Why your hardiness zone won't give you a frost date
Zone, frost dates and latitude are three different numbers that answer three different questions. Nearly every garden tool uses the first one for all three.
Photo: PumpkinSky · CC BY-SA 4.0, via Wikimedia Commons.
Three questions, three numbers
A garden planner has to answer three separate questions about where you live:
Will this perennial make it through the winter here?
When can I sow this annual, and will it finish before frost?
Will this onion form a bulb at my latitude?
The answers come from three different numbers. Your hardiness zone answers the first. Frost dates and accumulated heat answer the second. Latitude answers the third. Using one of them for all three is the most common mistake in garden tools, and it’s nearly everywhere.
What a zone is, and what it isn’t
A USDA hardiness zone captures exactly one thing: the average coldest night of the year. It’s a winter-survival number. It tells you whether a fig will live. It can’t tell you when to plant a tomato.
Hardiness zone is the correct variable for perennials, and only for perennials.
If you skip this, it still grows, just less well. The planner suggests it.
How it works: Zones encode average annual minimum winter temperature, which governs whether a woody perennial survives to spring. It is the wrong quantity for an annual, which never sees winter.
A zone that is too warm is a second, different mechanism. Mild winters do not kill a perennial; they fail to chill it. Deciduous fruit crops need a winter dormancy period to break bud and set fruit, so in a zone milder than a plant is adapted to it may survive and never crop. Only the cold end is enforced, because only winter survival follows from what a zone measures; the warm end gets advice and is never refused.
That warm limit is a property of the cultivar, and that limits what this rule can do. Chill requirement is published per variety and varies enormously within a species, so a species-level ceiling says "the modeled range does not reach your zone", never "no plant of this kind will crop for you". So above its range the planner still offers the plant, with advice to choose a cultivar rated for your zone.
Evidence: checked against 1 source in the research-based literature.
Winter cold-hardy cultivars adapted to the colder regions of Oregon (such as zones 5 to 7), may not grow normally in regions of the Willamette Valley (zone 8) where winters are warmer and plants receive less winter chill.
Plus 2 working notes on how the sources were read.
Two places in the same zone can have growing seasons weeks apart, because the coldest night of the year and the date of the last spring frost aren’t the same thing, and they aren’t closely tied to each other. A mild coastal spot and a place deep inland can share a zone number and have almost nothing else in common.
So this planner uses zone where zone applies, and nowhere else.
Scheduling is a frost question
When to sow depends on your actual frost dates and on how much heat the season builds up, which is what decides whether a crop can finish in time.
Don't sow or set out tender crops before your last frost date
If you skip this, the planting fails. The planner won’t allow it.
How it works: The season's accumulated growing degree days (a running total of its warmth) against the total the crop needs to mature, or the crop's days to maturity against the number of frost-free days.
Evidence: checked against 3 sources in the research-based literature.
Once you know the average dates of the last frost in spring and the first frost in fall, count the number of days between. If the “days to maturity” figure for a desired vegetable is a larger number of days and the “frost tolerance” column indicates “none,” the sad truth is that particular vegetable probably won’t grow to maturity.
… high elevation areas don’t accumulate enough “growing degree units” (GDUs, also called growing degree days and heat units) to support some of the most coveted warm season vegetables
Remedy
Start it indoors and transplant it, choose a variety with fewer days to maturity, or skip this crop at this site.
The second one asks whether the season is long enough for the plant to finish, which is a different question from whether it survives. A zone can’t answer it at all, and it’s the question that decides whether your melons ever ripen.
Frost dates are odds
There’s one more problem with frost dates the way they’re usually handed to you. “Last frost, May 15” gets presented as a fact, but it’s really a probability. It’s the date by which the frost has passed in some share of years, and which share almost never gets said out loud.
This planner shows you the odds instead of a single date, because the odds are what you’re betting against. Planting on the 50% date means you’re taking a coin flip. Waiting for the 10% date costs you two weeks of season. That’s a real trade, and you should get to make it yourself.
