and count weeks from those. The month names fall out of that, and they will not be the same months as your sister’s garden two states away.

What your growing zone actually tells you

The USDA Plant Hardiness Zone Map describes one thing: the average annual extreme minimum winter temperature. That is it. The current edition is the 2023 map, and USDA states plainly that zones are “based on the average annual extreme minimum winter temperature, displayed as 10-degree F zones and 5-degree F half zones.”

USDA also says the map is built on 1991 to 2020 weather data, and that this “does not represent the coldest it has ever been or ever will be in an area.”

Notice what is missing from that definition. Frost dates. Growing season length. Planting windows. None of it. A zone tells you whether a perennial is likely to survive your winter. It says nothing about when your last spring frost arrives.

The map’s own guidance is careful about this. USDA says the maps “should serve as general guides for growing perennial plants,” and that “many other environmental factors, in addition to hardiness zones, contribute to the success or failure of plants.”

Illinois Extension puts the two ideas side by side when it explains that planting dates “are determined two things: the first and last frost dates where you grow, and how much time it takes for a crop to mature.” Your zone is not one of those two things.

The proof is in the spread

If zones determined frost dates, two gardens in the same zone would frost at roughly the same time. They do not, and the gap can be enormous.

The National Weather Service office in Huntsville, Alabama publishes last-freeze records for stations across its area. Guntersville averages its last freeze on March 25. Valley Head, in the same NWS area and effectively the same zone, averages April 18. That is a 24 day spread with no zone difference between them.

The same effect shows up across a whole state. NC State publishes average first and last freeze dates by station. Charlotte, in the Piedmont, averages a last spring freeze on April 1 and a first fall freeze on November 5. Banner Elk, in the mountains at 3,770 feet, averages May 12 and October 3. Roughly six weeks apart, both in North Carolina.

Then there is zone 9, which contains coastal southern California, where NOAA notes the first freeze “should not occur until at least December,” and the Florida interior, which freezes most years. One zone label, two completely different gardens.

Look up two numbers before you plant anything

Write these down. They are the only calendar you need.

1. Your median last spring frost date. The date by which half of recent years have had their final spring freeze.
2. Your late last-frost date. The date that holds in nine years out of ten. This is your safe date.

Then add one more for the end of the season.

3. Your median first fall frost date. This tells you how much growing season you have left and when to start protecting things.

The difference between numbers 1 and 2 is where most garden failures live. The median is a coin flip. NOAA’s Climate.gov explains that its map shows “the average date on which the chances of freezing temperatures drop below 50 percent,” and that past that date, “temperatures have historically been more likely to stay above freezing than to fall below it again for the rest of the spring.” More likely is not the same as certain.

NOAA’s own worked example makes it concrete. For Boulder, Colorado, the 50 percent date is May 6. If you want the odds that a freeze has already happened to reach 90 percent, you wait until May 18. Twelve days of patience, and a much better chance your tomatoes survive.

The National Centers for Environmental Information frame the fall side the same way: “a freeze is expected to have happened on or before the listed date in five out of the 10 years recorded.”

Where to find your own dates

• NOAA NCEI U.S. Climate Normals at ncei.noaa.gov/products/land-based-station/us-climate-normals is the official source, covering almost 15,000 stations, and it includes agricultural normals with freeze probabilities and growing season length.
• The NCEI normals data search at ncei.noaa.gov/access/search/data-search/normals-annualseasonal-1991-2020 lets you pull, for a single station, the date past which the chance of freezing drops below 50, 40, 30, 20 and 10 percent.
• NOAA Climate.gov’s interactive last-spring-freeze map lets you click your nearest station rather than reading a table.
• Your local National Weather Service office publishes station-level frost and freeze climatology, including the earliest and latest freezes on record. Search for your office plus “frost freeze.”
• Your state climatologist, if it has one, is often the best-presented version. The Illinois State Climatologist frost dates page shows earliest, 1-in-10, median, 9-in-10 and latest dates at both 32F and 28F for spring and fall. That is the format worth copying.

Two definitions to keep straight, because sources use them differently. A freeze means 32F or below. A hard freeze means 28F or below. Frost itself is rarely measured at weather stations, so the 32F threshold stands in as a proxy, and Illinois notes that visible frost can appear when the reported air temperature is slightly above 32F.

Once you have your dates, count weeks instead of months

This is the part that transfers between zones. A zone 5 garden in the Pacific Northwest and a zone 5 garden in the upper Midwest share a zone label but not a planting calendar. They do share frost-relative timing.

A few of those deserve expansion.

Cool-season crops tolerate real cold. Iowa State is specific: many “tolerate a light frost, often surviving down to 28F or even 25F with little damage to flowers or leaves. This means these crops can be planted up to 3 or 4 weeks before the last frost date in spring, depending on the crop.” Lettuce, cabbage, cauliflower, broccoli, Brussels sprouts and onions are the crops UMN names.

Warm-season crops do not. Iowa State’s rule is flat: “Warm-season crops must be planted after the threat of frost has passed.” Tomatoes, eggplant, peppers, summer squash and basil all wait. UMN puts the Minnesota version at mid to late May.

Harden off anything you started indoors. Moving a windowsill seedling straight outside is how you lose it. Iowa State’s instruction is to acclimate plants over several days, starting “in a shady, protected location and then gradually expose them to longer periods of sunlight.”

