In Newfoundland and Labrador, cover crops go in twice: in spring once the ground can be worked, and in late summer early enough to take hold before frost. The only frost dates this page has come from one station in St. John’s, so treat them as a starting point rather than a deadline. What follows explains how to read those dates and where they stop applying. For the bigger picture, see Cover Crops for Newfoundland and Labrador Gardens.
The short answer for Newfoundland and Labrador
Plan on sowing between May 30 and October 17. Those are the average last spring frost and the average first fall frost at the St. John’s A station, taken from Environment and Climate Change Canada’s 1981-2010 climate normals.
Each date is a mean, and a mean is a midpoint, not a safe line. In about one year out of two, a frost still arrives after May 30. In about one year out of two, the first fall frost arrives before October 17. Subtract one date from the other and you get roughly 140 days between them, which describes an average year and promises nothing for any particular one. One more caution: these Canadian normals stop at 2010, while the US figures elsewhere on this site run 1991-2020, so don’t lay the two side by side.
Spring sowing
Frost-hardy cover crops can go in as soon as the soil can be worked, ahead of May 30, because a late frost does them little harm. Frost-tender types such as buckwheat belong after May 30, and since half of all years still frost later than that, a cautious gardener adds a margin rather than sowing on the date itself.
Late-summer and fall sowing
Work backward from October 17. A cover crop needs weeks of growth before frost to do its job, so that date is where you want plants established, not the day you sow. Sow once an early vegetable crop is out of the bed. This page has no species-by-species cutoff date, so choose the type first with The Best Cover Crops for Newfoundland and Labrador.
| Window | Anchor in the data | How to read it |
|---|---|---|
| Spring, frost-hardy types | Soil workable, before the May 30 mean | A late frost is tolerated |
| Spring, frost-tender types | After May 30, with a margin | Half of years still frost later |
| Late summer to fall | Established before the October 17 mean | Half of years frost earlier |
What will not work here
A 140-day average window is short, and it shrinks further once a vegetable crop has occupied the bed for most of it. Most failures come from ignoring that.
Sowing too late for the season you have
Tender, warm-season cover crops sown in late summer get caught by an autumn frost before they have done anything. Sowing right on October 17 fails too, because in about half of years frost has already come, and seed that germinates slowly in cold soil leaves almost no growth to protect the bed. Slow cover crops that need a long, warm run to mature simply do not fit a window this size.
Overwintering needs the same caution. The mean daily minimum of the coldest month at the reference station is -8.7 °C (16.4 °F). That is an average of nights, not a record low, so colder nights do occur. A marginal type sown late, with little top growth, is the first to be lost. Sown early enough to establish, a hardy type has a far better chance.
Dates borrowed from somewhere else
Calendars written for mainland Canada or for US states assume a longer, warmer season, and following them in this province means sowing too late. Even the St. John’s dates should not be carried everywhere. They come from a single station, and northern or upland districts run weeks behind it. Treating May 30 as a safe date in those places is a mistake, because the real last frost there falls well after it.
What changes across Newfoundland and Labrador
The reference city sits in Canadian hardiness zone 6a (Natural Resources Canada, 1991-2020 normals). That is the zone of one city, not of the province. A garden in northern Labrador or on high ground inland lives in a different climate, so there is no single answer for the whole territory. This page has only that one zone number, not the spread, and the map shows the rest.
Do not read 6a as a USDA zone. The Canadian system draws on seven climate variables and the USDA system on one, and Natural Resources Canada states plainly that there are no simple conversions between them. Its own example: Canadian zone 4a overlaps nine USDA zones, from 3a to 7a. To find where your garden falls, use the Canada’s Plant Hardiness Site map and look up your own municipality.
The shift is predictable even without the numbers. The farther north or higher up you are, the later the spring window opens and the earlier the fall one closes. Move spring sowing later, move late-summer sowing earlier, and lean toward the hardiest types. In a cold district the margin around each date matters more than the date.
Where to check this for your own county
Start with the Newfoundland and Labrador Department of Fisheries, Forestry and Agriculture. Look for two things: a local planting calendar for your region, and the cover crop varieties that have actually been trialled near you. A variety that thrived in one trial plot tells you far more than any general rule written from a single airport station.
A county or district recommendation beats a provincial one, and this page is the provincial one. It rests on a single station and a single hardiness reference. It cannot see the valley that holds frost, the exposed headland, or the sheltered garden behind a hill. Newfoundland and Labrador is not organized around counties in the way some US states are, so think in terms of your own community and region. If you find a local source with dates that differ from the ones here, trust the local one.
Checking this against your own garden
Your own observations outrank every date above. Three are worth watching, and none needs equipment.
- Soil warmth by feel. Push a hand into the bed and hold it there. If the soil still feels cold and clammy against your skin, hold off on tender seed. A bed that warms early is a sheltered one.
- The first true leaves. Watch a quick row of radishes or lettuce, or the weeds. Once seedlings put out real leaves, the bed is growing. If yours reach that stage later than a neighbour’s, shift your dates later.
- The first frost nip. Note when tender plants such as squash or basil first blacken in the fall. That is your own first-frost date, and a low corner will often blacken before a sheltered one.
Write those three dates down for a couple of seasons. After two years they will tell you more about your microclimate than a station sixty or a few hundred miles away ever could.
Some gardeners also time this by the Moon: when to plant cover crops by the moon, as almanac tradition rather than fact.
Common questions
Can I sow tender cover crops on May 30?
You can, but it is not guaranteed safe. May 30 is the average last spring frost, so in about one year out of two a frost still comes after it. Waiting a little longer, or covering seedlings on cold nights, lowers the risk.
Is October 17 the last day to sow?
No. It is the average first fall frost, and it falls earlier in about half of years. Aim to have plants established by then, which means sowing well before it. This page has no species-specific cutoff dates.
Do the St. John’s dates apply in Labrador?
Not directly. The figures come from one station in St. John’s, and northern or upland districts run weeks behind it. Use them as a rough reference only and shift your sowing later in spring and earlier in late summer.
Is hardiness zone 6a the same as a USDA zone?
No. It is a Canadian zone, built on seven climate variables, and Natural Resources Canada says there is no simple conversion to the USDA system. Check your own municipality on the national hardiness map.