The season tips over: harvest, store and start planning next year.
The month ahead
September is the tipping point. In the north and the uplands the first frosts arrive this month, while mild coastal lowlands crop on into October. It is the season of the main harvest: lifting and storing roots, onions and potatoes, gathering apples and autumn raspberries, and clearing the beds that have finished. It is also the best month of the year to plant ; garlic, autumn onion sets, overwintering salads and hardy green manures all go in now.
Lift maincrop potatoes before the slugs find them. Once the soil cools and stays damp, slug damage accelerates fast. Store undamaged tubers in the dark; any with fork marks or scab should be eaten first, not stored. Harvest apples and pears as they ripen ; lift and twist; if the fruit does not come away easily, it is not ready.
What to do in September
Sow indoors
Under cover: windowsill, heated propagator or greenhouse.
Sow outdoors
Direct into prepared ground (or under a cloche/fleece where noted).
Plant out
Move transplants, sets, tubers or cloves into their final spot.
Harvest
Ready to pick or lift this month (windows are approximate).
Fruit
Prune blackcurrants after fruiting: remove about a third of the oldest wood, cutting right down to the base. New shoots will emerge from below ground and carry next year's crop.
Bring tender herbs under cover before the first frost ; basil will blacken at the slightest chill, and rosemary in a pot is far safer against a wall or in a cold frame than standing in the open. Collect fallen leaves for leaf mould.
Key jobs in September
- Lift and store maincrop potatoes before slugs find them.
- Harvest apples, pears, autumn raspberries and maincrop vegetables.
- Plant autumn onion sets and garlic in free-draining ground.
- Sow overwintering salads and hardy green manures on empty beds.
- Plant bare-root fruit bushes and canes as they become available.
- Prune blackcurrants after fruiting: remove a third of the oldest wood.
- Bring tender herbs (basil, rosemary in pots) under cover before frost.
- Start planning next year's rotation while this season's lessons are fresh.
- Mow the lawn less frequently as growth slows; raise the cutting height.
- Collect fallen leaves for leaf mould; pile them in a wire cage and forget about them.
The Scottish picture this month
Cooling and often wet. The first autumn frosts return to the uplands and the north, while mild coastal lowlands stay frost-free into late October or November.
Frost: Upland and northern first frosts arrive mid-to-late September; lowlands and Islands still have weeks of frost-free growing left.
Region-aware toolFrost Date CalculatorYour local last and first frost bands: the single most useful number for timing everything above.Watch out for
Vine weevil (larvae)
The lethal stage: grubs eat roots from autumn to spring, worst in pots and containers. Check the roots of any plant that wilts for no clear reason.
Allium leaf minerRHS UK timing · uncertain in Scotland
Two egg-laying periods on alliums: March–April and late summer–autumn. Covering leeks, onions and garlic with fine mesh over those windows is the main defence.
UK timing, and conditional: the pest may not be established across all of Scotland, and where it is, Scottish generations have consistently lagged the UK ones. Treat these as a UK guide, not a Scottish forecast.
Leek mothRHS UK timing · uncertain in Scotland
Two UK generations: May–June and a more numerous August–October flight. Mesh over alliums is again the practical control.
UK timing, and conditional in the same way as allium leaf miner; presence in Scotland is uncertain and local timing is unverified.
Carrot flyRHS UK timing · uncertain in Scotland
The avoidance model: sowing after mid-May dodges the first generation, and lifting before late August dodges the second. A 60cm fleece/mesh barrier helps whenever carrots are up.
The generation timing is the UK model; the Scottish calendar is not separately sourced. Use it as an avoidance strategy, not a dated forecast.
Mint rustbroad seasonal window
Symptoms can appear from spring until autumn. Cut affected mint to the ground and never propagate from infected plants.
Timing is broad, close to "the whole growing season", so treat it as a standing watch rather than a monthly alert.
These problems track the weather, not the calendar. Their month-by-month activity isn't sourced, so we tie them to the conditions that trigger them rather than pretend a date:
Potato & tomato blight
Watch when: warm, humid, muggy spells in the main growing season
June onwards for the UK; Scottish onset not separately sourced, so we tie it to the weather. Watch potatoes and outdoor tomatoes after warm, wet spells.
Grey mould (botrytis)
Watch when: cool, damp, still air, especially under cover
Can occur year-round with latent infections; ventilation and airflow are the real controls. Peaks when growth is soft and air is stagnant.
Powdery mildew
Watch when: dry spells and drought stress at the roots
Driven by dry, warm weather and plants short of water. Keep roots watered and airflow open around courgettes and peas.
Leek rust
Watch when: warm, wet summers with poor airflow
A wet-summer disease that is genuinely more of a Scottish problem than the allium insects. Space alliums generously; driven by leaf wetness, not a date.
Sources
- RHS: grow-your-own guides: sowing, planting and harvest timing for all 25 crops (each window hand-derived from the crop's own RHS prose)
- GrowScotland: pest activity framing: which pests to flag each month is our own reading of RHS activity data. We surface only what RHS sources, and frame weather-driven diseases by trigger conditions rather than inventing a Scottish calendar