When to Start Cannabis Seeds Outdoors in the UK

The calendar is a harder enemy than weather, pests, or nutrient deficiencies. Every year, UK growers lose entire outdoor harvests not because of bad genetics or poor technique, but because they started too late and the first October frost arrived before the resin had finished building. The mistake is almost always the same: picking a strain, throwing seeds down in late May or June, and hoping for the best. The correct approach is to work backwards from your regional first-frost date, match that window to your strain's finish time, and build in a buffer. This guide gives you the exact start dates, regional frost data, and a named decision framework so you never lose a season to bad timing again.

What is the UK outdoor cannabis growing season?

The UK outdoor cannabis season runs from the last spring air frost to the first autumn air frost. In practice that window spans roughly late April to mid-October in the South West and South East, shrinking to late May to late September across much of Scotland and Northern Ireland.

According to Met Office 1991 to 2020 climate normals, the average last air frost in the South West falls around mid-April, while central Scotland averages a last frost in mid-May. The first autumn frost arrives in Scotland as early as late September, giving northern growers fewer than 18 frost-free weeks in many locations.

Wet autumns matter more than winter cold in most UK grow advice. September rain and cool nights arriving in week four of flowering is the dominant failure mode, not hard freezes. Mould resistance and a fast finish consistently beat raw cold tolerance as selection criteria for British outdoor genetics.

Autoflower vs photoperiod: which suits a UK season?

Autoflowering seeds flower based on age, not day length. They finish in 70 to 85 days from germination on any light schedule, making the maths simple: start date plus finish time equals harvest date.

Feminised photoperiod strains require shortening days to trigger flowering. In the UK, that shift happens naturally around the August solstice. A photoperiod plant started from seed in May will spend roughly 12 to 14 weeks in vegetative growth before flowering begins, pushing harvest to October or even November in slower genetics.

For growers above the Midlands, autoflowers are almost always the safer call. Below the Midlands, a fast-finishing feminised indica started in late April can work well, but the window is tight and mould-resistant genetics are non-negotiable.

The Simple Timing Rule Most UK Growers Miss

Outdoor start dates by UK region

The UK north-south gradient is the dominant factor. Last-frost dates shift by three to five weeks between Cornwall and the Scottish Highlands. Coastal areas are warmer in both spring and autumn; inland and elevated sites are colder and frost sooner.

South West England (Cornwall, Devon, Somerset)

Last frost: mid-April. First frost: late October to early November. This is the most generous UK window. Autoflowers can start outdoors direct from mid-May. Fast photoperiod indicas started in late April under fleece can finish by late September. A second autoflower run started in late June is possible in sheltered sites.

South East England and London

Last frost: late April. First frost: late October. Start autoflowers outdoors from mid-May; photoperiods from late April with cloche cover. Urban heat islands in London push first-frost dates later, giving an extra one to two weeks versus surrounding rural areas.

Midlands (East and West)

Last frost: early to mid-May. First frost: mid-October. The Midlands frost-free window runs roughly 22 to 23 weeks. Autoflowers started 1 May to 15 May finish by mid-August, well clear of autumn risks. Fast photoperiod indicas are possible but require mould-tolerant genetics for the damp September finish.

Yorkshire, North West, North East England

Last frost: mid-May. First frost: early to mid-October. The window narrows to 20 to 21 weeks and autumn rain arrives earlier. Autoflowers are strongly preferred; start no later than 20 May. Fast-flowering genetics finishing in 65 to 75 days are the smartest pick here.

Wales

Wales varies sharply by elevation. Coastal south Wales mirrors the South West; upland mid-Wales behaves like northern England. Use the Met Office regional data for your specific postcode. Mould resistance is the single most important trait for Welsh outdoor grows given the Atlantic rainfall pattern.

Scotland and Northern Ireland

Last frost: mid to late May in lowlands, June in Highland glens. First frost: late September to early October. The frost-free window can be as short as 16 to 17 weeks. Only autoflowers finishing in 70 to 75 days are reliably safe. Start seeds indoors in the first two weeks of May and transplant after the last frost. A poly-tunnel extends the season by two to three weeks for serious growers.

Regional worked examples with frost data

These five examples apply the 14-Day Buffer Rule with Met Office climate normals. All frost dates are approximate based on 1991 to 2020 averages.

UK outdoor cannabis calendar

Common timing mistakes UK outdoor growers make

After reviewing hundreds of public UK grower journals and the questions that come up most often in growing communities every spring, the same timing errors appear again and again. Here are the five most costly ones.

Mistake 1: Starting too late to save money on heating

Growers avoid heating propagation trays by waiting until June to start seeds. The result is an autoflower finishing in September with four weeks of mould risk layered on top. A small heat mat (typically £15 to £25) costs far less than a lost harvest. Start seeds indoors in early May regardless of outdoor temperatures.

Mistake 2: Transplanting before the last frost

Young cannabis seedlings in the first two to three weeks are highly vulnerable to frost. A single overnight frost at -2°C can kill or severely stress a seedling. Check your local Met Office data for the average last frost date, then wait until it has passed before transplanting outdoors. In the Midlands, that means waiting until at least 10 to 15 May in most years.

Mistake 3: Choosing a photoperiod strain without checking the finish time

A 70-day flowering photoperiod plant started from seed in May will not begin flowering until late July or early August in the UK, putting harvest at mid to late October. That is well inside the high-mould risk window for most of England, Wales, and all of Scotland. Check the total seed-to-harvest time, not just the flowering period.

Mistake 4: Ignoring the north-south gradient for the same start date advice

Generic "start in May" advice works for a grower in Devon but can leave a Glasgow grower with a harvest that runs into October frost. Your region determines your start date. What works in the South West is often three weeks too late for Scotland. Always calculate from your local frost data, not a national average.

Mistake 5: No buffer for weather variance

Breeders publish finish times as averages under controlled conditions. Real outdoor plants can run seven to ten days longer due to cooler nights, overcast skies, or temperature swings. The 14-Day Buffer Rule exists to absorb this variance. Cutting the buffer to five or six days to squeeze in a late start is the single most common reason experienced growers still lose crops.

If you want help matching start dates to genetics, our strain finder tool lets you filter by finish time and climate suitability to narrow down the right pick for your region.

Which strain type should you choose?

What happens if you choose wrong

Bottom line: the strain choice made in May determines whether September is a harvest celebration or a mould disaster. The decision takes five minutes; the consequences last a season.

Browse our full cannabis seed collection and use the finish-time filters to find strains that fit your region's safe window.

Frequently asked questions

Sources

• Met Office UK Climate Averages (1991, 2020 normals)
• Met Office Regional Climate Summaries
• RHS Plant Hardiness Ratings
• NIAB Crop Research
• Journal of Cannabis Research