Where a number comes from a model and wasn’t measured, the page says so, and tells you how far the data traveled to reach you:
Every derived climate value carries the tier, grade, station, and distance that produced it.
If you skip this, it still grows, just less well. The planner suggests it.
How it works: Frost percentiles exist only at weather stations: NOAA NCEI's published normals in the US, and odds worked out from a station's own daily record in NOAA's GHCN-Daily abroad. Values elsewhere are offsets or model output. A bare date conceals the difference between a measurement and an inference.
There’s a tempting fix here that this project blocks outright. Frost figures come from weather stations, your garden isn’t sitting at a station, and the obvious move is to correct for elevation: colder as you go up.
On the nights that matter, that gets it exactly backwards.
Frost dates can be downscaled by applying a standard lapse rate to elevation.
The planner won’t plan around this claim. What is known instead is below.
What is known: On clear still nights, which are exactly the nights that produce radiative frost, cold dense air drains downhill and pools. The valley floor is colder than the slope above it. A standard lapse rate predicts the opposite and makes frost dates worse in the terrain where accuracy matters most. That is why the planner uses PRISM, whose model accounts for temperature inversions, and never a lapse rate.
Evidence: checked against 1 source in the research-based literature.
Radiation frosts occur when the heat that has been stored in the upper soil surfaces is radiated back into the atmosphere at night. Wind speeds are generally low, and an inversion usually develops with temperatures near the ground surface falling to, or below
The frosts that bite happen on still, clear nights. On those nights the cold air slides downhill and pools in the low ground. The bottom of the dip is the frost pocket, and the slope above it is warmer. Correcting by elevation predicts the opposite of what really happens, and does it with confidence. Sometimes refusing a plausible-looking correction is the accurate thing to do.
And your own ground beats all of it
The model is a starting point. Once you’ve logged enough seasons on a particular patch, what happened there takes over from the model for that patch:
Your own frost records from 3+ seasons outrank any climate model for your plot
If you skip this, it still grows, just less well. The planner suggests it.
How it works: Garden-scale microclimate (south wall, frost pocket, raised-bed drainage, urban heat island) lives below 800m resolution and below 80m. No gridded product resolves it. The season log is a direct measurement on the exact ground in question.
Evidence: checked against 3 sources in the research-based literature.
… weather station temperature readings, often taken at local airports, most often do not reflect the temperature in a grower’s fields, especially in frost pockets. Growers should use their own thermometers
Generally, the corrections consisted of adding or subtracting a correction to the key station forecast. For example, a grower might subtract 0.5 °C from a key station forecast for a crop located in a low spot.
Your garden has its own little climate that no station knows about: a south-facing wall, a cold hollow, a hedge that keeps the wind off. After a few seasons of your own records, the planner trusts your ground over its own numbers, and tells you which one it’s going on.
If you skip this, the planting fails. The planner won’t allow it.
How it works: Bulbing fires when daylength crosses a cultivar's critical threshold (short ~10-12h, intermediate ~12-14h, long ~14-16h). A short-day cultivar grown too far from the equator crosses its low threshold early and bulbs prematurely on small plants, so bulbs stay small. A long-day cultivar grown too close to it never reaches its high threshold and fails to bulb. Changeover is around the 35th-38th parallel, north or south.
Evidence: checked against 2 sources in the research-based literature.
… onions will not form bulbs until daylength reaches the cultivar's critical threshold. Short-day onions bulb at approximately 10-12 hours of daylength ... Intermediate-day onions bulb at approximately 12-14 hours ... Long-day onions bulb at 14-16 hours of daylength.
Long day varieties are for northern growing areas (above the 35th latitude); short-day varieties are for southern. If short-day varieties are grown in our zone, they won't do well. They will not form bulbs of any decent size. And since the long day onions need more daylight, they will not do well
Remedy
Choose long-day onions far from the equator and short-day near it; the planner picks the group from your plot's latitude.
Onions form their bulbs based on how long the days are. A short-day onion planted far north will grow leaves all season and never bulb up, and no amount of zone information will warn you. It’s a latitude question, so it gets a latitude answer.