For the fall, work backwards. The Illinois State Climatologist’s rule of thumb is worth writing on the seed packet: “For tender plants, add two weeks to the average date in the spring to protect against the possibility of late-season frost. In the fall, subtract two weeks from the average date to be on guard against an early frost.”

If you are starting from scratch indoors, seed starting indoors for beginners walks through the setup, and the week-by-week zone 6 schedule shows what a full season looks like on paper. There is also a no-soil method using a bag if you would rather not deal with trays.

The same task lands in different months, which is the whole point

Here is when to start tomato seeds in five real places, using published figures. Same crop. Same job. Different month.

The Illinois rows come from Illinois Extension’s zone-banded summary. The Lancaster figure is Penn State Extension’s own statement that “our last date of frost is April 28 and first date of frost is October 11.” Huntsville and Charlotte come from NWS Huntsville and NC State respectively. The seed-starting months are calculated as about six weeks before the local frost date, which is the interval UMN uses.

Read down that table and the original premise collapses. Two places in the same zone can be six weeks apart. Zone 6B in Pennsylvania and 5b in Illinois share a tomato timing. The zone column is the least useful column in the table.

Soil temperature beats the calendar anyway

If counting weeks feels fussy, there is a better instrument, and extension services say so directly. UMN states it flatly: “Soil temperature is the best indicator for when to plant.”

Illinois Extension agrees and explains why the timeline advice exists at all: “A more direct measure is the soil temperature and that is really what this recommendation is getting at. Companies are trying to make it easier on us by giving us a timeline we can understand as opposed to telling us to measure soil temperature.”

Thresholds worth knowing:

• 40 to 50F: direct-seed peas, spinach, lettuce and radish, per UMN.
• Above 50F: “kale, radish, carrots or lettuce may all be seeded at this point,” per Illinois Extension.
• 60F: most annual flowers need this to germinate and grow.
• About 70F: zinnias, cosmos, cucumber and squash germinate best.

A cheap soil thermometer does more for your timing than any zone chart. If you want flowers specifically, these are the seeds worth direct sowing in early summer once the soil is genuinely warm, and these give you the most bloom for a small bed .

Containers, balconies and small spaces

The frost-relative rules above apply exactly the same way in a pot on a balcony, because frost does not check whether a plant is in the ground.

One thing you will read everywhere is that container roots sit “one zone colder” than in-ground roots, and that you should therefore treat containers as a zone colder. I could not verify that from an extension source, so this page does not repeat it. What extension material does support is duller and more useful: containers dry out faster and swing in temperature more than soil does, so they need more attention, not a different zone label. If a frost is coming, the practical move is the same either way, which is to move pots under cover or bring them in. There is a first frost checklist for pots and small gardens that walks through it.

For growing in limited space, container vegetables for small spaces covers what is actually worth the pot, and shade plants for containers handles the balconies that never get full sun.

Five things that trip people up

The “frost-free date” is a misleading name. Illinois Extension does not mince words: “I think most of us have heard the term frost-free date which leads many to think there is a definitive date when no more frost can be guaranteed. However, its not that simple and I really dislike the use of the term since its misleading.”

The average is a coin flip, and the record is later than you think. Huntsville averages its last freeze on April 1 and has recorded one as late as May 7, in 1944. Illinois Extension notes McLean County has had frost as late as May 28.

Microclimates beat zone maps inside your own yard. USDA: “Your entire yard could be somewhat warmer or cooler than the surrounding area because it is sheltered or exposed.” The Illinois State Climatologist adds that “open, grassy areas are usually the first to experience frost, while areas under trees are more protected,” and that frost arrives in town a little later than in the countryside. Your south-facing wall is not your neighbour’s low spot.

A zone change does not mean you should replant. USDA is explicit: “If your hardiness zone has changed in this edition of the USDA Plant Hardiness Zone Map (PHZM), it does not mean you should start removing plants from your garden or change what you are growing.”

Different agencies publish different numbers for the same place. Illinois Extension’s own material described central Illinois as “frost-free date: April 11 to 30” in 2021, and then in 2025 gave Champaign County a median last frost of April 15 with a 9-in-10 date of April 29. Same agency, same region, different framing. This is the strongest argument for writing down a range and a percentile rather than a single date.

If you only do one thing

Find your nearest weather station’s median last spring frost and its nine-in-ten date. Put both in your phone. Then, when a planting chart tells you to sow something “in April,” you will know whether that means the start of the month, the end of it, or not until May.

The calendar is local. The zone column never was.

Sources

• USDA Plant Hardiness Zone Map, 2023 edition, and its how to use the maps guidance
• NOAA NCEI U.S. Climate Normals , 1991 to 2020
• NOAA Climate.gov interactive map of the average last spring freeze
• NCEI: when to expect your last spring freeze and the first fall freeze
• Illinois State Climatologist: frost dates and growing season
• Illinois Extension: is it time to plant
• Iowa State University Yard and Garden: when can I plant flowers and vegetables
• UMN Extension: planting the vegetable garden
• Penn State Extension: succession planting in the raised bed garden
• NWS Huntsville spring freeze climatology
• NC State: average first and last frost dates

Plant hardiness zones describe winter cold, not frost timing. Frost dates come from your nearest station, and they change over short distances. Check your own before you sow.