Updated: 2026-07-31
The earliest date on record when the minimum temperature stayed above 0°C — the last frost day — is 11 April at the Aberfoyle station, the nearest Met Office measuring point to Glasgow (35.6 km away). That was in 1968, and it marks the absolute earliest window a gardener could have risked planting tomatoes outdoors without a freeze. In practice, that single early date is the optimistic end of the scale; most years, the safe window comes later.
Let’s be clear about what we’re measuring. The nearest Met Office weather station to Glasgow is Aberfoyle, in Perthshire-in-Central-Region, sitting at 28 m altitude. Its records, spanning from 1968 through 2001, are the best continuous temperature series we have for the Glasgow area. But Glasgow itself — with its urban heat, the River Clyde, and its position between hills and the Firth — can behave differently from Aberfoyle, sheltered in the Trossachs near a reservoir. So read these numbers as a reliable guide from the closest official thermometer, not a perfect forecast for your specific garden.
The key question for tomato growers is simple: when does the last frost hit? Tomatoes are tender plants; a single night below 0°C can blacken leaves and set the crop back weeks. The Met Office record from Aberfoyle shows the earliest last frost was 11 April 1968. That’s the standout date — the earliest spring day in the entire 1968–2001 record when the minimum temperature stayed above freezing.
But a single warm April night doesn't mean safe planting. What matters more is the pattern of minimum temperatures month by month, and that's where the monthly averages below tell a clearer story.
These are the average coldest temperatures for each month of the year, based on the full daily record at the station. They show how long the ground stays cold at night — the real constraint for tender crops:
| Month | Mean Minimum Temp (°C) |
|---|---|
| January | 0.1 |
| February | -0.9 |
| March | 0.1 |
| April | 1.6 |
| May | 4.5 |
| June | 7.0 |
| July | 9.0 |
| August | 9.3 |
| September | 7.0 |
| October | 5.0 |
| November | 1.4 |
| December | 0.7 |
Look at February: that's the coldest month of the record, with an average nightly minimum of -0.9°C. March creeps up to 0.1°C — still at freezing. April rises to 1.6°C, which is above freezing on average, but that’s a monthly mean; individual nights can still dip below zero. May's mean minimum of 4.5°C gives real confidence, and June’s 7.0°C is firmly in safe territory.
So what does this mean for planting? The 11 April 1968 date is the earliest possible last frost in the record — a lucky spring. But with February averaging -0.9°C and March at 0.1°C, frost is a real threat right up to the end of March. Even April, at 1.6°C average minimum, can produce a cold night that would kill a young tomato plant.
Tomatoes need two things: no frost and consistent soil warmth. The Aberfoyle data shows the average minimum temperature only climbs reliably above 4°C in May (4.5°C), and above 7°C in June (7.0°C). That’s the real signal. While 11 April 1968 was a one-off early gift, the typical safe planting window for tender crops like tomatoes in the Glasgow area, based on this station’s record, is from mid-May onward.
Here’s why the monthly table matters more than the headline date:
The 11 April 1968 date is the record — the single earliest last frost in the 1968–2001 series. But gardening by records is gambling. The monthly means tell a steadier story: April’s average low of 1.6°C is not enough margin for a plant that can’t survive a single degree below zero. May’s 4.5°C starts to be safe, and by June at 7.0°C, the risk is genuinely low.
If you want the earliest possible start, watch the forecast, not the calendar — the 1968 record shows an 11 April planting could have worked. But for the rest of us, the Aberfoyle record points to a safer window: late May at the earliest, early June with confidence. May's mean minimum of 4.5°C is the threshold where many gardeners transplant; June’s 7.0°C removes almost all doubt.
One caution from the data: February’s average of -0.9°C is a reminder that hard frost is possible well into the spring. Even if you see mild daytime temperatures in March (monthly average minimum 0.1°C), the nights can still bite. Tender plants belong indoors or under protection until the overnight lows, on average, stay well above freezing — which the Aberfoyle record shows first in May.
Glasgow gardeners have a short but workable season. The data from Aberfoyle, gathered daily from 1968 to 2001 by the Met Office, gives a clear answer: the earliest recorded safe date is 11 April, but the reliable window for planting tomatoes outdoors without frost protection is from mid-May onward, matching when mean minimum temperatures reach 4.5°C in May and 7.0°C in June. Watch the May nights, prepare for colder-than-average springs, and you’ll be harvesting while the record books still show that 1968 April as the one great exception.
Source: Met Office MIDAS (Integrated Data Archive System), UK national weather station network. Station: Aberfoyle (Perthshire-in-Central-Region, 28 m altitude), period 1968–2001. Data held at CEDA, Open Government Licence (OGL v3.0).
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