In New Brunswick, the cover crop calendar hangs on two dates: May 23, the average last spring frost, and October 2, the average first fall frost. Frost-tender spring sowings wait until after the first. Fall sowings need to be in the ground well before the second. Both dates come from the Moncton station and are midpoints, not guarantees.
The short answer for New Brunswick
Sow tender cover crops after May 23 and fall cover crops after harvest, ideally weeks ahead of October 2. Environment and Climate Change Canada’s 1981-2010 normals for Moncton give both dates as averages. In one year out of two the last spring frost falls later than May 23, and in one year out of two the first fall frost arrives before October 2. Neither date marks a safe day. Each is the middle of a window.
| Marker | Moncton average | What it means for sowing |
|---|---|---|
| Last spring frost | May 23 | Midpoint of the spring window. Tender seed sown on this date still has about even odds of meeting a frost. |
| First fall frost | October 2 | Midpoint of the fall window. In half of years the frost comes sooner. |
| Coldest month, mean daily minimum | -13.9 °C | The winter a fall-sown cover crop must survive, if it is meant to. |
Count the days between the two averages and you get about 132, or roughly 19 weeks. That is the entire frost-free stretch at this station, and it has to hold your vegetables, your harvest and your cover crop. Every week spent waiting for a late tomato bed to finish is a week taken from the fall window.
How the dates sort the species
Frost-tender covers such as buckwheat follow the spring date, so seed them after May 23 and, in a cold year, a week or two later. Frost-hardy covers tolerate a cold snap, so they can go in earlier in spring and later in fall. Oats die over winter. Rye lives through it. Which one you want depends on whether you need bare soil in April. The The Best Cover Crops for New Brunswick page covers the choice by type, and the parent guide, Cover Crops for New Brunswick Gardens, covers the reasons to plant them at all.
One caveat on the figures. Canadian normals cover 1981-2010, while the US figures elsewhere on this site cover 1991-2020, so don’t line the two up as if they were measured the same way.
What will not work here
A 19-week frost-free stretch rules out anything slow. A cover crop that needs a full season to reach a useful size will not get one, and a fall sowing that goes in after October 2 is a bet against the average. In half of years the frost has already arrived by then. Seed sown that late may germinate and sit as threadbare seedlings, doing little for the soil.
Sowing tender species early is the second failure. After a few mild days in April, the temptation is real, but May 23 is an average. In half of years frost falls after it, and a frost-tender cover crop sown ahead of it can be killed outright. Seed you have to buy twice is an expensive way to learn the calendar.
Cold is the third problem. The mean daily minimum of the coldest month at Moncton is -13.9 °C (6.9 °F, if you want to compare with a US garden). That is an average night in the worst month, not a record, so some nights run well below it. A cover crop that was never meant to overwinter will not, and even a hardy one sown too late can enter that cold too small to survive it. Treat “overwinters in the catalog” as a claim to test against your own fall, not as a promise.
Waiting for rain and perfect conditions also costs more here than in a longer-season state. In a place with a long autumn, a missed week is a shrug. Here, when the window is already short, a missed week can be the difference between a cover crop that establishes and one that doesn’t. Sow when the bed is free and the soil is workable, and accept a rough seedbed rather than waiting for a perfect one.
Every figure on this page also comes from one station. Moncton sits in the southeast of the province. A page that gave you one date for all of New Brunswick would be inventing precision.
What changes across New Brunswick
The reference city, Moncton, sits in zone 5a on the Canadian plant hardiness system, using Natural Resources Canada’s 1991-2020 normals. That is the zone of the reference city, not of the province. This page does not have the full spread of zones across New Brunswick, so there is no single answer to give. What the data does tell us is that northern and upland districts run weeks behind Moncton, which means later springs and earlier falls.
Shift accordingly. If you garden in the north or at elevation, treat May 23 as a date you will probably reach later and October 2 as a date you will probably meet sooner. Your window is narrower than Moncton’s 19 weeks, and the fall sowing is where it hurts most. Favor frost-hardy covers that can be seeded late, and get them in the day a bed clears.
A note on the zone system itself. The Canadian zones draw on seven climate variables, whereas the USDA system relies on one. Natural Resources Canada says plainly that there is no simple conversion between the two, and that Canadian zone 4a overlaps nine different USDA zones. Don’t read “5a” as a USDA zone, and don’t compare it with the US zones elsewhere on this site.
To find which zone applies to your own municipality, use the map on Natural Resources Canada’s Plant Hardiness Site. We won’t tell you your zone from here, because a hillside and a river valley a few miles apart can differ.
Where to check this for your own county
Start with Agriculture and Agri-Food Canada, the federal department. The provincial agriculture department is the more specific stop, but its site refuses automated checks, so we link the federal one, which we have verified. This page does not have a New Brunswick county calendar to hand you, and it will not borrow another region’s.
When you get there, look for two things. First, a local planting calendar, ideally one built on frost dates from a station near you rather than from Moncton. Second, the cover crop varieties that have actually been trialled in your county, with notes on how they did in a cold or short season. A trial result from nearby is worth more than a dozen catalog descriptions.
A county recommendation beats a provincial one. This page is the provincial one: it uses a single reference station and a single zone, and it gives you the shape of the calendar, not the day. If an advisor, a farm supplier or a local trial tells you something different for your area, believe them.
Checking this against your own garden
Your own observation outranks every date above. Three things are worth watching, and none needs an instrument.
- Soil warmth by feel. Push your hand into the bed in the morning. If it still feels cold and heavy, tender seed will sit. A bed that warms sooner than your neighbor’s is a sign of a sheltered or south-facing spot.
- The first male flowers. On squash and similar crops, the first flush of male blooms tells you how fast your site warms. If it arrives later than the calendar suggests, shift your fall sowing earlier.
- The first true leaves. When seedlings put out their first true leaves, note the date. Slow seedlings point to a cool pocket, such as a hollow where cold air settles. That is where frost will come first in fall.
Write the dates down for a season or two. Your notes will be worth more than the Moncton averages, because they describe the one garden you actually plant.
Some gardeners also time this by the Moon: when to plant cover crops by the moon, as almanac tradition rather than fact.
Common questions
Is May 23 a safe date to sow tender cover crops?
No. It is the average last spring frost at the Moncton station, so in one year out of two a frost falls later. Treat it as the midpoint of a window and wait longer in a cold spring or a northern district.
Can I sow a cover crop after October 2?
You can, but you are betting against the average. In half of years the first fall frost has already come by then. A frost-hardy cover is the only reasonable choice, and its results will be modest.
Do these dates apply to the whole province?
No. They come from the Moncton station, and northern or upland districts run weeks behind it. Use them as a starting point and check a local source for your own area.
Is New Brunswick’s zone 5a the same as USDA zone 5a?
No. The 5a here is on the Canadian plant hardiness system, which uses seven climate variables. The USDA system uses one, and Natural Resources Canada states that there are no simple conversions between the two.